February 20, 2006
@ 09:14 PM

Patrick Logan has a post on the recently re-ignited discussion on REST vs. SOAP entitled REST and SOAP where he writes

Update: Mike Champion makes an analogy between messaging technologies (SOAP/WSDL and HTTP) and road vehicle types (trucks and cars). Unfortunately this is an arbitrary analogy. That is, saying that SOAP/WSDL is best "to haul a lot of heavy stuff securely and reliably, use a truck" does not make it so. The question is how to make an objective determination.

Mike is fond of implying that you need to use WS-* if you want security and reliability while REST/POX is only good for simple scenarios. I agree with Patrick Logan that this seems to be an arbitrary determination not backed by empirical evidence. As an end user, the fact that my bank allows me to make financial transactions using REST (i.e. making withdrawals and transfers from their website) is one counter example to the argument that REST isn't good enough for secure and reliable transactions. If it is good enough for banks why isn't it good enough for us?

Of course, the bank's website is only the externally focused aspect of the service and they probably do use systems that ensure reliability and security internally that go beyond the capabilities of the Web's family of protocols and formats. However as someone who builds services that enable tens of millions of end users communicate with each other on a daily basis I find it hard to imagine how WS-* technologies would significanlty improve the situation for folks in my situation.

For example, take the post Clemens Vasters entitled The case of the missing "durable messaging" feature where he writes

I just got a comment from Oran about the lack of durable messaging in WCF and the need for a respective extensibility point. Well... the thing is: Durable messaging is there; use the MSMQ bindings. One of the obvious "problems" with durable messaging that's only based on WS-ReliableMessaging is that that spec (intentionally) does not make any assertions about the behavior of the respective endpoints.

There is no rule saying: "the received message MUST be written do disk". WS-ReliableMessaging is as reliable (and unreliable in case of very long-lasting network failures or an endpoint outright crashing) and plays the same role as TCP. The mapping is actually pretty straightforward like this: WS-Addressing = IP, WS-ReliableMessaging = TCP.

So if you do durable messaging on one end and the other end doesn't do it, the sum of the gained reliability doesn't add up to anything more than it was before.

The funny thing about Clemens's post is that scenarios like the hard drive of a server crashing are the exact kind of reliability issues that concern us in the services we build at MSN Windows Live. It's cool that specs like WS-ReliableMessaging allow me to specify semantics like AtMostOnce (messages must be delivered at most once or result in an error) and InOrder (messages must be delivered in the order they were sent) but this only scratches the surface of what it takes to build a reliable world class service. At best WS-* means you don't have to reinvent the building blocks when building a service that has some claims around reliability and security. However the specifications and tooling aren't mature yet. In the meantime, many of us have services to build.   

I tend to agree with Don's original point in his Pragmatics post. REST vs. SOAP is mainly about reach of services and not much else. If you know the target platform of the consumers of your service is going to be .NET or some other platform with rich WS-* support then you should use SOAP/WSDL/WS-*. On the other hand, if you can't guarantee the target platform of your customers then you should build a Plain Old XML over HTTP (POX/HTTP) or REST web service.


Categories: XML Web Services
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