Despite the name, Web service technology offers several advantages in non-Web environments. For example, Web service technology facilitates the integration of J2EE components with .NET components within an enterprise or department in a straightforward manner. But as shown, Web services can be implemented in Web environments, too, on top of basic Web technologies such as HTTP, Simple Mail Transfer Protocol (SMTP), and so on. Representational State Transfer (REST) is a specific architectural style introduced in [F00]. Simply put, it is the architecture of the Web.
Consequently, the question arises about how Web services compare to the Web, or how the corresponding underlying architectural styles SOA and REST compare.
3.10.1 "Representational" in REST
The basic concept of the REST architecture is that of a resource. A resource is any piece of information that you can identify uniquely. In REST, requesters and services exchange messages that typically contain both data and metadata. The data part of a message corresponds to the resource in a particular representation as described by the accompanying metadata (format), which might also contain additional processing directives. You can exchange a resource in multiple representations. Both communication partners might agree to a particular representation in advance.
For example, the data of a message might be information about the current weather in New York City (the resource). The data might be rendered as an HTML document, and the language in which this document is encoded might be German. This makes up the representation of the resource "current weather in New York City." The processing directives might indicate that the data should not be cached because it changes frequently.
3.10.2 "State Transfer" in REST
Services in REST do not maintain the state of an interaction with a requester; that is, if an interaction requires a state, all states must be part of the messages exchanged. By being stateless, services increase their reliability by simplifying recovery processing. (A failed service can recover quickly.) Furthermore, scalability of such services is improved because the services do not need to persist state and do not consume memory, representing active interactions. Both reliability and scalability are required properties of the Web. By following the REST architectural style, you can meet these requirements.
3.10.3 REST Interface Structure
REST assumes a simple interface to manipulate resources in a generic manner. This interface basically supports the create, retrieve, update, and delete (CRUD) method. The metadata of the corresponding messages contains the method name and the identifier of the resource that the method targets. Except for the retrieval method, the message includes a representation of the resource. Therefore, messages are self-describing.
Identifiers being included in the messages is fundamental in REST. It implies further benefits of this architectural style. For example, by making the identifier of the resource explicit, REST furnishes caching strategies at various levels and at proper intermediaries along the message path. An intermediary might determine that it has a valid copy of the target resource available at its side and can satisfy a retrieval request without passing the request further on. This contributes to the scalability of the overall environment.
If you are familiar with HTTP and URIs, you will certainly recognize how REST maps onto these technologies.
3.10.4 REST and Web Services
At its heart, the discussion of REST versus Web services revolves around the advantage and disadvantages of generic CRUD interfaces and custom-defined interfaces.
Proponents of REST argue against Web service technology because custom-defined Web service interfaces do not automatically result in reliability and scalability of the implementing Web services or cacheability of results, as discussed earlier. For example, caching is prohibited mainly because neither identifiers of resources nor the semantics of operations are made explicit in messages that represent Web service operations. Consequently, an intermediary cannot determine the target resource of a request message and whether a request represents a retrieval or an update of a resource. Thus, an intermediary cannot maintain its cache accordingly.
Proponents of Web service technology argue against REST because quality of service is only rudimentally addressed in REST. Scenarios in which SOA is applied require qualities of services such as reliable transport of messages, transactional interactions, and selective encryption of parts of the data exchanged. Furthermore, a particular message exchange between a requester and a service might be carried out in SOA over many different transport protocols along its message path—with transport protocols not even supported by the Web. Thus, the tight coupling of the Web architecture to HTTP (and a few other transport protocols) prohibits meeting this kind of end-to-end qualities of service requirement. Metadata that corresponds to qualities of services cannot—in contrast to what REST assumes—be expected as metadata of the transport protocols along the whole message path. Therefore, this metadata must be part of the payload of the messages. This is exactly what Web service technology addresses from the outset, especially via the header architecture of SOAP.
From an architectural perspective, it is not "either REST or Web services." Both technologies have areas of applicability. As a rule of thumb, REST is preferable in problem domains that are query intense or that require exchange of large grain chunks of data. SOA in general and Web service technology as described in this book in particular is preferable in areas that require asynchrony and various qualities of services. Finally, SOA-based custom interfaces enable straightforward creation of new services based on choreography.
You can even mix both architectural styles in a pure Web environment. For example, you can use a regular HTTP GET request to solicit a SOAP representation of a resource that the URL identifies and specifies in the HTTP message. In that manner, benefits from both approaches are combined. The combination allows use of the SOAP header architecture in the response message to built-in quality of service that HTTP does not support (such as partial encryption of the response). It also supports the benefits of REST, such as caching the SOAP response.