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Time to balance WS-* and REST

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The argument rages on, with the REST advocates mostly talking loudest. Here, for instance, are ringing condemnations of WS-* from an IBM employee and a Microsoft employee. It's positively alarming to see how closely they agree. Just for balance, here's Steve Vinoski's take. He's a practical distributed systems architect, an expert in CORBA and most other types of middleware, and quite impartial. But perhaps we should give the last word to Sanjiva Weerawarana, an ex-IBMer who now seems inclined to call down "a plague on both your houses". He's right, of course: WS-* and REST both have their strengths and weaknesses, and neither is perfect for everything.

When confused by too much detail, it pays to go back to first principles. First of all, we should ask ourselves, what are web services exactly? Sad to say, the honest answer is that no one knows - or, at least, not everyone agrees. Some definitions stress the use of XML, while others insist on SOAP or WSDL. Some implementations directly support CICS, Java EE or grids, while others are deliberately lightweight and easy to use.

The next question is whether web services should be designed to fit in as painlessly as possible with existing object-oriented applications and frameworks, or to work as efficiently and cleanly as possible across existing networks? The two objectives are hard to reconcile, if not actually incompatible. Then perhaps we should ask how much it should cost to use web services? Should it be free, as originally envisaged, or should it require the purchase of complicated, expensive proprietary tools and middleware?

Without answering these questions, it is impossible to choose between WS-*, XML/HTTP, REST, and other technical approaches. Even distributed object systems like CORBA, COM+, and RMI work well in certain particular types of application - for instance, those running on fast, reliable networks within a single administration and security domain. They lie at one end of the spectrum, with REST at the far end and WS-* in the middle. REST is particularly suitable for very large networks with huge numbers of clients and servers, where clients may also be servers and vice versa, and where it is convenient to model all operations as reads and writes directed to individual URIs. As for WS-*, its sweet spot lies in systems bigger than those that use distributed objects, and whose networks are further flung, slower, less reliable, and possibly divided between different administrative domains.

Don't expect conclusive answers to this many sided debate any time soon. When it comes to web-scale distributed systems, we simply don't know enough yet about what works and what doesn't. Besides, web services are still a solution in search of new problems. That's always harder than trying to solve a known set of requirements.®

Tom Welsh is a senior consultant with Cutter Consortium's Enterprise Architecture advisory service. Tom has been following OMG and its specifications since 1992.

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