Over the years, updating our Licensed ZapThink Architect (LZA) course has given us ample time to explore advances in SOA and Cloud Computing. Now that we’re working on version 9 of the course, we’re taking a closer look at REST-based SOA. Of course, ZapThink has discussed REST for several years now, but recently we’ve seen some fascinating REST-based SOA case studies, both at startups as well as within the US Federal Government.
Nevertheless, while many architects believe that as an architectural style, REST is simpler and more straightforward that Web Services-based SOA, our research is turning up continued confusion over the principles of REST and how best to implement them. Everybody seems to get the basics—operate on resources at URIs with the four HTTP-centric operations GET, POST, PUT, and DELETE—but most people seem to miss the subtleties. Combine that confusion with the fact that you can do REST without SOA, the specifics of REST-based SOA are even more elusive, as we must pare down the essentials of both REST and SOA to understand the true nature of the combined approach. How, therefore, should we handle Service abstractions, contracts, and compositions – arguably, the essence of SOA – in a REST-based SOA world?
Where is the Service Abstraction?
At the center of the SOA approach is the notion of a Service abstraction. REST resources are abstractions as well, but resources are abstractions of capabilities or entities on the server, which is not quite the same thing as a Service abstraction. In SOA, the Service abstraction supports Business Services, which represent flexible, business-centric capabilities. A Business Service may abstract multiple Service interfaces, where routing and transformation operations on intermediaries present a loosely coupled façade.
Most RESTafarians, however, don’t think at this level. They are thinking of clients (e.g. browsers) accessing resources at URIs which return representations. A representation is an HTML page, an XML file, a video, etc. The business context is lost in a sea of URI formats and Internet media types.
What RESTafarians often overlook is that the intermediary pattern is actually one of the core architectural constraints of REST. URIs need not point directly to resources; it is perfectly OK for an intermediary to resolve the URI into a physical endpoint. After all, that’s what DNS servers do!
From the SOA perspective, we can rely upon the intermediary to execute routing rules and transformations as necessary to support the business abstraction. Furthermore, we can establish and enforce the policy (as a part of our SOA governance framework) that the only allowed way to access resources is via endpoints on an intermediary. From the REST perspective, think DNS server on steroids: instead of simply resolving URLs to IP addresses, resolve any formal URI structure to physical resource endpoints by following a rich set of transformation and routing metadata.
Where is the Contract?
At the technical level, a Service is a contracted interface or an abstraction of contracted interfaces. Web Services have contracts that comply with WSDL, but there’s no equivalent of WSDL for REST resources. True, resources have uniform interfaces that the four HTTP operations define, but simply knowing you can GET a resource or POST to a resource doesn’t tell you anything about what that resource is supposed to do. Accessing a resource does give you a representation of that resource, however. Representations can comply with standard Internet media types (formerly known as MIME types), but even the media type specification is insufficient to qualify as a contract.
Sun Microsystems tried to promote the Web Application Description Language (WADL) as a RESTafarian alternative to WSDL, but work on WADL has largely petered out now that Sun is part of Oracle. The point to WADL was more to stub out REST resources in Java than to provide an implementation-neutral contract language in any case.
Where, then, is the contract? Let’s look at a simple REST example: the simplest, of course, being the Web itself. Let’s say you are filling in a form on a Web page and then hit submit. Where is the contract?
The form method is POST, and the POST data are the information that you filled into the form. The resource is identified by the form action URL. So far so good. Have you found the contract yet?
In this example, the contract is the Web form itself. The form specifies and constrains the POST data you may input, and specifies the form action, which is a hyperlink to the next resource. You browsed to the page with the Web form by following an earlier link or loading a URL for a resource that returned that Web page as a representation of that resource.
Remember, a REST application is a set of resources that return representations that link to other resources – in other words, hypermedia. One resource returns one or more representations (Web pages, XML files, etc.) that contain links to other resources, and it is those hyperlinks (and their associated metadata) that specify the application behavior.
While a Web page with a form is the simplest and most common example of how to contract POST data, we can generalize that form however we like, depending on what type of client we want to support. For machine-to-machine interactions, for example (that is, when the client is not simply a browser), the first resource may return an XML representation that provides a contracted interface to the client for POSTing to the linked resource. How your resource builds that representation is up to you.
In Web Services-based SOA we store the contract metadata in a centralized registry/repository. In REST-based SOA each resource is responsible for returning contract metadata either for itself or for any resource it hyperlinks to. As a result, we may not able to obtain contracts for resources we’re not (yet) able to access, but on the other hand, we can code our resources to dynamically generate contracts if we wish. In REST-based SOA, therefore, contract changes can be automated, where in Web Services-based SOA, contract change is a complex, manual process that requires rigorous governance.
Where is the Composition?
The third core characteristic of SOA we look for is the ability to compose Services into applications. Such compositions might be orchestrations, when they have a pre-defined flow, or choreographies, when the order of steps in the composition is not determined ahead of time.
A REST application, of course, is an example of a composition of resources. From the SOA perspective, furthermore, a REST application is a workflow – that is, a composition with human steps. We can also consider such compositions to be choreographies, because the order of steps depends upon which links the user clicks. Users may click links in a different order every time they work their way through the application.
The question still remains: how do we create automated orchestrations in the REST world? The answer is simpler than it looks. In REST, everything can be a resource. Therefore, orchestrations can be resources as well. An orchestration resource might return a BPEL representation or a BPMN representation or perhaps a simplified representation of an orchestration that doesn’t have the baggage of either BPEL or BPMN. If anything, establishing a pre-defined orchestration is simpler than a hypermedia composition, because the orchestration logic is static, while with a hypermedia composition, the underlying resource logic may change the composition logic on the fly. Just because we don’t have to fix our application state transitions ahead of time doesn’t mean we’re not allowed to.
The ZapThink Take
Over the more than ten years ZapThink has been writing about SOA, we’ve fought the battle to explain what SOA really was, fighting the deluge of misinformation from profit-seeking vendors and ill-informed industry analysts. Fortunately, this time vendors aren’t trying to coopt REST to sell software the way they did SOA, to be sure, but the fact still remains that there is extensive confusion and misinformation about REST, just as there still is for SOA.
Mix the two together, therefore, and you’re just asking for trouble. But the effort is worth the trouble, for one simple fact: done right, REST-based SOA actually works. It paves a path to the agile architecture that we’ve been seeking since we first dipped our toe into the ocean of distributed computing. Of course, there’s a catch: the “done right” bit. The devil is in the details.
ZapThink will be providing far more detail on this topic over the next year, in our ZapFlash newsletters, in the next version of our LZA course, and in our Podcasts and at our events. Our new REST-based SOA module in the LZA course in particular breaks new ground and lays the groundwork for simpler, more successful approaches to SOA. Hope to see you at one of our events or classes soon!