Today, we had the OIOREST workshop, a workshop on the OIOREST initiative, we’re a part of at Trifork. The workshop was meant to inform about REST in the Danish public sector and to hear what users and developers had to say. The workshop went pretty well – not a lot was changed, but at least we were (more or less) confirmed in our view that REST does have its place.
The workshop was centered around a couple of Open Space sessions. One of the topics discussed was “REST vs WS-*”. While working with OIOREST, this topic has come up again and again: What is REST in relation to SOAP and WS-*, and when should each be used? This has been the basis for many heated discussions, and today was not much different. Most people say something like “REST is for simple things, but when things get too complicated, you’ll want to use WS-*”. I can’t really figure out what to think about this. First of all, what is simple and what is complicated? Complicated scenarios often include transactions and reliability. Distributed transactions are evil, period. Reliability is handled in REST by adhering to the HTTP verbs and keeping operations idempotent, so that’s not really a good argument either. Another argument against REST is “now we’ve finally developed all these standards for SOAP, why should we then do the same for REST?”. Ignoring that REST is older than SOAP and WS-*, somehow this argument also feels wrong.
I agree that REST is not for everything: if you want a RPC interface, don’t use REST. If you work in an organization which is committed to SOAP-based web services, don’t use REST (at least not openly). If you have the need for hierarchical data in the message header, REST doesn’t quite fit. If you want to use only GET/PUT/DELETE/POST, it will for example, be hard to implement transfers between two accounts in REST.
Now, everything is possible, and all of this can be modeled in REST, but it’s starting to look wrong and diverge from “real” REST. Something like WS-Security seems hard to implement in a nice way in REST. WS-Security makes it possible to sign and/or encrypt XML payloads transparently. The signature is placed in the SOAP header, and the payload format does not have to support digital signatures directly. If you need end-to-end security, this is a pretty nice thing, and REST does not have something similar. Some payload formats, for example Atom Publishing Protocol, supports signatures as a part of the payload itself. WS-Security still has the advantage that no matter what format you’re using, you can use the same mechanism for signing and checking signatures.
WS-Security has some potential, but I’ve also seen a lot of cases where WS-Security could be replaced by plain SSL instead together with a simple Authorization header. So, the original question is still valid: what are the scenarios where SOAP/WS-* simply cannot be avoided? My guess is that there is no good answer for this, just as there is no good answer to the question “when should we use Ruby instead of Java?”, or “when should we use PostgreSQL instead of MySQL?”. The answers to these questions will depend on who you are, what your experience is, where you’re working, and so on. The only way is probably to try some different scenarios in REST and SOAP and see what approach is most successful – which is what we’re in the process of doing with the Danish Nature and Environment Portal, but more about that another day.