SOA is the all about loosely coupled services, right? The Web service behind WSDL is an opaque black box, i.e., it might be implemented by an EJB today and by some COTS package tomorrow. XML must to be used for passing data between layers; we can‘t assume that service provider and service consumer understand the same binary protocol.

What does it mean to a typical J2EE application? For the sake of the discussion, let‘s assume that our application has rich domain object model (say, POJOs with Hibernate persistence), EJB session fasade classes and Struts/JSP on the front-end. Domain classes are directly used in the UI tier; there are no DTOs.

Now, some business functions of our application turned out to be pretty useful so we now want to build Web services to allow the entire organization (or may be even business partners) to benefit from these functions.

“Pure“ SOA would require us to change the UI so that it would rely on Web services API and XML instead of POJOs to communicate with the back-end. Potentially, we‘d also have some kind of ESB /messaging solution in the middle. With that, the service becomes fully de-coupled from the UI and we‘re well on our way to SOA nirvana. Now the implementation of the Web service can be changed without affecting anything on the client‘s side.

But if you look at this approach closely, it immediately raises some nagging questions:

• Both UI and back-end still depend on the same object model. So whenever object model classes change, we want to re-deploy both the service and the UI component.
• If data model changes, object model, UI and may be APIs will be affected anyway.
• We loose the ability to call the service from other EJBs (still co-deployed with the UI) as part of the same transaction unless some kind of proprietary SOAP binding is implemented (and then we need two different WSDL files, one for external consumers, one for internal).
• There are also some “minor“ issues, like concurrency. Let‘s say each updatable POJO has a version number and the UI keeps the object in session for each form. This is not exactly the best model for Web services that are supposed to be stateless; perhaps we should not assume that all service consumers will be able to comply with that.

The list can go on, but I‘ll stop here. Doesn‘t look like we‘ve accomplished much by “SOA-izing” our application, does it? Perhaps, what we should‘ve done is to leave the application alone and build the service just for the external consumers. Oh, and it would be nice to know what their requirements are before we do that, may be we should not assume that one size will fit all of them.

SOA may mean different things to different people (to me, SOA is just an architectural style), but one thing is certain – SOA is being touted as the next generation enterprise integration technology and architecture. And yes, SOA is not all about Web services; however, Web services (SOAP and WSDL specifically) are immediately brought into conversation when SOA is mentioned.

But do Web services today provide necessary enterprise-level capabilities?

Take transaction support. Ideally, we should be able to wrap any access to a remote resource in a transaction since remote resources are prone to failing. In J2EE , this is certainly supported for JMS , JDBC and RMI . Yes, two-phase commit comes at a price, but it‘s nice to have it available when it‘s needed. Now, SOA is all about remote calls (synchronous or asynchronous, does not matter). So where is the transaction support? Well, there is WS-AtomicTransaction specification that in theory should provide it. But where are the products that support it today? Also, this specification is not currently part of any WS-I profile, so interoperability is a big question.

WS-ReliableMessaging is in the similar situation, and so the rule of thumb today is that Web services should conform to WS-I basic profile, which does not include any of the advanced specifications.

In theory, one could use SOAP over JMS supported by many J2EE vendors today which immediately makes SOA implementation more robust. But this approach is hardly interoperable with non-Java technologies and what is it really buying for J2EE applications? JMS is already an abstraction of the messaging middleware, so why do we need another abstraction on the top of it?

I concede that describing JMS destinations using WSDL might be useful for some environments, but in many cases it is simply overkill (if you must use BPEL , use the BPELJ variant which directly supports JMS ).

Using JAX-RPC or WSIF to communicate with JMS does not sound like a good idea either. These APIs are not JMS -aware, so, for example, how do I get/set messageID or correlationID?

I do realize that developing true enterprise-level integration technologies takes time, and so perhaps in a couple years most Web services products will provide WS-Transaction and WS-ReliableMessaging support out of the box. But are Web services really ready to take on the enterprise today?