Question

I am trying to learn different web services to understand what they are used for. But, I am confused as these terms overlap frequently. I would appreciate if someone can tell me briefly what are the differences between these terminologies. I came across some sources such as http://java.sun.com/developer/Books/j2ee/jws/ch07.pdf, however I am not able to distinctly differentiate them.

Answer 1

Web Service: a standards-conforming invocable service, might be written in Java, might be written in some other technology, e.g. .NET. Key point is that there is a standard language, WSDL, that describes the service. The WSDL contains information about the transport, protocol, where the service is running, the available operations and the payloads flowing to and fro.

Most Web Services you will encounter use SOAP messages (a particular XML format) over an HTTP protocol, and so the WSDL will contain the URL of where to invoke the service. More generally other message formats and protocols are possible – we’ll come back to that point in a moment.

Suppose you have the WSDL for a service that you wish to invoke from a Java program, then in principle you can write Java to format a suitable XML payload and squirt the message down an HTTP connection. Perfectly doable, but very tedious, almost all the code is boilerplate code, which can be generated from the the WSDL.

Equally, if you want to create a Web Service in Java then you might begin by writing the WSDL, but once again there’s lots of standard boilerplate code for reading HTTP, parsing XML etc. So in both cases you benefit from standard Java APIs for doing all that work. There have been several such APIs, JAX/RPC and JAX/WS are two such, and JAX/WS is the more recent and easier to use.

An alternative integration technology is to use messaging, there are many vendors who provide message queuing products, so it’s perfectly possible to send messages from (say) Java to (say) C++, you just need to agree on the message format (no standard WSDL there to tell you).

JMS is a Java programming API that provides an abstraction about specific vendors’ messaging products. If working in Java, JMS gives you portability across messaging products. The Java EE standard requires that Java EE App Server vendors provide a messaging infrastructure – I believe that as business systems get large, they nearly always need some asynchronous messaging facilities.

There are various possibilities for applying WSDL-like approaches to messaging. For example it is possible to write a Web Service using SOAP/JMS rather than SOAP/HTTP. JAXM is an emerging API in this space. Frankly I’m unclear as to its relationship to SOAP/JMS and standards such as WS-ReliableMessaging.