is simply an empty set of constraints. 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 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. 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 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. 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. 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. Rest uses a resource identifier to identify the particular resource involved in an interaction between components. 5.3.2 Connector View A connector view of an architecture concentrates on the mechanics of the communication between components.
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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. The generic connector interface allows components to be placed on the stream based on the properties of each request or response. 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. 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. Allowing features to be downloaded after deployment improves system extensibility. 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. This simplifies clients by reducing the number of features required to be pre-implemented. 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. This chapter introduced the Representational State Transfer (rest) architectural style for distributed hypermedia systems. Three types of view-process, connector, and data-are useful for illuminating the design principles of rest. Developers of Web implementations had already exceeded the early design.
For a rest-based architecture, we are particularly interested in the constraints that define the generic resource interface. 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. 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. 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. This section provides a general overview of rest by walking through the process of deriving it as an architectural style.