REST-Based SOA: an Iconoclastic Approach
Architecture that actually works is more important than following a dogmatic approach
By: Jason Bloomberg
Aug. 7, 2011 09:00 AM
At ZapThink we’re proud to be iconoclasts. After all, building agile architectures requires critical appraisal—and frequent dismissal—of traditionally held beliefs. We know this role places us among the heretics who dare challenge established dogma.
In fact, the whole notion of agility has long suffered this battle between iconoclasm and dogmatism. As we discuss in our Licensed ZapThink Architect course, the Agile Manifesto embodies an iconoclastic approach to software development dogma – and yet, so many people have become dogmatic about the Agile Manifesto itself, entirely missing its point!
ZapThink once more jumped into this iconoclasm-masquerading-as-dogma fray with our recent ZapFlash, How I Became a REST “Convert.” We explained how you can implement SOA following REST principles, and how such an approach introduces important simplifications and efficiencies as compared to a Web Services-based approach.
Get out the torches and pitchforks! ZapThink is at it again! Not only are we flying in the face of established SOA dogma, we’re taking on REST dogma as well! Never mind that the original vision for the REST movement was inherently iconoclastic. In fact, it’s no wonder that RESTafarians use religious metaphors in their discussions (although Rastafarianism is a far cry indeed from the Catholic metaphors we’re leveraging in this ZapFlash).
To quote our beloved Agile Manifesto, we want to favor responding to change over following a plan—even if that plan is how SOA or even REST is “supposed to be done.” The goal is stuff that actually addresses the business problem, not some adherence to official dogma (another Agile principle, in case you didn’t notice). Therefore, it came as no surprise to us when an organization contacted us and let us know that they are taking the REST-based SOA approach we discussed in the ZapFlash, and that it actually works.
The Iconoclastic REST-based SOA of the US Coast Guard
In the Web Services world, when a WSDL file specifies a document style interface, then the constraints on the input and output messages fall into one or more schema definitions, instead of the SOAP message structure itself. As far as SOAP is concerned, the body of the SOAP message contains a payload consisting of whatever document you like. The advantage of the document style over the tightly coupled remote procedure call (RPC) style is it allows for flexible versioning of Services: many changes to a Service do not force a new contract version.
However, as anyone who has monkeyed with Web Services can attest, the operations of a Service still cause issues, even when it’s a document style interface. We can’t seem to get away from Web Services’ tightly coupled RPC heritage. And furthermore, XML schemas are strongly typed, which introduces a further undesirable level of tight coupling.
To resolve such issues, REST moves the operations out of any formal contract, instead relying upon the GET, POST, PUT and DELETE operations best known from HTTP. Even so, there’s no requirement in REST that resources are necessarily documents. True, URLs that point to documents are a common and familiar pattern, but URLs could as easily point to abstracted method calls as well.
SPEAR takes document-centricity to the next level. For SPEAR, the document is the interface. It has meaning both for human and machine consumption. It’s self-describing and removes the need for defining a specific, formal contract. Instead, the USCG provides a basic header/body document structure. The header contains elements like requester, type, ID, timestamps, and status (request, response, publication or error, for example). The body contains different parts depending upon the type of Service. Request/response Services, for example, include elements like request, response, publication, and exception. As a result, SPEAR’s Service contracts (if you even decide to call them that) consist of a simple document structure and core REST operations—nothing more.
Maintaining State the Service-Oriented Way
This approach to state is a simple example of a new trend in REST-based state management: Hypermedia as the Engine of Application State (HATEOAS). With HATEOAS, documents and the hyperlinks they contain represent all the state information a distributed environment requires. Want to know the next step in a process? Simply follow the appropriate link.
In the case of SPEAR, the USCG has established an internal standard URI representation: domain://provider:context/resource. For example, service://uscg.mda.vesselCrew:test/sla. The domain represents a taxonomy of resource types, including system://, organization://, geography://, and service://. The provider component represents the address to the provider of the resource. The context (squeezed in where ports go in a URL), represents the business context like :test, :dev, :stage, etc.
The ESB then resolves URIs to the physical endpoint references, acting as a routing engine that delivers the Business Service abstraction. Any hyperlink to such a URI, therefore, points to a document that contains all the state information the system requires, and furthermore, the architecture offers late binding to Services as a key feature. There is no build time mapping to any particular resource. Instead of using strongly typed schemas, they rely upon dynamic, loose typing in the documents. Such an approach is decidedly not Web Services-friendly.
It’s also important to note that state maintenance in the Web Services world has always been problematic. There are basically three ways to main state information in interactions among inherently stateless Services: rely upon the Service consumer to maintain a correlation ID (either not broadly adopted or relies upon an underlying protocol like HTTP cookies); rely upon the underlying execution environment (vendor dependent); or place state information into the message. Unfortunately, Web Services offer no standard way of accomplishing the latter task, requiring SOA teams to customize their SOAP headers—which is usually a deal killer.
The SPEAR approach, however, includes state information in the message, because, of course, the message is the document. As a result, they are taking a fully Service-oriented approach to maintaining state. They are able to do so in spite of leveraging an ESB because they selected the Fiorano ESB, which has long maintained state in a fully message-centric manner, instead of spawning threads to keep track of state as in other vendors’ ESBs.
Asynchronicity as the General Case
In the SPEAR architecture, the listener acts as the handler, retaining messages at their last stop. The senders simply fire and forget. Messages are sent to the Fiorano bus, not the destination, allowing for dynamic routing of messages as well as publish/subscribe. In this way the ESB routes, filters, and enforces policy via the URIs themselves.
Publish/subscribe is a broadcast push exchange pattern: many consumers can subscribe to the same topic, and the ESB pushes messages to subscribers as appropriate. Even when a single consumer queries a resource, the ESB follows a push pattern, putting documents onto a queue for the consumer to pick up. As a result, SPEAR can support different consumer behaviors, depending upon the use case. If the consumer is accessing Services via unreliable network connections, or simply requires low message volumes and can tolerate some latency, then the consumer can simply poll the Service. In other cases, when latency is an issue or when there are higher message volumes, the ESB can perform a true push to the consumer via a JMS interface.
OK, hold on just one minute. JMS? I thought we were talking about REST, you know, over HTTP. How can JMS fit into this picture?
Yes, the USCG is being iconoclastic once again. The ESB is abstracting the endpoint, while at the same time providing a push-based messaging infrastructure. If the consumer wants the resource to push a real-time stream of large messages to it, then the URI should resolve to a JMS endpoint. If not, or if the consumer doesn’t support JMS (typically when it’s running over HTTP) then the resource will resolve to an HTTP endpoint.
The ZapThink Take
There are two important morals here. First, REST-based SOA is alive and well, and offers straightforward solutions to many of the knottier problems that Web Services-based SOA has suffered from. But even more importantly, the success the USCG has achieved shows that any architectural approach is nothing more than a loose collection of best practices. It’s up to the architect to know which best practices are best for solving their particular problems. Avoid a dogmatic approach, and select the right tool for the job, even if that brands you as an iconoclast.
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