Wednesday, September 8, 2010

Versioning WSDL interfaces in an OSGi world

Some time ago (2007, I think) I wrote a short paper with Oliver Wulff on WSDL versioning. Recently, I shared this paper with some smart OSGi-savvy ServiceMix users, and it raised a discussion about how does WSDL interface versioning mix with the versioning concepts in OSGi? Below is an extract of an email response I sent back to them: for me, the bottom line is that WSDL interface versioning (in fact, any interface versioning) is a separate concern to the way we version implementations.

For anyone out there who is implementing Web Services using OSGi runtimes like FUSE ESB (built on Apache ServiceMix and Apache Karaf), you might find the thoughts below of interest.

From my side, I think the most important point is that my paper concerns WSDL versioning, that is, the versioning of an *interface*; the versioning of an OSGi bundle is, from a service consumer's perspective, an *implementation* detail and a totally separate concern. I believe you should consider the versioning of the interface and the implementation as two *separate* evolutionary paths.

So, I could have, say, version 1 of an 'foo' web service, in namespace, defined in foo-v1.0.wsdl. We might generate the Java JAX-WS/JAXB-B classes, and then package as a bundle foo-ws-api-1.0.jar. I could implement it today in bundle foo-v1.0-impl-1.0.0.jar, and then fix a bug later in foo-v1.0-impl-1.0.1.jar, add a cool new feature in foo-v1.0-impl-1.1.0.jar, and then do a completely new, high-performance implementation in foo-v1.0-impl-2.0.0.jar. All of these implementation bundles would implement *the same* version of the WSDL!!

Now, imagine we need to do a minor change to the web service interface: say add a new operation, or add a new parameter to a method. We can do this *without* breaking on the wire interoperability by creating the new PortType in foo-v1.1.wsdl, but still in the namespace We would deploy the JAX-WS content for this in foo-ws-api-1.1.jar. We could do a new implementation, and put the implementation of this in foo-v1.0-impl-1.0.0.jar. Now, we can deploy the new version of both the interface and the implementation, and, if we've been careful, the new deployment will handle requests existing consumers of version 1.0 of the interface. KEEP IN MIND THAT WHILE THIS IS POSSIBLE TO DO ... (sorry for shouting!) it's actually quite tricky to understand and I think that the complexity overweighs the benefits for most users. So, it's probably better to try to keep WSDL interface's versioning at major release numbers.

Now, with this in mind, we can consider what happens when we deploy a whole new version of the WSDL interface! We can deliver foo-v2.0.wsdL, with portTypes and all in namespace We can deploy this API as bundle foo-ws-api-2.0.jar, and deliver our first implementation bundle as foo-v2.0-impl-1.0.0.jar. And, later, when we fix some bugs or add some features, we can deploy the new implementation as foo-v2.0-impl-1.0.1.jar.

Get the picture? For me, the rate of change of an interface should be different to that of an implementation - ideally, it should be significantly slower, changing maybe on a yearly base. We need to apply different versioning semantics, and evolve them at different rates to the implementation bundles.

Hope that helps!


1 comment:

Mike @ said...

Hey, I just wanted to compliment you on your treatment of this topic. I had been thinking about this same issue concerning the evolution of our web services, and how to handle the tension between interface vs. implementation advancement.

I agree with you that there should be separate versioning for the WSDL/schemas and for the underlying execution context (i.e. implementation) that supports the interface. I have read others who advocate that a web service contract is *more* that just an interface; considerations like QoS, performance, pre & post conditions apply and influence the interaction between client and server. This can also be true, but it greatly complicates the maintenance of the underlying implementation. IMO fixing a bug or changing the code structure of the implementation generally shouldn't influence the message semantics, so I don't see a huge need to impact clients and make them adapt to changes that are probably extremely innocuous to them.

Thanks again, I appreciate the thoughts on this topic!