26 | 10 |
11 |12 | Architectural Styles and the Design of Network-based Software Architectures
13 |14 | by
15 |16 | Roy Thomas Fielding
17 |18 | Doctor of Philosophy in Information and Computer Science
19 |20 | University of California, Irvine, 2000
21 |22 | Professor Richard N. Taylor, Chair
23 |24 |
25 |26 | The World Wide Web has succeeded in large part because its software architecture has been designed to meet the needs of an Internet-scale distributed hypermedia system. The Web has been iteratively developed over the past ten years through a series of modifications to the standards that define its architecture. In order to identify those aspects of the Web that needed improvement and avoid undesirable modifications, a model for the modern Web architecture was needed to guide its design, definition, and deployment.
27 |28 | Software architecture research investigates methods for determining how best to partition a system, how components identify and communicate with each other, how information is communicated, how elements of a system can evolve independently, and how all of the above can be described using formal and informal notations. My work is motivated by the desire to understand and evaluate the architectural design of network-based application software through principled use of architectural constraints, thereby obtaining the functional, performance, and social properties desired of an architecture. An architectural style is a named, coordinated set of architectural constraints.
29 |30 | This dissertation defines a framework for understanding software architecture via architectural styles and demonstrates how styles can be used to guide the architectural design of network-based application software. A survey of architectural styles for network-based applications is used to classify styles according to the architectural properties they induce on an architecture for distributed hypermedia. I then introduce the Representational State Transfer (REST) architectural style and describe how REST has been used to guide the design and development of the architecture for the modern Web.
31 |32 | REST emphasizes scalability of component interactions, generality of interfaces, independent deployment of components, and intermediary components to reduce interaction latency, enforce security, and encapsulate legacy systems. I describe the software engineering principles guiding REST and the interaction constraints chosen to retain those principles, contrasting them to the constraints of other architectural styles. Finally, I describe the lessons learned from applying REST to the design of the Hypertext Transfer Protocol and Uniform Resource Identifier standards, and from their subsequent deployment in Web client and server software.
33 | 34 | 35 | -------------------------------------------------------------------------------- /OEBPS/conclusions.htm: -------------------------------------------------------------------------------- 1 | 3 | 4 | 5 |10 | Each one of us has, somewhere in his heart, the dream to make a living world, a universe. Those of us who have been trained as architects have this desire perhaps at the very center of our lives: that one day, somewhere, somehow, we shall build one building which is wonderful, beautiful, breathtaking, a place where people can walk and dream for centuries.13 |
11 | -- Christopher Alexander [3] 12 |
14 | At the beginning of our efforts within the Internet Engineering Taskforce to define the existing Hypertext Transfer Protocol (HTTP/1.0) [19] and design the extensions for the new standards of HTTP/1.1 [42] and Uniform Resource Identifiers (URI) [21], I recognized the need for a model of how the World Wide Web should work. This idealized model of the interactions within an overall Web application, referred to as the Representational State Transfer (REST) architectural style, became the foundation for the modern Web architecture, providing the guiding principles by which flaws in the preexisting architecture could be identified and extensions validated prior to deployment.
15 |16 | REST is a coordinated set of architectural constraints that attempts to minimize latency and network communication while at the same time maximizing the independence and scalability of component implementations. This is achieved by placing constraints on connector semantics where other styles have focused on component semantics. REST enables the caching and reuse of interactions, dynamic substitutability of components, and processing of actions by intermediaries, thereby meeting the needs of an Internet-scale distributed hypermedia system.
17 |18 | The following contributions to the field of Information and Computer Science have been made as part of this dissertation:
19 |30 | The modern Web is one instance of a REST-style architecture. Although Web-based applications can include access to other styles of interaction, the central focus of its protocol and performance concerns is distributed hypermedia. REST elaborates only those portions of the architecture that are considered essential for Internet-scale distributed hypermedia interaction. Areas for improvement of the Web architecture can be seen where existing protocols fail to express all of the potential semantics for component interaction, and where the details of syntax can be replaced with more efficient forms without changing the architecture capabilities. Likewise, proposed extensions can be compared to REST to see if they fit within the architecture; if not, it is more efficient to redirect that functionality to a system running in parallel with a more applicable architectural style.
31 |32 | In an ideal world, the implementation of a software system would exactly match its design. Some features of the modern Web architecture do correspond exactly to their design criteria in REST, such as the use of URI [21] as resource identifiers and the use of Internet media types [48] to identify representation data formats. However, there are also some aspects of the modern Web protocols that exist in spite of the architectural design, due to legacy experiments that failed (but must be retained for backwards compatibility) and extensions deployed by developers unaware of the architectural style. REST provides a model not only for the development and evaluation of new features, but also for the identification and understanding of broken features.
33 |34 | The World Wide Web is arguably the world's largest distributed application. Understanding the key architectural principles underlying the Web can help explain its technical success and may lead to improvements in other distributed applications, particularly those that are amenable to the same or similar methods of interaction. REST contributes both the rationale behind the modern Web's software architecture and a significant lesson in how software engineering principles can be systematically applied in the design and evaluation of a real software system.
35 |36 | For network-based applications, system performance is dominated by network communication. For a distributed hypermedia system, component interactions consist of large-grain data transfers rather than computation-intensive tasks. The REST style was developed in response to those needs. Its focus upon the generic connector interface of resources and representations has enabled intermediate processing, caching, and substitutability of components, which in turn has allowed Web-based applications to scale from 100,000 requests/day in 1994 to 600,000,000 requests/day in 1999.
37 |38 | The REST architectural style has been validated through six years of development of the HTTP/1.0 [19] and HTTP/1.1 [42] standards, elaboration of the URI [21] and relative URL [40] standards, and successful deployment of several dozen independently developed, commercial-grade software systems within the modern Web architecture. It has served as both a model for design guidance and as an acid test for architectural extensions to the Web protocols.
39 |40 | Future work will focus on extending the architectural guidance toward the development of a replacement for the HTTP/1.x protocol family, using a more efficient tokenized syntax, but without losing the desirable properties identified by REST. The needs of wireless devices, which have many characteristics in common with the principles behind REST, will motivate further enhancements for application-level protocol design and architectures involving active intermediaries. There has also been some interest in extending REST to consider variable request priorities, differentiated quality-of-service, and representations consisting of continuous data streams, such as those generated by broadcast audio and video sources.
41 | 42 | 43 | -------------------------------------------------------------------------------- /OEBPS/content.opf: -------------------------------------------------------------------------------- 1 | 2 |12 | Excuse me ... did you say `knives'?15 |
13 | -- City Gent #1 (Michael Palin), The Architects Sketch [111] 14 |
16 | As predicted by Perry and Wolf [105], software architecture has been a focal point for software engineering research in the 1990s. The complexity of modern software systems have necessitated a greater emphasis on componentized systems, where the implementation is partitioned into independent components that communicate to perform a desired task. Software architecture research investigates methods for determining how best to partition a system, how components identify and communicate with each other, how information is communicated, how elements of a system can evolve independently, and how all of the above can be described using formal and informal notations.
17 |18 | A good architecture is not created in a vacuum. All design decisions at the architectural level should be made within the context of the functional, behavioral, and social requirements of the system being designed, which is a principle that applies equally to both software architecture and the traditional field of building architecture. The guideline that "form follows function" comes from hundreds of years of experience with failed building projects, but is often ignored by software practitioners. The funny bit within the Monty Python sketch, cited above, is the absurd notion that an architect, when faced with the goal of designing an urban block of flats (apartments), would present a building design with all the components of a modern slaughterhouse. It might very well be the best slaughterhouse design ever conceived, but that would be of little comfort to the prospective tenants as they are whisked along hallways containing rotating knives.
19 |20 | The hyperbole of The Architects Sketch may seem ridiculous, but consider how often we see software projects begin with adoption of the latest fad in architectural design, and only later discover whether or not the system requirements call for such an architecture. Design-by-buzzword is a common occurrence. At least some of this behavior within the software industry is due to a lack of understanding of why a given set of architectural constraints is useful. In other words, the reasoning behind good software architectures is not apparent to designers when those architectures are selected for reuse.
21 |22 | This dissertation explores a junction on the frontiers of two research disciplines in computer science: software and networking. Software research has long been concerned with the categorization of software designs and the development of design methodologies, but has rarely been able to objectively evaluate the impact of various design choices on system behavior. Networking research, in contrast, is focused on the details of generic communication behavior between systems and improving the performance of particular communication techniques, often ignoring the fact that changing the interaction style of an application can have more impact on performance than the communication protocols used for that interaction. My work is motivated by the desire to understand and evaluate the architectural design of network-based application software through principled use of architectural constraints, thereby obtaining the functional, performance, and social properties desired of an architecture. When given a name, a coordinated set of architectural constraints becomes an architectural style.
23 |24 | The first three chapters of this dissertation define a framework for understanding software architecture via architectural styles, revealing how styles can be used to guide the architectural design of network-based application software. Common architectural styles are surveyed and classified according to the architectural properties they induce when applied to an architecture for network-based hypermedia. This classification is used to identify a set of architectural constraints that could be used to improve the architecture of the early World Wide Web.
25 |26 | Architecting the Web requires an understanding of its requirements, as we shall discuss in Chapter 4. The Web is intended to be an Internet-scale distributed hypermedia system, which means considerably more than just geographical dispersion. The Internet is about interconnecting information networks across organizational boundaries. Suppliers of information services must be able to cope with the demands of anarchic scalability and the independent deployment of software components. Distributed hypermedia provides a uniform means of accessing services through the embedding of action controls within the presentation of information retrieved from remote sites. An architecture for the Web must therefore be designed with the context of communicating large-grain data objects across high-latency networks and multiple trust boundaries.
27 |28 | Chapter 5 introduces and elaborates the Representational State Transfer (REST) architectural style for distributed hypermedia systems. REST provides a set of architectural constraints that, when applied as a whole, emphasizes scalability of component interactions, generality of interfaces, independent deployment of components, and intermediary components to reduce interaction latency, enforce security, and encapsulate legacy systems. I describe the software engineering principles guiding REST and the interaction constraints chosen to retain those principles, contrasting them to the constraints of other architectural styles.
29 |30 | Over the past six years, the REST architectural style has been used to guide the design and development of the architecture for the modern Web, as presented in Chapter 6. This work was done in conjunction with my authoring of the Internet standards for the Hypertext Transfer Protocol (HTTP) and Uniform Resource Identifiers (URI), the two specifications that define the generic interface used by all component interactions on the Web.
31 |32 | Like most real-world systems, not all components of the deployed Web architecture obey every constraint present in its architectural design. REST has been used both as a means to define architectural improvements and to identify architectural mismatches. Mismatches occur when, due to ignorance or oversight, a software implementation is deployed that violates the architectural constraints. While mismatches cannot be avoided in general, it is possible to identify them before they become standardized. Several mismatches within the modern Web architecture are summarized in Chapter 6, along with analyses of why they arose and how they deviate from REST.
33 |34 | In summary, this dissertation makes the following contributions to software research within the field of Information and Computer Science:
35 |15 | This chapter continues our discussion of background material by focusing on network-based application architectures and describing how styles can be used to guide their architectural design.
16 |18 | Architecture is found at multiple levels within software systems. This dissertation examines the highest level of abstraction in software architecture, where the interactions among components are capable of being realized in network communication. We limit our discussion to styles for network-based application architectures in order to reduce the dimensions of variance among the styles studied.
19 |21 | The primary distinction between network-based architectures and software architectures in general is that communication between components is restricted to message passing [6], or the equivalent of message passing if a more efficient mechanism can be selected at run-time based on the location of components [128].
22 |23 | Tanenbaum and van Renesse [127] make a distinction between distributed systems and network-based systems: a distributed system is one that looks to its users like an ordinary centralized system, but runs on multiple, independent CPUs. In contrast, network-based systems are those capable of operation across a network, but not necessarily in a fashion that is transparent to the user. In some cases it is desirable for the user to be aware of the difference between an action that requires a network request and one that is satisfiable on their local system, particularly when network usage implies an extra transaction cost [133]. This dissertation covers network-based systems by not limiting the candidate styles to those that preserve transparency for the user.
24 |26 | Another restriction on the scope of this dissertation is that we limit our discussion to application architectures, excluding the operating system, networking software, and some architectural styles that would only use a network for system support (e.g., process control styles [53]). Applications represent the "business-aware" functionality of a system [131].
27 |28 | Application software architecture is an abstraction level of an overall system, in which the goals of a user action are representable as functional architectural properties. For example, a hypermedia application must be concerned with the location of information pages, performing requests, and rendering data streams. This is in contrast to a networking abstraction, where the goal is to move bits from one location to another without regard to why those bits are being moved. It is only at the application level that we can evaluate design trade-offs based on the number of interactions per user action, the location of application state, the effective throughput of all data streams (as opposed to the potential throughput of a single data stream), the extent of communication being performed per user action, etc.
29 |31 | One of the goals of this dissertation is to provide design guidance for the task of selecting or creating the most appropriate architecture for a given application domain, keeping in mind that an architecture is the realization of an architectural design and not the design itself. An architecture can be evaluated by its run-time characteristics, but we would obviously prefer an evaluation mechanism that could be applied to the candidate architectural designs before having to implement all of them. Unfortunately, architectural designs are notoriously hard to evaluate and compare in an objective manner. Like most artifacts of creative design, architectures are normally presented as a completed work, as if the design simply sprung fully-formed from the architect's mind. In order to evaluate an architectural design, we need to examine the design rationale behind the constraints it places on a system, and compare the properties derived from those constraints to the target application's objectives.
32 |33 | The first level of evaluation is set by the application's functional requirements. For example, it makes no sense to evaluate the design of a process control architecture against the requirements of a distributed hypermedia system, since the comparison is moot if the architecture would not function. Although this will eliminate some candidates, in most cases there will remain many other architectural designs that are capable of meeting the application's functional needs. The remainder differ by their relative emphasis on the non-functional requirements--the degree to which each architecture would support the various non-functional architectural properties that have been identified as necessary for the system. Since properties are created by the application of architectural constraints, it is possible to evaluate and compare different architectural designs by identifying the constraints within each architecture, evaluating the set of properties induced by each constraint, and comparing the cumulative properties of the design to those properties required of the application.
34 |35 | As described in the previous chapter, an architectural style is a coordinated set of architectural constraints that has been given a name for ease of reference. Each architectural design decision can be seen as an application of a style. Since the addition of a constraint may derive a new style, we can think of the space of all possible architectural styles as a derivation tree, with its root being the null style (empty set of constraints). When their constraints do not conflict, styles can be combined to form hybrid styles, eventually culminating in a hybrid style that represents a complete abstraction of the architectural design. An architectural design can therefore be analyzed by breaking-down its set of constraints into a derivation tree and evaluating the cumulative effect of the constraints represented by that tree. If we understand the properties induced by each basic style, then traversing the derivation tree gives us an understanding of the overall design's architectural properties. The specific needs of an application can then be matched against the properties of the design. Comparison becomes a relatively simple matter of identifying which architectural design satisfies the most desired properties for that application.
36 |37 | Care must be taken to recognize when the effects of one constraint may counteract the benefits of some other constraint. Nevertheless, it is possible for an experienced software architect to build such a derivation tree of architectural constraints for a given application domain, and then use the tree to evaluate many different architectural designs for applications within that domain. Thus, building a derivation tree provides a mechanism for architectural design guidance.
38 |39 | The evaluation of architectural properties within a tree of styles is specific to the needs of a particular application domain because the impact of a given constraint is often dependent on the application characteristics. For example, the pipe-and-filter style enables several positive architectural properties when used within a system that requires data transformations between components, whereas it would add nothing but overhead to a system that consists only of control messages. Since it is rarely useful to compare architectural designs across different application domains, the simplest means of ensuring consistency is to make the tree domain-specific.
40 |41 | Design evaluation is frequently a question of choosing between trade-offs. Perry and Wolf [105] describe a method of recognizing trade-offs explicitly by placing a numeric weight against each property to indicate its relative importance to the architecture, thus providing a normalized metric for comparing candidate designs. However, in order to be a meaningful metric, each weight would have to be carefully chosen using an objective scale that is consistent across all properties. In practice, no such scale exists. Rather than having the architect fiddle with weight values until the result matches their intuition, I prefer to present all of the information to the architect in a readily viewable form, and let the architect's intuition be guided by the visual pattern. This will be demonstrated in the next chapter.
42 |44 | This section describes the architectural properties used to differentiate and classify architectural styles in this dissertation. It is not intended to be a comprehensive list. I have included only those properties that are clearly influenced by the restricted set of styles surveyed. Additional properties, sometimes referred to as software qualities, are covered by most textbooks on software engineering (e.g., [58]). Bass et al. [9] examine qualities in regards to software architecture.
45 |47 | One of the main reasons to focus on styles for network-based applications is because component interactions can be the dominant factor in determining user-perceived performance and network efficiency. Since the architectural style influences the nature of those interactions, selection of an appropriate architectural style can make the difference between success and failure in the deployment of a network-based application.
48 |49 | The performance of a network-based application is bound first by the application requirements, then by the chosen interaction style, followed by the realized architecture, and finally by the implementation of each component. In other words, software cannot avoid the basic cost of achieving the application needs; e.g., if the application requires that data be located on system A and processed on system B, then the software cannot avoid moving that data from A to B. Likewise, an architecture cannot be any more efficient than its interaction style allows; e.g., the cost of multiple interactions to move the data from A to B cannot be any less than that of a single interaction from A to B. Finally, regardless of the quality of an architecture, no interaction can take place faster than a component implementation can produce data and its recipient can consume data.
50 |52 | Network performance measures are used to describe some attributes of communication. Throughput is the rate at which information, including both application data and communication overhead, is transferred between components. Overhead can be separated into initial setup overhead and per-interaction overhead, a distinction which is useful for identifying connectors that can share setup overhead across multiple interactions (amortization). Bandwidth is a measure of the maximum available throughput over a given network link. Usable bandwidth refers to that portion of bandwidth which is actually available to the application.
53 |54 | Styles impact network performance by their influence on the number of interactions per user action and the granularity of data elements. A style that encourages small, strongly typed interactions will be efficient in an application involving small data transfers among known components, but will cause excessive overhead within applications that involve large data transfers or negotiated interfaces. Likewise, a style that involves the coordination of multiple components arranged to filter a large data stream will be out of place in an application that primarily requires small control messages.
55 |57 | User-perceived performance differs from network performance in that the performance of an action is measured in terms of its impact on the user in front of an application rather than the rate at which the network moves information. The primary measures for user-perceived performance are latency and completion time.
58 |59 | Latency is the time period between initial stimulus and the first indication of a response. Latency occurs at several points in the processing of a network-based application action: 1) the time needed for the application to recognize the event that initiated the action; 2) the time required to setup the interactions between components; 3) the time required to transmit each interaction to the components; 4) the time required to process each interaction on those components; and, 5) the time required to complete sufficient transfer and processing of the result of the interactions before the application is able to begin rendering a usable result. It is important to note that, although only (3) and (5) represent actual network communication, all five points can be impacted by the architectural style. Furthermore, multiple component interactions per user action are additive to latency unless they take place in parallel.
60 |61 | Completion is the amount of time taken to complete an application action. Completion time is dependent upon all of the aforementioned measures. The difference between an action's completion time and its latency represents the degree to which the application is incrementally processing the data being received. For example, a Web browser that can render a large image while it is being received provides significantly better user-perceived performance than one that waits until the entire image is completely received prior to rendering, even though both experience the same network performance.
62 |63 | It is important to note that design considerations for optimizing latency will often have the side-effect of degrading completion time, and vice versa. For example, compression of a data stream can produce a more efficient encoding if the algorithm samples a significant portion of the data before producing the encoded transformation, resulting in a shorter completion time to transfer the encoded data across the network. However, if this compression is being performed on-the-fly in response to a user action, then buffering a large sample before transfer may produce an unacceptable latency. Balancing these trade-offs can be difficult, particularly when it is unknown whether the recipient cares more about latency (e.g., Web browsers) or completion (e.g., Web spiders).
64 |66 | An interesting observation about network-based applications is that the best application performance is obtained by not using the network. This essentially means that the most efficient architectural styles for a network-based application are those that can effectively minimize use of the network when it is possible to do so, through reuse of prior interactions (caching), reduction of the frequency of network interactions in relation to user actions (replicated data and disconnected operation), or by removing the need for some interactions by moving the processing of data closer to the source of the data (mobile code).
67 |68 | The impact of the various performance issues is often related to the scope of distribution for the application. The benefits of a style under local conditions may become drawbacks when faced with global conditions. Thus, the properties of a style must be framed in relation to the interaction distance: within a single process, across processes on a single host, inside a local-area network (LAN), or spread across a wide-area network (WAN). Additional concerns become evident when interactions across a WAN, where a single organization is involved, are compared to interactions across the Internet, involving multiple trust boundaries.
69 |71 | Scalability refers to the ability of the architecture to support large numbers of components, or interactions among components, within an active configuration. Scalability can be improved by simplifying components, by distributing services across many components (decentralizing the interactions), and by controlling interactions and configurations as a result of monitoring. Styles influence these factors by determining the location of application state, the extent of distribution, and the coupling between components.
72 |73 | Scalability is also impacted by the frequency of interactions, whether the load on a component is distributed evenly over time or occurs in peaks, whether an interaction requires guaranteed delivery or a best-effort, whether a request involves synchronous or asynchronous handling, and whether the environment is controlled or anarchic (i.e., can you trust the other components?).
74 |76 | The primary means by which architectural styles induce simplicity is by applying the principle of separation of concerns to the allocation of functionality within components. If functionality can be allocated such that the individual components are substantially less complex, then they will be easier to understand and implement. Likewise, such separation eases the task of reasoning about the overall architecture. I have chosen to lump the qualities of complexity, understandability, and verifiability under the general property of simplicity, since they go hand-in-hand for a network-based system.
77 |78 | Applying the principle of generality to architectural elements also improves simplicity, since it decreases variation within an architecture. Generality of connectors leads to middleware [22].
79 |81 | Modifiability is about the ease with which a change can be made to an application architecture. Modifiability can be further broken down into evolvability, extensibility, customizability, configurability, and reusability, as described below. A particular concern of network-based systems is dynamic modifiability [98], where the modification is made to a deployed application without stopping and restarting the entire system.
82 |83 | Even if it were possible to build a software system that perfectly matches the requirements of its users, those requirements will change over time just as society changes over time. Because the components participating in a network-based application may be distributed across multiple organizational boundaries, the system must be prepared for gradual and fragmented change, where old and new implementations coexist, without preventing the new implementations from making use of their extended capabilities.
84 |86 | Evolvability represents the degree to which a component implementation can be changed without negatively impacting other components. Static evolution of components generally depends on how well the architectural abstraction is enforced by the implementation, and thus is not something unique to any particular architectural style. Dynamic evolution, however, can be influenced by the style if it includes constraints on the maintenance and location of application state. The same techniques used to recover from partial failure conditions in a distributed system [133] can be used to support dynamic evolution.
87 |89 | Extensibility is defined as the ability to add functionality to a system [108]. Dynamic extensibility implies that functionality can be added to a deployed system without impacting the rest of the system. Extensibility is induced within an architectural style by reducing the coupling between components, as exemplified by event-based integration.
90 |92 | Customizability refers to the ability to temporarily specialize the behavior of an architectural element, such that it can then perform an unusual service. A component is customizable if it can be extended by one client of that component's services without adversely impacting other clients of that component [50]. Styles that support customization may also improve simplicity and scalability, since service components can be reduced in size and complexity by directly implementing only the most frequent services and allowing infrequent services to be defined by the client. Customizability is a property induced by the remote evaluation and code-on-demand styles.
93 |95 | Configurability is related to both extensibility and reusability in that it refers to post-deployment modification of components, or configurations of components, such that they are capable of using a new service or data element type. The pipe-and-filter and code-on-demand styles are two examples that induce configurability of configurations and components, respectively.
96 |98 | Reusability is a property of an application architecture if its components, connectors, or data elements can be reused, without modification, in other applications. The primary mechanisms for inducing reusability within architectural styles is reduction of coupling (knowledge of identity) between components and constraining the generality of component interfaces. The uniform pipe-and-filter style exemplifies these types of constraints.
99 |101 | Styles can also influence the visibility of interactions within a network-based application by restricting interfaces via generality or providing access to monitoring. Visibility in this case refers to the ability of a component to monitor or mediate the interaction between two other components. Visibility can enable improved performance via shared caching of interactions, scalability through layered services, reliability through reflective monitoring, and security by allowing the interactions to be inspected by mediators (e.g., network firewalls). The mobile agent style is an example where the lack of visibility may lead to security concerns.
102 |103 | This usage of the term visibility differs from that in Ghezzi et al. [58], where they are referring to visibility into the development process rather than the product.
104 |106 | Software is portable if it can run in different environments [58]. Styles that induce portability include those that move code along with the data to be processed, such as the virtual machine and mobile agent styles, and those that constrain the data elements to a set of standardized formats.
107 |109 | Reliability, within the perspective of application architectures, can be viewed as the degree to which an architecture is susceptible to failure at the system level in the presence of partial failures within components, connectors, or data. Styles can improve reliability by avoiding single points of failure, enabling redundancy, allowing monitoring, or reducing the scope of failure to a recoverable action.
110 |112 | This chapter examined the scope of the dissertation by focusing on network-based application architectures and describing how styles can be used to guide their architectural design. It also defined the set of architectural properties that will be used throughout the rest of the dissertation for the comparison and evaluation of architectural styles.
113 |114 | The next chapter presents a survey of common architectural styles for network-based application software within a classification framework that evaluates each style according to the architectural properties it would induce if applied to an architecture for a prototypical network-based hypermedia system.
115 | 116 | 117 | 118 | 119 | -------------------------------------------------------------------------------- /OEBPS/page-template.xpgt: -------------------------------------------------------------------------------- 1 |15 | This chapter introduces and elaborates the Representational State Transfer (REST) architectural style for distributed hypermedia systems, describing the software engineering principles guiding REST and the interaction constraints chosen to retain those principles, while contrasting them to the constraints of other architectural styles. REST is a hybrid style derived from several of the network-based architectural styles described in Chapter 3 and combined with additional constraints that define a uniform connector interface. The software architecture framework of Chapter 1 is used to define the architectural elements of REST and examine sample process, connector, and data views of prototypical architectures.
16 |18 | The design rationale behind the Web architecture can be described by an architectural style consisting of the set of constraints applied to elements within the architecture. By examining the impact of each constraint as it is added to the evolving style, we can identify the properties induced by the Web's constraints. Additional constraints can then be applied to form a new architectural style that better reflects the desired properties of a modern Web architecture. This section provides a general overview of REST by walking through the process of deriving it as an architectural style. Later sections will describe in more detail the specific constraints that compose the REST style.
19 |21 | There are two common perspectives on the process of architectural design, whether it be for buildings or for software. The first is that a designer starts with nothing--a blank slate, whiteboard, or drawing board--and builds-up an architecture from familiar components until it satisfies the needs of the intended system. The second is that a designer starts with the system needs as a whole, without constraints, and then incrementally identifies and applies constraints to elements of the system in order to differentiate the design space and allow the forces that influence system behavior to flow naturally, in harmony with the system. Where the first emphasizes creativity and unbounded vision, the second emphasizes restraint and understanding of the system context. REST has been developed using the latter process. Figures 5-1 through 5-8 depict this graphically in terms of how the applied constraints would differentiate the process view of an architecture as the incremental set of constraints is applied.
22 |23 | The Null style (Figure 5-1) is simply an empty set of constraints. From an architectural perspective, the null style describes a system in which there are no distinguished boundaries between components. It is the starting point for our description of REST.
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26 | 29 | The first constraints added to our hybrid style are those of the client-server architectural style (Figure 5-2), described in Section 3.4.1. Separation of concerns is the principle behind the client-server constraints. By separating the user interface concerns from the data storage concerns, we improve the portability of the user interface across multiple platforms and improve scalability by simplifying the server components. Perhaps most significant to the Web, however, is that the separation allows the components to evolve independently, thus supporting the Internet-scale requirement of multiple organizational domains.
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32 | 35 | We next add a constraint to the client-server interaction: communication must be stateless in nature, as in the client-stateless-server (CSS) style of Section 3.4.3 (Figure 5-3), such that each request from client to server must contain all of the information necessary to understand the request, and cannot take advantage of any stored context on the server. Session state is therefore kept entirely on the client.
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38 | 40 | This constraint induces the properties of visibility, reliability, and scalability. Visibility is improved because a monitoring system does not have to look beyond a single request datum in order to determine the full nature of the request. Reliability is improved because it eases the task of recovering from partial failures [133]. Scalability is improved because not having to store state between requests allows the server component to quickly free resources, and further simplifies implementation because the server doesn't have to manage resource usage across requests.
41 |42 | Like most architectural choices, the stateless constraint reflects a design trade-off. The disadvantage is that it may decrease network performance by increasing the repetitive data (per-interaction overhead) sent in a series of requests, since that data cannot be left on the server in a shared context. In addition, placing the application state on the client-side reduces the server's control over consistent application behavior, since the application becomes dependent on the correct implementation of semantics across multiple client versions.
43 |45 | In order to improve network efficiency, we add cache constraints to form the client-cache-stateless-server style of Section 3.4.4 (Figure 5-4). Cache constraints require that the data within a response to a request be implicitly or explicitly labeled as cacheable or non-cacheable. If a response is cacheable, then a client cache is given the right to reuse that response data for later, equivalent requests.
46 |
48 | 50 | The advantage of adding cache constraints is that they have the potential to partially or completely eliminate some interactions, improving efficiency, scalability, and user-perceived performance by reducing the average latency of a series of interactions. The trade-off, however, is that a cache can decrease reliability if stale data within the cache differs significantly from the data that would have been obtained had the request been sent directly to the server.
51 |52 | The early Web architecture, as portrayed by the diagram in Figure 5-5 [11], was defined by the client-cache-stateless-server set of constraints. That is, the design rationale presented for the Web architecture prior to 1994 focused on stateless client-server interaction for the exchange of static documents over the Internet. The protocols for communicating interactions had rudimentary support for non-shared caches, but did not constrain the interface to a consistent set of semantics for all resources. Instead, the Web relied on the use of a common client-server implementation library (CERN libwww) to maintain consistency across Web applications.
53 |
55 | 57 | Developers of Web implementations had already exceeded the early design. In addition to static documents, requests could identify services that dynamically generated responses, such as image-maps [Kevin Hughes] and server-side scripts [Rob McCool]. Work had also begun on intermediary components, in the form of proxies [79] and shared caches [59], but extensions to the protocols were needed in order for them to communicate reliably. The following sections describe the constraints added to the Web's architectural style in order to guide the extensions that form the modern Web architecture.
58 |60 | The central feature that distinguishes the REST architectural style from other network-based styles is its emphasis on a uniform interface between components (Figure 5-6). By applying the software engineering principle of generality to the component interface, the overall system architecture is simplified and the visibility of interactions is improved. Implementations are decoupled from the services they provide, which encourages independent evolvability. The trade-off, though, is that a uniform interface degrades efficiency, since information is transferred in a standardized form rather than one which is specific to an application's needs. The REST interface is designed to be efficient for large-grain hypermedia data transfer, optimizing for the common case of the Web, but resulting in an interface that is not optimal for other forms of architectural interaction.
61 |
63 | 65 | In order to obtain a uniform interface, multiple architectural constraints are needed to guide the behavior of components. REST is defined by four interface constraints: identification of resources; manipulation of resources through representations; self-descriptive messages; and, hypermedia as the engine of application state. These constraints will be discussed in Section 5.2.
66 |68 | In order to further improve behavior for Internet-scale requirements, we add layered system constraints (Figure 5-7). As described in Section 3.4.2, the layered system style allows an architecture to be composed of hierarchical layers by constraining component behavior such that each component cannot "see" beyond the immediate layer with which they are interacting. By restricting knowledge of the system to a single layer, we place a bound on the overall system complexity and promote substrate independence. Layers can be used to encapsulate legacy services and to protect new services from legacy clients, simplifying components by moving infrequently used functionality to a shared intermediary. Intermediaries can also be used to improve system scalability by enabling load balancing of services across multiple networks and processors.
69 |
71 | 73 | The primary disadvantage of layered systems is that they add overhead and latency to the processing of data, reducing user-perceived performance [32]. For a network-based system that supports cache constraints, this can be offset by the benefits of shared caching at intermediaries. Placing shared caches at the boundaries of an organizational domain can result in significant performance benefits [136]. Such layers also allow security policies to be enforced on data crossing the organizational boundary, as is required by firewalls [79].
74 |75 | The combination of layered system and uniform interface constraints induces architectural properties similar to those of the uniform pipe-and-filter style (Section 3.2.2). Although REST interaction is two-way, the large-grain data flows of hypermedia interaction can each be processed like a data-flow network, with filter components selectively applied to the data stream in order to transform the content as it passes [26]. Within REST, intermediary components can actively transform the content of messages because the messages are self-descriptive and their semantics are visible to intermediaries.
76 |78 | The final addition to our constraint set for REST comes from the code-on-demand style of Section 3.5.3 (Figure 5-8). REST allows client functionality to be extended by downloading and executing code in the form of applets or scripts. This simplifies clients by reducing the number of features required to be pre-implemented. Allowing features to be downloaded after deployment improves system extensibility. However, it also reduces visibility, and thus is only an optional constraint within REST.
79 |
81 | 83 | The notion of an optional constraint may seem like an oxymoron. However, it does have a purpose in the architectural design of a system that encompasses multiple organizational boundaries. It means that the architecture only gains the benefit (and suffers the disadvantages) of the optional constraints when they are known to be in effect for some realm of the overall system. For example, if all of the client software within an organization is known to support Java applets [45], then services within that organization can be constructed such that they gain the benefit of enhanced functionality via downloadable Java classes. At the same time, however, the organization's firewall may prevent the transfer of Java applets from external sources, and thus to the rest of the Web it will appear as if those clients do not support code-on-demand. An optional constraint allows us to design an architecture that supports the desired behavior in the general case, but with the understanding that it may be disabled within some contexts.
84 |86 | REST consists of a set of architectural constraints chosen for the properties they induce on candidate architectures. Although each of these constraints can be considered in isolation, describing them in terms of their derivation from common architectural styles makes it easier to understand the rationale behind their selection. Figure 5-9 depicts the derivation of REST's constraints graphically in terms of the network-based architectural styles examined in Chapter 3.
87 |
89 | 92 | The Representational State Transfer (REST) style is an abstraction of the architectural elements within a distributed hypermedia system. REST ignores the details of component implementation and protocol syntax in order to focus on the roles of components, the constraints upon their interaction with other components, and their interpretation of significant data elements. It encompasses the fundamental constraints upon components, connectors, and data that define the basis of the Web architecture, and thus the essence of its behavior as a network-based application.
93 |95 | Unlike the distributed object style [31], where all data is encapsulated within and hidden by the processing components, the nature and state of an architecture's data elements is a key aspect of REST. The rationale for this design can be seen in the nature of distributed hypermedia. When a link is selected, information needs to be moved from the location where it is stored to the location where it will be used by, in most cases, a human reader. This is unlike many other distributed processing paradigms [6, 50], where it is possible, and usually more efficient, to move the "processing agent" (e.g., mobile code, stored procedure, search expression, etc.) to the data rather than move the data to the processor.
96 |97 | A distributed hypermedia architect has only three fundamental options: 1) render the data where it is located and send a fixed-format image to the recipient; 2) encapsulate the data with a rendering engine and send both to the recipient; or, 3) send the raw data to the recipient along with metadata that describes the data type, so that the recipient can choose their own rendering engine.
98 |99 | Each option has its advantages and disadvantages. Option 1, the traditional client-server style [31], allows all information about the true nature of the data to remain hidden within the sender, preventing assumptions from being made about the data structure and making client implementation easier. However, it also severely restricts the functionality of the recipient and places most of the processing load on the sender, leading to scalability problems. Option 2, the mobile object style [50], provides information hiding while enabling specialized processing of the data via its unique rendering engine, but limits the functionality of the recipient to what is anticipated within that engine and may vastly increase the amount of data transferred. Option 3 allows the sender to remain simple and scalable while minimizing the bytes transferred, but loses the advantages of information hiding and requires that both sender and recipient understand the same data types.
100 |101 | REST provides a hybrid of all three options by focusing on a shared understanding of data types with metadata, but limiting the scope of what is revealed to a standardized interface. REST components communicate by transferring a representation of a resource in a format matching one of an evolving set of standard data types, selected dynamically based on the capabilities or desires of the recipient and the nature of the resource. Whether the representation is in the same format as the raw source, or is derived from the source, remains hidden behind the interface. The benefits of the mobile object style are approximated by sending a representation that consists of instructions in the standard data format of an encapsulated rendering engine (e.g., Java [45]). REST therefore gains the separation of concerns of the client-server style without the server scalability problem, allows information hiding through a generic interface to enable encapsulation and evolution of services, and provides for a diverse set of functionality through downloadable feature-engines.
102 |103 | REST's data elements are summarized in Table 5-1.
104 || Data Element | 108 |Modern Web Examples | 109 |
|---|---|
| resource | 111 |the intended conceptual target of a hypertext reference | 112 |
| resource identifier | 114 |URL, URN | 115 |
| representation | 117 |HTML document, JPEG image | 118 |
| representation metadata | 120 |media type, last-modified time | 121 |
| resource metadata | 123 |source link, alternates, vary | 124 |
| control data | 126 |if-modified-since, cache-control | 127 |
131 | The key abstraction of information in REST is a resource. Any information that can be named can be a resource: a document or image, a temporal service (e.g. "today's weather in Los Angeles"), a collection of other resources, a non-virtual object (e.g. a person), and so on. In other words, any concept that might be the target of an author's hypertext reference must fit within the definition of a resource. A resource is a conceptual mapping to a set of entities, not the entity that corresponds to the mapping at any particular point in time.
132 |133 | More precisely, a resource R is a temporally varying membership function MR(t), which for time t maps to a set of entities, or values, which are equivalent. The values in the set may be resource representations and/or resource identifiers. A resource can map to the empty set, which allows references to be made to a concept before any realization of that concept exists -- a notion that was foreign to most hypertext systems prior to the Web [61]. Some resources are static in the sense that, when examined at any time after their creation, they always correspond to the same value set. Others have a high degree of variance in their value over time. The only thing that is required to be static for a resource is the semantics of the mapping, since the semantics is what distinguishes one resource from another.
134 |135 | For example, the "authors' preferred version" of an academic paper is a mapping whose value changes over time, whereas a mapping to "the paper published in the proceedings of conference X" is static. These are two distinct resources, even if they both map to the same value at some point in time. The distinction is necessary so that both resources can be identified and referenced independently. A similar example from software engineering is the separate identification of a version-controlled source code file when referring to the "latest revision", "revision number 1.2.7", or "revision included with the Orange release."
136 |137 | This abstract definition of a resource enables key features of the Web architecture. First, it provides generality by encompassing many sources of information without artificially distinguishing them by type or implementation. Second, it allows late binding of the reference to a representation, enabling content negotiation to take place based on characteristics of the request. Finally, it allows an author to reference the concept rather than some singular representation of that concept, thus removing the need to change all existing links whenever the representation changes (assuming the author used the right identifier).
138 |139 | REST uses a resource identifier to identify the particular resource involved in an interaction between components. REST connectors provide a generic interface for accessing and manipulating the value set of a resource, regardless of how the membership function is defined or the type of software that is handling the request. The naming authority that assigned the resource identifier, making it possible to reference the resource, is responsible for maintaining the semantic validity of the mapping over time (i.e., ensuring that the membership function does not change).
140 |141 | Traditional hypertext systems [61], which typically operate in a closed or local environment, use unique node or document identifiers that change every time the information changes, relying on link servers to maintain references separately from the content [135]. Since centralized link servers are an anathema to the immense scale and multi-organizational domain requirements of the Web, REST relies instead on the author choosing a resource identifier that best fits the nature of the concept being identified. Naturally, the quality of an identifier is often proportional to the amount of money spent to retain its validity, which leads to broken links as ephemeral (or poorly supported) information moves or disappears over time.
142 |144 | REST components perform actions on a resource by using a representation to capture the current or intended state of that resource and transferring that representation between components. A representation is a sequence of bytes, plus representation metadata to describe those bytes. Other commonly used but less precise names for a representation include: document, file, and HTTP message entity, instance, or variant.
145 |146 | A representation consists of data, metadata describing the data, and, on occasion, metadata to describe the metadata (usually for the purpose of verifying message integrity). Metadata is in the form of name-value pairs, where the name corresponds to a standard that defines the value's structure and semantics. Response messages may include both representation metadata and resource metadata: information about the resource that is not specific to the supplied representation.
147 |148 | Control data defines the purpose of a message between components, such as the action being requested or the meaning of a response. It is also used to parameterize requests and override the default behavior of some connecting elements. For example, cache behavior can be modified by control data included in the request or response message.
149 |150 | Depending on the message control data, a given representation may indicate the current state of the requested resource, the desired state for the requested resource, or the value of some other resource, such as a representation of the input data within a client's query form, or a representation of some error condition for a response. For example, remote authoring of a resource requires that the author send a representation to the server, thus establishing a value for that resource that can be retrieved by later requests. If the value set of a resource at a given time consists of multiple representations, content negotiation may be used to select the best representation for inclusion in a given message.
151 |152 | The data format of a representation is known as a media type [48]. A representation can be included in a message and processed by the recipient according to the control data of the message and the nature of the media type. Some media types are intended for automated processing, some are intended to be rendered for viewing by a user, and a few are capable of both. Composite media types can be used to enclose multiple representations in a single message.
153 |154 | The design of a media type can directly impact the user-perceived performance of a distributed hypermedia system. Any data that must be received before the recipient can begin rendering the representation adds to the latency of an interaction. A data format that places the most important rendering information up front, such that the initial information can be incrementally rendered while the rest of the information is being received, results in much better user-perceived performance than a data format that must be entirely received before rendering can begin.
155 |156 | For example, a Web browser that can incrementally render a large HTML document while it is being received provides significantly better user-perceived performance than one that waits until the entire document is completely received prior to rendering, even though the network performance is the same. Note that the rendering ability of a representation can also be impacted by the choice of content. If the dimensions of dynamically-sized tables and embedded objects must be determined before they can be rendered, their occurrence within the viewing area of a hypermedia page will increase its latency.
157 |159 | REST uses various connector types, summarized in Table 5-2, to encapsulate the activities of accessing resources and transferring resource representations. The connectors present an abstract interface for component communication, enhancing simplicity by providing a clean separation of concerns and hiding the underlying implementation of resources and communication mechanisms. The generality of the interface also enables substitutability: if the users' only access to the system is via an abstract interface, the implementation can be replaced without impacting the users. Since a connector manages network communication for a component, information can be shared across multiple interactions in order to improve efficiency and responsiveness.
160 || Connector | 164 |Modern Web Examples | 165 |
|---|---|
| client | 167 |libwww, libwww-perl | 168 |
| server | 170 |libwww, Apache API, NSAPI | 171 |
| cache | 173 |browser cache, Akamai cache network | 174 |
| resolver | 176 |bind (DNS lookup library) | 177 |
| tunnel | 179 |SOCKS, SSL after HTTP CONNECT | 180 |
183 | All REST interactions are stateless. That is, each request contains all of the information necessary for a connector to understand the request, independent of any requests that may have preceded it. This restriction accomplishes four functions: 1) it removes any need for the connectors to retain application state between requests, thus reducing consumption of physical resources and improving scalability; 2) it allows interactions to be processed in parallel without requiring that the processing mechanism understand the interaction semantics; 3) it allows an intermediary to view and understand a request in isolation, which may be necessary when services are dynamically rearranged; and, 4) it forces all of the information that might factor into the reusability of a cached response to be present in each request.
184 |185 | The connector interface is similar to procedural invocation, but with important differences in the passing of parameters and results. The in-parameters consist of request control data, a resource identifier indicating the target of the request, and an optional representation. The out-parameters consist of response control data, optional resource metadata, and an optional representation. From an abstract viewpoint the invocation is synchronous, but both in and out-parameters can be passed as data streams. In other words, processing can be invoked before the value of the parameters is completely known, thus avoiding the latency of batch processing large data transfers.
186 |187 | The primary connector types are client and server. The essential difference between the two is that a client initiates communication by making a request, whereas a server listens for connections and responds to requests in order to supply access to its services. A component may include both client and server connectors.
188 |189 | A third connector type, the cache connector, can be located on the interface to a client or server connector in order to save cacheable responses to current interactions so that they can be reused for later requested interactions. A cache may be used by a client to avoid repetition of network communication, or by a server to avoid repeating the process of generating a response, with both cases serving to reduce interaction latency. A cache is typically implemented within the address space of the connector that uses it.
190 |191 | Some cache connectors are shared, meaning that its cached responses may be used in answer to a client other than the one for which the response was originally obtained. Shared caching can be effective at reducing the impact of "flash crowds" on the load of a popular server, particularly when the caching is arranged hierarchically to cover large groups of users, such as those within a company's intranet, the customers of an Internet service provider, or Universities sharing a national network backbone. However, shared caching can also lead to errors if the cached response does not match what would have been obtained by a new request. REST attempts to balance the desire for transparency in cache behavior with the desire for efficient use of the network, rather than assuming that absolute transparency is always required.
192 |193 | A cache is able to determine the cacheability of a response because the interface is generic rather than specific to each resource. By default, the response to a retrieval request is cacheable and the responses to other requests are non-cacheable. If some form of user authentication is part of the request, or if the response indicates that it should not be shared, then the response is only cacheable by a non-shared cache. A component can override these defaults by including control data that marks the interaction as cacheable, non-cacheable or cacheable for only a limited time.
194 |195 | A resolver translates partial or complete resource identifiers into the network address information needed to establish an inter-component connection. For example, most URI include a DNS hostname as the mechanism for identifying the naming authority for the resource. In order to initiate a request, a Web browser will extract the hostname from the URI and make use of a DNS resolver to obtain the Internet Protocol address for that authority. Another example is that some identification schemes (e.g., URN [124]) require an intermediary to translate a permanent identifier to a more transient address in order to access the identified resource. Use of one or more intermediate resolvers can improve the longevity of resource references through indirection, though doing so adds to the request latency.
196 |197 | The final form of connector type is a tunnel, which simply relays communication across a connection boundary, such as a firewall or lower-level network gateway. The only reason it is modeled as part of REST and not abstracted away as part of the network infrastructure is that some REST components may dynamically switch from active component behavior to that of a tunnel. The primary example is an HTTP proxy that switches to a tunnel in response to a CONNECT method request [71], thus allowing its client to directly communicate with a remote server using a different protocol, such as TLS, that doesn't allow proxies. The tunnel disappears when both ends terminate their communication.
198 |200 | REST components, summarized in Table 5-3, are typed by their roles in an overall application action.
201 || Component | 205 |Modern Web Examples | 206 |
|---|---|
| origin server | 208 |Apache httpd, Microsoft IIS | 209 |
| gateway | 211 |Squid, CGI, Reverse Proxy | 212 |
| proxy | 214 |CERN Proxy, Netscape Proxy, Gauntlet | 215 |
| user agent | 217 |Netscape Navigator, Lynx, MOMspider | 218 |
221 | A user agent uses a client connector to initiate a request and becomes the ultimate recipient of the response. The most common example is a Web browser, which provides access to information services and renders service responses according to the application needs.
222 |223 | An origin server uses a server connector to govern the namespace for a requested resource. It is the definitive source for representations of its resources and must be the ultimate recipient of any request that intends to modify the value of its resources. Each origin server provides a generic interface to its services as a resource hierarchy. The resource implementation details are hidden behind the interface.
224 |225 | Intermediary components act as both a client and a server in order to forward, with possible translation, requests and responses. A proxy component is an intermediary selected by a client to provide interface encapsulation of other services, data translation, performance enhancement, or security protection. A gateway (a.k.a., reverse proxy) component is an intermediary imposed by the network or origin server to provide an interface encapsulation of other services, for data translation, performance enhancement, or security enforcement. Note that the difference between a proxy and a gateway is that a client determines when it will use a proxy.
226 |228 | Now that we have an understanding of the REST architectural elements in isolation, we can use architectural views [105] to describe how the elements work together to form an architecture. Three types of view--process, connector, and data--are useful for illuminating the design principles of REST.
229 |231 | A process view of an architecture is primarily effective at eliciting the interaction relationships among components by revealing the path of data as it flows through the system. Unfortunately, the interaction of a real system usually involves an extensive number of components, resulting in an overall view that is obscured by the details. Figure 5-10 provides a sample of the process view from a REST-based architecture at a particular instance during the processing of three parallel requests.
232 |
234 | 236 | REST's client-server separation of concerns simplifies component implementation, reduces the complexity of connector semantics, improves the effectiveness of performance tuning, and increases the scalability of pure server components. Layered system constraints allow intermediaries--proxies, gateways, and firewalls--to be introduced at various points in the communication without changing the interfaces between components, thus allowing them to assist in communication translation or improve performance via large-scale, shared caching. REST enables intermediate processing by constraining messages to be self-descriptive: interaction is stateless between requests, standard methods and media types are used to indicate semantics and exchange information, and responses explicitly indicate cacheability.
237 |238 | Since the components are connected dynamically, their arrangement and function for a particular application action has characteristics similar to a pipe-and-filter style. Although REST components communicate via bidirectional streams, the processing of each direction is independent and therefore susceptible to stream transducers (filters). The generic connector interface allows components to be placed on the stream based on the properties of each request or response.
239 |240 | Services may be implemented using a complex hierarchy of intermediaries and multiple distributed origin servers. The stateless nature of REST allows each interaction to be independent of the others, removing the need for an awareness of the overall component topology, an impossible task for an Internet-scale architecture, and allowing components to act as either destinations or intermediaries, determined dynamically by the target of each request. Connectors need only be aware of each other's existence during the scope of their communication, though they may cache the existence and capabilities of other components for performance reasons.
241 |243 | A connector view of an architecture concentrates on the mechanics of the communication between components. For a REST-based architecture, we are particularly interested in the constraints that define the generic resource interface.
244 |245 | Client connectors examine the resource identifier in order to select an appropriate communication mechanism for each request. For example, a client may be configured to connect to a specific proxy component, perhaps one acting as an annotation filter, when the identifier indicates that it is a local resource. Likewise, a client can be configured to reject requests for some subset of identifiers.
246 |247 | REST does not restrict communication to a particular protocol, but it does constrain the interface between components, and hence the scope of interaction and implementation assumptions that might otherwise be made between components. For example, the Web's primary transfer protocol is HTTP, but the architecture also includes seamless access to resources that originate on pre-existing network servers, including FTP [107], Gopher [7], and WAIS [36]. Interaction with those services is restricted to the semantics of a REST connector. This constraint sacrifices some of the advantages of other architectures, such as the stateful interaction of a relevance feedback protocol like WAIS, in order to retain the advantages of a single, generic interface for connector semantics. In return, the generic interface makes it possible to access a multitude of services through a single proxy. If an application needs the additional capabilities of another architecture, it can implement and invoke those capabilities as a separate system running in parallel, similar to how the Web architecture interfaces with "telnet" and "mailto" resources.
248 |250 | A data view of an architecture reveals the application state as information flows through the components. Since REST is specifically targeted at distributed information systems, it views an application as a cohesive structure of information and control alternatives through which a user can perform a desired task. For example, looking-up a word in an on-line dictionary is one application, as is touring through a virtual museum, or reviewing a set of class notes to study for an exam. Each application defines goals for the underlying system, against which the system's performance can be measured.
251 |252 | Component interactions occur in the form of dynamically sized messages. Small or medium-grain messages are used for control semantics, but the bulk of application work is accomplished via large-grain messages containing a complete resource representation. The most frequent form of request semantics is that of retrieving a representation of a resource (e.g., the "GET" method in HTTP), which can often be cached for later reuse.
253 |254 | REST concentrates all of the control state into the representations received in response to interactions. The goal is to improve server scalability by eliminating any need for the server to maintain an awareness of the client state beyond the current request. An application's state is therefore defined by its pending requests, the topology of connected components (some of which may be filtering buffered data), the active requests on those connectors, the data flow of representations in response to those requests, and the processing of those representations as they are received by the user agent.
255 |256 | An application reaches a steady-state whenever it has no outstanding requests; i.e., it has no pending requests and all of the responses to its current set of requests have been completely received or received to the point where they can be treated as a representation data stream. For a browser application, this state corresponds to a "web page," including the primary representation and ancillary representations, such as in-line images, embedded applets, and style sheets. The significance of application steady-states is seen in their impact on both user-perceived performance and the burstiness of network request traffic.
257 |258 | The user-perceived performance of a browser application is determined by the latency between steady-states: the period of time between the selection of a hypermedia link on one web page and the point when usable information has been rendered for the next web page. The optimization of browser performance is therefore centered around reducing this communication latency.
259 |260 | Since REST-based architectures communicate primarily through the transfer of representations of resources, latency can be impacted by both the design of the communication protocols and the design of the representation data formats. The ability to incrementally render the response data as it is received is determined by the design of the media type and the availability of layout information (visual dimensions of in-line objects) within each representation.
261 |262 | An interesting observation is that the most efficient network request is one that doesn't use the network. In other words, the ability to reuse a cached response results in a considerable improvement in application performance. Although use of a cache adds some latency to each individual request due to lookup overhead, the average request latency is significantly reduced when even a small percentage of requests result in usable cache hits.
263 |264 | The next control state of an application resides in the representation of the first requested resource, so obtaining that first representation is a priority. REST interaction is therefore improved by protocols that "respond first and think later." In other words, a protocol that requires multiple interactions per user action, in order to do things like negotiate feature capabilities prior to sending a content response, will be perceptively slower than a protocol that sends whatever is most likely to be optimal first and then provides a list of alternatives for the client to retrieve if the first response is unsatisfactory.
265 |266 | The application state is controlled and stored by the user agent and can be composed of representations from multiple servers. In addition to freeing the server from the scalability problems of storing state, this allows the user to directly manipulate the state (e.g., a Web browser's history), anticipate changes to that state (e.g., link maps and prefetching of representations), and jump from one application to another (e.g., bookmarks and URI-entry dialogs).
267 |268 | The model application is therefore an engine that moves from one state to the next by examining and choosing from among the alternative state transitions in the current set of representations. Not surprisingly, this exactly matches the user interface of a hypermedia browser. However, the style does not assume that all applications are browsers. In fact, the application details are hidden from the server by the generic connector interface, and thus a user agent could equally be an automated robot performing information retrieval for an indexing service, a personal agent looking for data that matches certain criteria, or a maintenance spider busy patrolling the information for broken references or modified content [39].
269 |271 | Bass, et al. [9] devote a chapter on architecture for the World Wide Web, but their description only encompasses the implementation architecture within the CERN/W3C developed libwww (client and server libraries) and Jigsaw software. Although those implementations reflect many of the design constraints of REST, having been developed by people familiar with the Web's architectural design and rationale, the real WWW architecture is independent of any single implementation. The modern Web is defined by its standard interfaces and protocols, not how those interfaces and protocols are implemented in a given piece of software.
272 |273 | The REST style draws from many preexisting distributed process paradigms [6, 50], communication protocols, and software fields. REST component interactions are structured in a layered client-server style, but the added constraints of the generic resource interface create the opportunity for substitutability and inspection by intermediaries. Requests and responses have the appearance of a remote invocation style, but REST messages are targeted at a conceptual resource rather than an implementation identifier.
274 |275 | Several attempts have been made to model the Web architecture as a form of distributed file system (e.g., WebNFS) or as a distributed object system [83]. However, they exclude various Web resource types or implementation strategies as being "not interesting," when in fact their presence invalidates the assumptions that underlie such models. REST works well because it does not limit the implementation of resources to certain predefined models, allowing each application to choose an implementation that best matches its own needs and enabling the replacement of implementations without impacting the user.
276 |277 | The interaction method of sending representations of resources to consuming components has some parallels with event-based integration (EBI) styles. The key difference is that EBI styles are push-based. The component containing the state (equivalent to an origin server in REST) issues an event whenever the state changes, whether or not any component is actually interested in or listening for such an event. In the REST style, consuming components usually pull representations. Although this is less efficient when viewed as a single client wishing to monitor a single resource, the scale of the Web makes an unregulated push model infeasible.
278 |279 | The principled use of the REST style in the Web, with its clear notion of components, connectors, and representations, relates closely to the C2 architectural style [128]. The C2 style supports the development of distributed, dynamic applications by focusing on structured use of connectors to obtain substrate independence. C2 applications rely on asynchronous notification of state changes and request messages. As with other event-based schemes, C2 is nominally push-based, though a C2 architecture could operate in REST's pull style by only emitting a notification upon receipt of a request. However, the C2 style lacks the intermediary-friendly constraints of REST, such as the generic resource interface, guaranteed stateless interactions, and intrinsic support for caching.
280 |282 | This chapter introduced the Representational State Transfer (REST) architectural style for distributed hypermedia systems. REST provides a set of architectural constraints that, when applied as a whole, emphasizes scalability of component interactions, generality of interfaces, independent deployment of components, and intermediary components to reduce interaction latency, enforce security, and encapsulate legacy systems. I described the software engineering principles guiding REST and the interaction constraints chosen to retain those principles, while contrasting them to the constraints of other architectural styles.
283 |284 | The next chapter presents an evaluation of the REST architecture through the experience and lessons learned from applying REST to the design, specification, and deployment of the modern Web architecture. This work included authoring the current Internet standards-track specifications of the Hypertext Transfer Protocol (HTTP/1.1) and Uniform Resource Identifiers (URI), and implementing the architecture through the libwww-perl client protocol library and Apache HTTP server.
285 | 286 | 287 | 288 | 289 | -------------------------------------------------------------------------------- /OEBPS/software_arch.htm: -------------------------------------------------------------------------------- 1 | 3 | 4 | 5 | 6 |14 | In spite of the interest in software architecture as a field of research, there is little agreement among researchers as to what exactly should be included in the definition of architecture. In many cases, this has led to important aspects of architectural design being overlooked by past research. This chapter defines a self-consistent terminology for software architecture based on an examination of existing definitions within the literature and my own insight with respect to network-based application architectures. Each definition, highlighted within a box for ease of reference, is followed by a discussion of how it is derived from, or compares to, related research.
15 || 17 | A software architecture is an abstraction of the run-time elements of a software system during some phase of its operation. A system may be composed of many levels of abstraction and many phases of operation, each with its own software architecture. |
19 | At the heart of software architecture is the principle of abstraction: hiding some of the details of a system through encapsulation in order to better identify and sustain its properties [117]. A complex system will contain many levels of abstraction, each with its own architecture. An architecture represents an abstraction of system behavior at that level, such that architectural elements are delineated by the abstract interfaces they provide to other elements at that level [9]. Within each element may be found another architecture, defining the system of sub-elements that implement the behavior represented by the parent element's abstract interface. This recursion of architectures continues down to the most basic system elements: those that cannot be decomposed into less abstract elements.
20 |21 | In addition to levels of architecture, a software system will often have multiple operational phases, such as start-up, initialization, normal processing, re-initialization, and shutdown. Each operational phase has its own architecture. For example, a configuration file will be treated as a data element during the start-up phase, but won't be considered an architectural element during normal processing, since at that point the information it contained will have already been distributed throughout the system. It may, in fact, have defined the normal processing architecture. An overall description of a system architecture must be capable of describing not only the operational behavior of the system's architecture during each phase, but also the architecture of transitions between phases.
22 |23 | Perry and Wolf [105] define processing elements as "transformers of data," while Shaw et al. [118] describe components as "the locus of computation and state." This is further clarified in Shaw and Clements [122]: "A component is a unit of software that performs some function at run-time. Examples include programs, objects, processes, and filters." This raises an important distinction between software architecture and what is typically referred to as software structure: the former is an abstraction of the run-time behavior of a software system, whereas the latter is a property of the static software source code. Although there are advantages to having the modular structure of the source code match the decomposition of behavior within a running system, there are also advantages to having independent software components be implemented using parts of the same code (e.g., shared libraries). We separate the view of software architecture from that of the source code in order to focus on the software's run-time characteristics independent of a given component's implementation. Therefore, architectural design and source code structural design, though closely related, are separate design activities. Unfortunately, some descriptions of software architecture fail to make this distinction (e.g., [9]).
24 || 26 | A software architecture is defined by a configuration of architectural elements--components, connectors, and data--constrained in their relationships in order to achieve a desired set of architectural properties. |
28 | A comprehensive examination of the scope and intellectual basis for software architecture can be found in Perry and Wolf [105]. They present a model that defines a software architecture as a set of architectural elements that have a particular form, explicated by a set of rationale. Architectural elements include processing, data, and connecting elements. Form is defined by the properties of the elements and the relationships among the elements -- that is, the constraints on the elements. The rationale provides the underlying basis for the architecture by capturing the motivation for the choice of architectural style, the choice of elements, and the form.
29 |30 | My definitions for software architecture are an elaborated version of those within the Perry and Wolf [105] model, except that I exclude rationale. Although rationale is an important aspect of software architecture research and of architectural description in particular, including it within the definition of software architecture would imply that design documentation is part of the run-time system. The presence or absence of rationale can influence the evolution of an architecture, but, once constituted, the architecture is independent of its reasons for being. Reflective systems [80] can use the characteristics of past performance to change future behavior, but in doing so they are replacing one lower-level architecture with another lower-level architecture, rather than encompassing rationale within those architectures.
31 |32 | As an illustration, consider what happens to a building if its blueprints and design plans are burned. Does the building immediately collapse? No, since the properties by which the walls sustain the weight of the roof remain intact. An architecture has, by design, a set of properties that allow it to meet or exceed the system requirements. Ignorance of those properties may lead to later changes which violate the architecture, just as the replacement of a load-bearing wall with a large window frame may violate the structural stability of a building. Thus, instead of rationale, our definition of software architecture includes architectural properties. Rationale explicates those properties, and lack of rationale may result in gradual decay or degradation of the architecture over time, but the rationale itself is not part of the architecture.
33 |34 | A key feature of the model in Perry and Wolf [105] is the distinction of the various element types. Processing elements are those that perform transformations on data, data elements are those that contain the information that is used and transformed, and connecting elements are the glue that holds the different pieces of the architecture together. I use the more prevalent terms of components and connectors to refer to processing and connecting elements, respectively.
35 |36 | Garlan and Shaw [53] describe an architecture of a system as a collection of computational components together with a description of the interactions between these components--the connectors. This model is expanded upon in Shaw et al. [118]: The architecture of a software system defines that system in terms of components and of interactions among those components. In addition to specifying the structure and topology of the system, the architecture shows the intended correspondence between the system requirements and elements of the constructed system. Further elaboration of this definition can be found in Shaw and Garlan [121].
37 |38 | What is surprising about the Shaw et al. [118] model is that, rather than defining the software's architecture as existing within the software, it is defining a description of the software's architecture as if that were the architecture. In the process, software architecture as a whole is reduced to what is commonly found in most informal architecture diagrams: boxes (components) and lines (connectors). Data elements, along with many of the dynamic aspects of real software architectures, are ignored. Such a model is incapable of adequately describing network-based software architectures, since the nature, location, and movement of data elements within the system is often the single most significant determinant of system behavior.
39 || 41 | A component is an abstract unit of software instructions and internal state that provides a transformation of data via its interface. |
43 | Components are the most easily recognized aspect of software architecture. Perry and Wolf's [105] processing elements are defined as those components that supply the transformation on the data elements. Garlan and Shaw [53] describe components simply as the elements that perform computation. Our definition attempts to be more precise in making the distinction between components and the software within connectors.
44 |45 | A component is an abstract unit of software instructions and internal state that provides a transformation of data via its interface. Example transformations include loading into memory from secondary storage, performing some calculation, translating to a different format, encapsulation with other data, etc. The behavior of each component is part of the architecture insofar as that behavior can be observed or discerned from the point of view of another component [9]. In other words, a component is defined by its interface and the services it provides to other components, rather than by its implementation behind the interface. Parnas [101] would define this as the set of assumptions that other architectural elements can make about the component.
46 || 48 | A connector is an abstract mechanism that mediates communication, coordination, or cooperation among components. |
50 | Perry and Wolf [105] describe connecting elements vaguely as the glue that holds the various pieces of the architecture together. A more precise definition is provided by Shaw and Clements [122]: A connector is an abstract mechanism that mediates communication, coordination, or cooperation among components. Examples include shared representations, remote procedure calls, message-passing protocols, and data streams.
51 |52 | Perhaps the best way to think about connectors is to contrast them with components. Connectors enable communication between components by transferring data elements from one interface to another without changing the data. Internally, a connector may consist of a subsystem of components that transform the data for transfer, perform the transfer, and then reverse the transformation for delivery. However, the external behavioral abstraction captured by the architecture ignores those details. In contrast, a component may, but not always will, transform data from the external perspective.
53 || 55 | A datum is an element of information that is transferred from a component, or received by a component, via a connector. |
57 | As noted above, the presence of data elements is the most significant distinction between the model of software architecture defined by Perry and Wolf [105] and the model used by much of the research labelled software architecture [1, 5, 9, 53, 56, 117-122, 128]. Boasson [24] criticizes current software architecture research for its emphasis on component structures and architecture development tools, suggesting that more focus should be placed on data-centric architectural modeling. Similar comments are made by Jackson [67].
58 |59 | A datum is an element of information that is transferred from a component, or received by a component, via a connector. Examples include byte-sequences, messages, marshalled parameters, and serialized objects, but do not include information that is permanently resident or hidden within a component. From the architectural perspective, a "file" is a transformation that a file system component might make from a "file name" datum received on its interface to a sequence of bytes recorded within an internally hidden storage system. Components can also generate data, as in the case of a software encapsulation of a clock or sensor.
60 |61 | The nature of the data elements within a network-based application architecture will often determine whether or not a given architectural style is appropriate. This is particularly evident in the comparison of mobile code design paradigms [50], where the choice must be made between interacting with a component directly or transforming the component into a data element, transferring it across a network, and then transforming it back to a component that can be interacted with locally. It is impossible to evaluate such an architecture without considering data elements at the architectural level.
62 || 64 | A configuration is the structure of architectural relationships among components, connectors, and data during a period of system run-time. |
66 | Abowd et al. [1] define architectural description as supporting the description of systems in terms of three basic syntactic classes: components, which are the locus of computation; connectors, which define the interactions between components; and configurations, which are collections of interacting components and connectors. Various style-specific concrete notations may be used to represent these visually, facilitate the description of legal computations and interactions, and constrain the set of desirable systems.
67 |68 | Strictly speaking, one might think of a configuration as being equivalent to a set of specific constraints on component interaction. For example, Perry and Wolf [105] include topology in their definition of architectural form relationships. However, separating the active topology from more general constraints allows an architect to more easily distinguish the active configuration from the potential domain of all legitimate configurations. Additional rationale for distinguishing configurations within architectural description languages is presented in Medvidovic and Taylor [86].
69 |71 | The set of architectural properties of a software architecture includes all properties that derive from the selection and arrangement of components, connectors, and data within the system. Examples include both the functional properties achieved by the system and non-functional properties, such as relative ease of evolution, reusability of components, efficiency, and dynamic extensibility, often referred to as quality attributes [9].
72 |73 | Properties are induced by the set of constraints within an architecture. Constraints are often motivated by the application of a software engineering principle [58] to an aspect of the architectural elements. For example, the uniform pipe-and-filter style obtains the qualities of reusability of components and configurability of the application by applying generality to its component interfaces -- constraining the components to a single interface type. Hence, the architectural constraint is "uniform component interface," motivated by the generality principle, in order to obtain two desirable qualities that will become the architectural properties of reusable and configurable components when that style is instantiated within an architecture.
74 |75 | The goal of architectural design is to create an architecture with a set of architectural properties that form a superset of the system requirements. The relative importance of the various architectural properties depends on the nature of the intended system. Section 2.3 examines the properties that are of particular interest to network-based application architectures.
76 || 78 | An architectural style is a coordinated set of architectural constraints that restricts the roles/features of architectural elements and the allowed relationships among those elements within any architecture that conforms to that style. |
80 | Since an architecture embodies both functional and non-functional properties, it can be difficult to directly compare architectures for different types of systems, or for even the same type of system set in different environments. Styles are a mechanism for categorizing architectures and for defining their common characteristics [38]. Each style provides an abstraction for the interactions of components, capturing the essence of a pattern of interaction by ignoring the incidental details of the rest of the architecture [117].
81 |82 | Perry and Wolf [105] define architectural style as an abstraction of element types and formal aspects from various specific architectures, perhaps concentrating on only certain aspects of an architecture. An architectural style encapsulates important decisions about the architectural elements and emphasizes important constraints on the elements and their relationships. This definition allows for styles that focus only on the connectors of an architecture, or on specific aspects of the component interfaces.
83 |84 | In contrast, Garlan and Shaw [53], Garlan et al. [56], and Shaw and Clements [122] all define style in terms of a pattern of interactions among typed components. Specifically, an architectural style determines the vocabulary of components and connectors that can be used in instances of that style, together with a set of constraints on how they can be combined [53]. This restricted view of architectural styles is a direct result of their definition of software architecture -- thinking of architecture as a formal description, rather than as a running system, leads to abstractions based only in the shared patterns of box and line diagrams. Abowd et al. [1] go further and define this explicitly as viewing the collection of conventions that are used to interpret a class of architectural descriptions as defining an architectural style.
85 |86 | New architectures can be defined as instances of specific styles [38]. Since architectural styles may address different aspects of software architecture, a given architecture may be composed of multiple styles. Likewise, a hybrid style can be formed by combining multiple basic styles into a single coordinated style.
87 |88 | Some architectural styles are often portrayed as "silver bullet" solutions for all forms of software. However, a good designer should select a style that matches the needs of the particular problem being solved [119]. Choosing the right architectural style for a network-based application requires an understanding of the problem domain [67] and thereby the communication needs of the application, an awareness of the variety of architectural styles and the particular concerns they address, and the ability to anticipate the sensitivity of each interaction style to the characteristics of network-based communication [133].
89 |90 | Unfortunately, using the term style to refer to a coordinated set of constraints often leads to confusion. This usage differs substantially from the etymology of style, which would emphasize personalization of the design process. Loerke [76] devotes a chapter to denigrating the notion that personal stylistic concerns have any place in the work of a professional architect. Instead, he describes styles as the critics' view of past architecture, where the available choice of materials, the community culture, or the ego of the local ruler were responsible for the architectural style, not the designer. In other words, Loerke views the real source of style in traditional building architecture to be the set of constraints applied to the design, and attaining or copying a specific style should be the least of the designer's goals. Since referring to a named set of constraints as a style makes it easier to communicate the characteristics of common constraints, we use architectural styles as a method of abstraction, rather than as an indicator of personalized design.
91 |93 | In parallel with the software engineering research in architectural styles, the object-oriented programming community has been exploring the use of design patterns and pattern languages to describe recurring abstractions in object-based software development. A design pattern is defined as an important and recurring system construct. A pattern language is a system of patterns organized in a structure that guides the patterns' application [70]. Both concepts are based on the writings of Alexander et al. [3, 4] with regard to building architecture.
94 |95 | The design space of patterns includes implementation concerns specific to the techniques of object-oriented programming, such as class inheritance and interface composition, as well as the higher-level design issues addressed by architectural styles [51]. In some cases, architectural style descriptions have been recast as architectural patterns [120]. However, a primary benefit of patterns is that they can describe relatively complex protocols of interactions between objects as a single abstraction [91], thus including both constraints on behavior and specifics of the implementation. In general, a pattern, or pattern language in the case of multiple integrated patterns, can be thought of as a recipe for implementing a desired set of interactions among objects. In other words, a pattern defines a process for solving a problem by following a path of design and implementation choices [34].
96 |97 | Like software architectural styles, the software patterns research has deviated somewhat from its origin in building architecture. Indeed, Alexander's notion of patterns centers not on recurring arrangements of architectural elements, but rather on the recurring pattern of events--human activity and emotion--that take place within a space, with the understanding that a pattern of events cannot be separated from the space where it occurs [3]. Alexander's design philosophy is to identify patterns of life that are common to the target culture and determine what architectural constraints are needed to differentiate a given space such that it enables the desired patterns to occur naturally. Such patterns exist at multiple levels of abstraction and at all scales.
98 |104 |99 | As an element in the world, each pattern is a relationship between a certain context, a certain system of forces which occurs repeatedly in that context, and a certain spatial configuration which allows these forces to resolve themselves.
100 |101 | As an element of language, a pattern is an instruction, which shows how this spatial configuration can be used, over and over again, to resolve the given system of forces, wherever the context makes it relevant.
102 |103 | The pattern is, in short, at the same time a thing, which happens in the world, and the rule which tells us how to create that thing, and when we must create it. It is both a process and a thing; both a description of a thing which is alive, and a description of the process which will generate that thing. [3]
105 | In many ways, Alexander's patterns have more in common with software architectural styles than the design patterns of OOPL research. An architectural style, as a coordinated set of constraints, is applied to a design space in order to induce the architectural properties that are desired of the system. By applying a style, an architect is differentiating the software design space in the hope that the result will better match the forces inherent in the application, thus leading to system behavior that enhances the natural pattern rather than conflicting with it.
106 |108 | An architectural viewpoint is often application-specific and varies widely based on the application domain. ... we have seen architectural viewpoints that address a variety of issues, including: temporal issues, state and control approaches, data representation, transaction life cycle, security safeguards, and peak demand and graceful degradation. No doubt there are many more possible viewpoints. [70]109 |
110 | In addition to the many architectures within a system, and the many architectural styles from which the architectures are composed, it is also possible to view an architecture from many different perspectives. Perry and Wolf [105] describe three important views in software architecture: processing, data, and connection views. A process view emphasizes the data flow through the components and some aspects of the connections among the components with respect to the data. A data view emphasizes the processing flow, with less emphasis on the connectors. A connection view emphasizes the relationship between components and the state of communication.
111 |112 | Multiple architectural views are common within case studies of specific architectures [9]. One architectural design methodology, the 4+1 View Model [74], organizes the description of a software architecture using five concurrent views, each of which addresses a specific set of concerns.
113 |115 | I include here only those areas of research that define software architecture or describe software architectural styles. Other areas for software architecture research include architectural analysis techniques, architecture recovery and re-engineering, tools and environments for architectural design, architecture refinement from specification to implementation, and case studies of deployed software architectures [55]. Related work in the areas of style classification, distributed process paradigms, and middleware are discussed in Chapter 3.
116 |118 | Most early research on software architecture was concentrated on design methodologies. For example, object-oriented design [25] advocates a way to structure problems that leads naturally to an object-based architecture (or, more accurately, does not lead naturally to any other form of architecture). One of the first design methodologies to emphasize design at the architectural level is Jackson System Development [30]. JSD intentionally structures the analysis of a problem so that it leads to a style of architecture that combines pipe-and-filter (data flow) and process control constraints. These design methodologies tend to produce only one style of architecture.
119 |120 | There has been some initial work investigating methodologies for the analysis and development of architectures. Kazman et al. have described design methods for eliciting the architectural aspects of a design through scenario-based analysis with SAAM [68] and architectural trade-off analysis via ATAM [69]. Shaw [119] compares a variety of box-and-arrow designs for an automobile cruise control system, each done using a different design methodology and encompassing several architectural styles.
121 |123 | Shaw [117] advocates the development of architectural handbooks along the same lines as traditional engineering disciplines. The object-oriented programming community has taken the lead in producing catalogs of design patterns, as exemplified by the "Gang of Four" book [51] and the essays edited by Coplien and Schmidt [33].
124 |125 | Software design patterns tend to be more problem-oriented than architectural styles. Shaw [120] presents eight example architectural patterns based on the architectural styles described in [53], including information on the kinds of problems best suited to each architecture. Buschmann et al. [28] provide a comprehensive examination of the architectural patterns common to object-based development. Both references are purely descriptive and make no attempt to compare or illustrate the differences among architectural patterns.
126 |127 | Tepfenhart and Cusick [129] use a two dimensional map to differentiate among domain taxonomies, domain models, architectural styles, frameworks, kits, design patterns, and applications. In the topology, design patterns are predefined design structures used as building blocks for a software architecture, whereas architectural styles are sets of operational characteristics that identify an architectural family independent of application domain. However, they fail to define architecture itself.
128 |130 | Reference models are developed to provide conceptual frameworks for describing architectures and showing how components are related to each other [117]. The Object Management Architecture (OMA), developed by the OMG [96] as a reference model for brokered distributed object architectures, specifies how objects are defined and created, how client applications invoke objects, and how objects can be shared and reused. The emphasis is on management of distributed objects, rather than efficient application interaction.
131 |132 | Hayes-Roth et al. [62] define domain-specific software architecture (DSSA) as comprising: a) a reference architecture, which describes a general computational framework for a significant domain of applications, b) a component library, which contains reusable chunks of domain expertise, and c) an application configuration method for selecting and configuring components within the architecture to meet particular application requirements. Tracz [130] provides a general overview of DSSA.
133 |134 | DSSA projects have been successful at transferring architectural decisions to running systems by restricting the software development space to a specific architectural style that matches the domain requirements [88]. Examples include ADAGE [10] for avionics, AIS [62] for adaptive intelligent systems, and MetaH [132] for missile guidance, navigation, and control systems. DSSA emphasize reuse of components within a common architectural domain, rather than selecting an architectural style that is specific to each system.
135 |137 | Most of the recent published work regarding software architectures is in the area of architecture description languages (ADL). An ADL is, according to Medvidovic and Taylor [86], a language that provides features for the explicit specification and modeling of a software system's conceptual architecture, including at a minimum: components, component interfaces, connectors, and architectural configurations.
138 |139 | Darwin is a declarative language which is intended to be a general purpose notation for specifying the structure of systems composed of diverse components using diverse interaction mechanisms [81]. Darwin's interesting qualities are that it allows the specification of distributed architectures and dynamically composed architectures [82].
140 |141 | UniCon [118] is a language and associated toolset for composing an architecture from a restricted set of component and connector examples. Wright [5] provides a formal basis for specifying the interactions between architectural components by specifying connector types by their interaction protocols.
142 |143 | Like design methodologies, ADLs often introduce specific architectural assumptions that may impact their ability to describe some architectural styles, and may conflict with the assumptions in existing middleware [38]. In some cases, an ADL is designed specifically for a single architectural style, thus improving its capacity for specialized description and analysis at the cost of generality. For example, C2SADEL [88] is an ADL designed specifically to describe architectures developed in the C2 style [128]. In contrast, ACME [57] is an ADL that attempts to be as generic as possible, but with the trade-off being that it doesn't support style-specific analysis and the building of actual applications; rather, its focus is on the interchange among analysis tools.
144 |146 | Abowd et al. [1] claim that architectural styles can be described formally in terms of a small set of mappings from the syntactic domain of architectural descriptions (box-and-line diagrams) to the semantic domain of architectural meaning. However, this assumes that the architecture is the description, rather than an abstraction of a running system.
147 |148 | Inverardi and Wolf [65] use the Chemical Abstract Machine (CHAM) formalism to model software architecture elements as chemicals whose reactions are controlled by explicitly stated rules. It specifies the behavior of components according to how they transform available data elements and uses composition rules to propagate the individual transformations into an overall system result. While this is an interesting model, it is unclear as to how CHAM could be used to describe any form of architecture whose purpose goes beyond transforming a data stream.
149 |150 | Rapide [78] is a concurrent, event-based simulation language specifically designed for defining and simulating system architectures. The simulator produces a partially-ordered set of events that can be analyzed for conformance to the architectural constraints on interconnection. Le Métayer [75] presents a formalism for the definition of architectures in terms of graphs and graph grammars.
151 |153 | This chapter examined the background for this dissertation. Introducing and formalizing a consistent set of terminology for software architecture concepts is necessary to avoid the confusion between architecture and architecture description that is common in the literature, particularly since much of the prior research on architecture excludes data as an important architectural element. I concluded with a survey of other research related to software architecture and architectural styles.
154 |155 | The next two chapters continue our discussion of background material by focusing on network-based application architectures and describing how styles can be used to guide their architectural design, followed by a survey of common architectural styles using a classification methodology that highlights the architectural properties induced when the styles are applied to an architecture for network-based hypermedia.
156 | 157 | 158 | 159 | 160 | -------------------------------------------------------------------------------- /OEBPS/toc.ncx: -------------------------------------------------------------------------------- 1 | 2 |14 | This chapter presents the requirements of the World Wide Web architecture and the problems faced in designing and evaluating proposed improvements to its key communication protocols. I use the insights garnered from the survey and classification of architectural styles for network-based hypermedia systems to hypothesize methods for developing an architectural style that would be used to guide the design of improvements for the modern Web architecture.
15 |17 | Berners-Lee [20] writes that the "Web's major goal was to be a shared information space through which people and machines could communicate." What was needed was a way for people to store and structure their own information, whether permanent or ephemeral in nature, such that it could be usable by themselves and others, and to be able to reference and structure the information stored by others so that it would not be necessary for everyone to keep and maintain local copies.
18 |19 | The intended end-users of this system were located around the world, at various university and government high-energy physics research labs connected via the Internet. Their machines were a heterogeneous collection of terminals, workstations, servers and supercomputers, requiring a hodge podge of operating system software and file formats. The information ranged from personal research notes to organizational phone listings. The challenge was to build a system that would provide a universally consistent interface to this structured information, available on as many platforms as possible, and incrementally deployable as new people and organizations joined the project.
20 |22 | Since participation in the creation and structuring of information was voluntary, a low entry-barrier was necessary to enable sufficient adoption. This applied to all users of the Web architecture: readers, authors, and application developers.
23 |24 | Hypermedia was chosen as the user interface because of its simplicity and generality: the same interface can be used regardless of the information source, the flexibility of hypermedia relationships (links) allows for unlimited structuring, and the direct manipulation of links allows the complex relationships within the information to guide the reader through an application. Since information within large databases is often much easier to access via a search interface rather than browsing, the Web also incorporated the ability to perform simple queries by providing user-entered data to a service and rendering the result as hypermedia.
25 |26 | For authors, the primary requirement was that partial availability of the overall system must not prevent the authoring of content. The hypertext authoring language needed to be simple and capable of being created using existing editing tools. Authors were expected to keep such things as personal research notes in this format, whether directly connected to the Internet or not, so the fact that some referenced information was unavailable, either temporarily or permanently, could not be allowed to prevent the reading and authoring of information that was available. For similar reasons, it was necessary to be able to create references to information before the target of that reference was available. Since authors were encouraged to collaborate in the development of information sources, references needed to be easy to communicate, whether in the form of e-mail directions or written on the back of a napkin at a conference.
27 |28 | Simplicity was also a goal for the sake of application developers. Since all of the protocols were defined as text, communication could be viewed and interactively tested using existing network tools. This enabled early adoption of the protocols to take place in spite of the lack of standards.
29 |31 | While simplicity makes it possible to deploy an initial implementation of a distributed system, extensibility allows us to avoid getting stuck forever with the limitations of what was deployed. Even if it were possible to build a software system that perfectly matches the requirements of its users, those requirements will change over time just as society changes over time. A system intending to be as long-lived as the Web must be prepared for change.
32 |34 | Hypermedia is defined by the presence of application control information embedded within, or as a layer above, the presentation of information. Distributed hypermedia allows the presentation and control information to be stored at remote locations. By its nature, user actions within a distributed hypermedia system require the transfer of large amounts of data from where the data is stored to where it is used. Thus, the Web architecture must be designed for large-grain data transfer.
35 |36 | The usability of hypermedia interaction is highly sensitive to user-perceived latency: the time between selecting a link and the rendering of a usable result. Since the Web's information sources are distributed across the global Internet, the architecture needs to minimize network interactions (round-trips within the data transfer protocols).
37 |39 | The Web is intended to be an Internet-scale distributed hypermedia system, which means considerably more than just geographical dispersion. The Internet is about interconnecting information networks across multiple organizational boundaries. Suppliers of information services must be able to cope with the demands of anarchic scalability and the independent deployment of software components.
40 |42 | Most software systems are created with the implicit assumption that the entire system is under the control of one entity, or at least that all entities participating within a system are acting towards a common goal and not at cross-purposes. Such an assumption cannot be safely made when the system runs openly on the Internet. Anarchic scalability refers to the need for architectural elements to continue operating when they are subjected to an unanticipated load, or when given malformed or maliciously constructed data, since they may be communicating with elements outside their organizational control. The architecture must be amenable to mechanisms that enhance visibility and scalability.
43 |44 | The anarchic scalability requirement applies to all architectural elements. Clients cannot be expected to maintain knowledge of all servers. Servers cannot be expected to retain knowledge of state across requests. Hypermedia data elements cannot retain "back-pointers," an identifier for each data element that references them, since the number of references to a resource is proportional to the number of people interested in that information. Particularly newsworthy information can also lead to "flash crowds": sudden spikes in access attempts as news of its availability spreads across the world.
45 |46 | Security of the architectural elements, and the platforms on which they operate, also becomes a significant concern. Multiple organizational boundaries implies that multiple trust boundaries could be present in any communication. Intermediary applications, such as firewalls, should be able to inspect the application interactions and prevent those outside the security policy of the organization from being acted upon. The participants in an application interaction should either assume that any information received is untrusted, or require some additional authentication before trust can be given. This requires that the architecture be capable of communicating authentication data and authorization controls. However, since authentication degrades scalability, the architecture's default operation should be limited to actions that do not need trusted data: a safe set of operations with well-defined semantics.
47 |49 | Multiple organizational boundaries also means that the system must be prepared for gradual and fragmented change, where old and new implementations co-exist without preventing the new implementations from making use of their extended capabilities. Existing architectural elements need to be designed with the expectation that later architectural features will be added. Likewise, older implementations need to be easily identified so that legacy behavior can be encapsulated without adversely impacting newer architectural elements. The architecture as a whole must be designed to ease the deployment of architectural elements in a partial, iterative fashion, since it is not possible to force deployment in an orderly manner.
50 |52 | In late 1993, it became clear that more than just researchers would be interested in the Web. Adoption had occurred first in small research groups, spread to on-campus dorms, clubs, and personal home pages, and later to the institutional departments for campus information. When individuals began publishing their personal collections of information, on whatever topics they might feel fanatic about, the social network-effect launched an exponential growth of websites that continues today. Commercial interest in the Web was just beginning, but it was clear by then that the ability to publish on an international scale would be irresistible to businesses.
53 |54 | Although elated by its success, the Internet developer community became concerned that the rapid growth in the Web's usage, along with some poor network characteristics of early HTTP, would quickly outpace the capacity of the Internet infrastructure and lead to a general collapse. This was worsened by the changing nature of application interactions on the Web. Whereas the initial protocols were designed for single request-response pairs, new sites used an increasing number of in-line images as part of the content of Web pages, resulting in a different interaction profile for browsing. The deployed architecture had significant limitations in its support for extensibility, shared caching, and intermediaries, which made it difficult to develop ad-hoc solutions to the growing problems. At the same time, commercial competition within the software market led to an influx of new and occasionally contradictory feature proposals for the Web's protocols.
55 |56 | Working groups within the Internet Engineering Taskforce were formed to work on the Web's three primary standards: URI, HTTP, and HTML. The charter of these groups was to define the subset of existing architectural communication that was commonly and consistently implemented in the early Web architecture, identify problems within that architecture, and then specify a set of standards to solve those problems. This presented us with a challenge: how do we introduce a new set of functionality to an architecture that is already widely deployed, and how do we ensure that its introduction does not adversely impact, or even destroy, the architectural properties that have enabled the Web to succeed?
57 |59 | The early Web architecture was based on solid principles--separation of concerns, simplicity, and generality--but lacked an architectural description and rationale. The design was based on a set of informal hypertext notes [14], two early papers oriented towards the user community [12, 13], and archived discussions on the Web developer community mailing list (www-talk@info.cern.ch). In reality, however, the only true description of the early Web architecture was found within the implementations of libwww (the CERN protocol library for clients and servers), Mosaic (the NCSA browser client), and an assortment of other implementations that interoperated with them.
60 |61 | An architectural style can be used to define the principles behind the Web architecture such that they are visible to future architects. As discussed in Chapter 1, a style is a named set of constraints on architectural elements that induces the set of properties desired of the architecture. The first step in my approach, therefore, is to identify the constraints placed within the early Web architecture that are responsible for its desirable properties.
62 || 63 | Hypothesis I: The design rationale behind the WWW architecture can be described by an architectural style consisting of the set of constraints applied to the elements within the Web architecture. 64 | |
66 | Additional constraints can be applied to an architectural style in order to extend the set of properties induced on instantiated architectures. The next step in my approach is to identify the properties desirable in an Internet-scale distributed hypermedia system, select additional architectural styles that induce those properties, and combine them with the early Web constraints to form a new, hybrid architectural style for the modern Web architecture.
67 || 68 | Hypothesis II: Constraints can be added to the WWW architectural style to derive a new hybrid style that better reflects the desired properties of a modern Web architecture. 69 | |
71 | Using the new architectural style as a guide, we can compare proposed extensions and modifications to the Web architecture against the constraints within the style. Conflicts indicate that the proposal would violate one or more of the design principles behind the Web. In some cases, the conflict could be removed by requiring the use of a specific indicator whenever the new feature is used, as is often done for HTTP extensions that impact the default cacheability of a response. For severe conflicts, such as a change in the interaction style, the same functionality would either be replaced with a design more conducive to the Web's style, or the proposer would be told to implement the functionality as a separate architecture running in parallel to the Web.
72 || 73 | Hypothesis III: Proposals to modify the Web architecture can be compared to the updated WWW architectural style and analyzed for conflicts prior to deployment. 74 | |
76 | Finally, the updated Web architecture, as defined by the revised protocol standards that have been written according to the guidelines of the new architectural style, is deployed through participation in the development of the infrastructure and middleware software that make up the majority of Web applications. This included my direct participation in software development for the Apache HTTP server project and the libwww-perl client library, as well as indirect participation in other projects by advising the developers of the W3C libwww and jigsaw projects, the Netscape Navigator, Lynx, and MSIE browsers, and dozens of other implementations, as part of the IETF discourse.
77 |78 | Although I have described this approach as a single sequence, it is actually applied in a non-sequential, iterative fashion. That is, over the past six years I have been constructing models, adding constraints to the architectural style, and testing their affect on the Web's protocol standards via experimental extensions to client and server software. Likewise, others have suggested the addition of features to the architecture that were outside the scope of my then-current model style, but not in conflict with it, which resulted in going back and revising the architectural constraints to better reflect the improved architecture. The goal has always been to maintain a consistent and correct model of how I intend the Web architecture to behave, so that it could be used to guide the protocol standards that define appropriate behavior, rather than to create an artificial model that would be limited to the constraints originally imagined when the work began.
79 |81 | This chapter presented the requirements of the World Wide Web architecture and the problems faced in designing and evaluating proposed improvements to its key communication protocols. The challenge is to develop a method for designing improvements to an architecture such that the improvements can be evaluated prior to their deployment. My approach is to use an architectural style to define and improve the design rationale behind the Web's architecture, to use that style as the acid test for proving proposed extensions prior to their deployment, and to deploy the revised architecture via direct involvement in the software development projects that have created the Web's infrastructure.
82 |83 | The next chapter introduces and elaborates the Representational State Transfer (REST) architectural style for distributed hypermedia systems, as it has been developed to represent the model for how the modern Web should work. REST provides a set of architectural constraints that, when applied as a whole, emphasizes scalability of component interactions, generality of interfaces, independent deployment of components, and intermediary components to reduce interaction latency, enforce security, and encapsulate legacy systems.
84 | 85 | 86 | 87 | 88 | -------------------------------------------------------------------------------- /README.md: -------------------------------------------------------------------------------- 1 | # What is this? 2 | 3 | Roy Fielding's [REST dissertation](http://www.ics.uci.edu/~fielding/pubs/dissertation/top.htm) reformatted to .epub. 4 | 5 | # How to download? 6 | 7 | An .epub book is basically some HTML and metadata in a zip file. 8 | 9 | * Download the [epub](https://github.com/downloads/csabapalfi/rest-epub/rest-dissertation.epub) I created 10 | 11 | * or you can download the repository as [zip](https://github.com/csabapalfi/rest-epub/zipball/master) then rename the file to .epub 12 | 13 | # References 14 | 15 | Entirely based on the following work: 16 | 17 | Fielding, Roy Thomas. Architectural Styles and the Design of Network-based Software Architectures. 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