onsdag, december 05, 2007

Maintaining Changes to Web Services

A common problem that occurs when working with Web Services is how-to maintaining changes to the WSDL and/or XSD files that are used by the service. The problem occurs when performing changes to the WSDL (or XSD) files like removing operations, renaming operations, changing the structure of data types etc. As you might always not be aware of which clients that are using your service(s); you want to make sure that they continues to work even after you have implemented changes to your service. As support for dealing with this has not been built into the Web services architecture / standards; it is basically left to each implementer to deal with this problem.

One way of dealing with this problem is to use Namespaces versions. The approach for this is as:

1. Use different XML namespaces for different version of the service.
2. Send a specific namespace value along with every SOAP message and result.
3. Based on this value a Web Service implementation (mediator) can then correctly determine what to do with the incoming message.

You first need to ensure that the namespace for the XML elements resulting from that document is unique, for example, suppose that you now are using a namespace like:


You then need to change this to:


Where the numbers are year, month and date (it is not likely that you will alter the namespace more than once a day...).

Now each request will be accompanied with a reference to a namespace thus leaving it up to the Web Services to deal with what to do with requests that come in for any particular namespace.

Next thing is to decide what to do with the various types of incoming requests. One approach here is to generate an error if a request for an older namespace is received, and leave it up to the client to deal with this. Another approach is to use a mediator, as mentioned earlier in the post. The task of the mediator is to determine what to do with Web service requests that come in for any particular namespace. This is done by examination of the date stamp on the namespace (introduced in the previous steps) and then route requests from the older namespace to the older version of the Web service, while routing requests from the newer namespace to the new version of the Web service. This means that the URL to the Web Service (the mediator) will always be the same to your external clients, while you are routing it to different services internally.

As a mediator you could either use a Web Service that you write the code yourself for to handle this, or you could look into using our ESB (Enterprise Service Bus) product for this task.