Comparing Joe Gregorio’s RESTful Partial Updates to WS-ResourceTransfer

Joe Gregorio just proposed a way to do RESTful partial updates. I am not in that boat anymore but, along with my then-colleagues from HP, Microsoft, IBM and Intel, I have spent a fair bit of time trying to address the same problem, albeit in a SOAP-based way. That was WS-ResourceTransfer (WS-RT) which has been out as a draft since summer 2006. In a way, Joe’s proposal is to AtomPub what WS-ResourceTransfer is to WS-Transfer, retrofitting a partial resource update on top of a “full update” mechanism. Because of this, I read his proposal with interest. I have mentioned before that WS-RT isn’t the best-looking cow in the corral so I was ready to like Joe’s presumably simpler approach.

I don’t think it meets the bill for partial update requirements in IT management scenarios.

This is not a REST versus SOAP kind of thing and I am not about to launch in a “how do you do end to end encryption and reliable messaging” tirade. I think it is perfectly possible to meet most management scenarios in a RESTful way. And BTW, I think most management scenarios do not need partial updates at all.

But for those that do, there is just too little flexibility in Joe’s proposal. Not that it means it’s a bad proposal, I don’t have much of an idea of what his use cases are. The proposal might be perfectly adequate for them. But when I read his proposal, it’s IT management I was mentally trying to apply it to and it came short in that regard.

Joe’s proposal requires the server to annotate any element that can be updated. On the positive side, this “puts the server firmly in control of what sub-sections of a document it is willing to handle partial updates on” which can indeed be useful. On the negative side it is not very flexible. If you are interacting with a desired-state controller, the rules that govern what you can and cannot change are often a lot more complex than “you can change X, you can’t change Y”. Look at SML validation for an example.

Another aspect is that the requester has to explicitly name the elements to replace. That could make for a long list. And it creates a risk of race conditions. If I want to change all the elements that have an attribute “foo” with a value “bar” I need to retrieve them first so that I can find their xml:id. Then I need to send a message to update them. But if someone changed them in the meantime, they may not have the “bar” value anymore and I am going to end up updating elements that should not be updated. Again, not necessarily a problem in some domains. An update mechanism that lets you point at the target set via something like XPath helps prevent this round-tripping (at a significant complexity cost unfortunately, something WS-RT tries to address with simplified dialects of XPath).

Joe volunteers another obvious limitation when he writes that “this doesn’t solve all the partial update scenarios, for example this doesn’t help if you have a long sub-list that you want to append to”. Indeed. And it’s even worse if you care about element order. Not something that we normally care much about in IT management (UML, MOF, etc don’t have a notion of property order) but the overuse of XSD in system modeling has resulted in order being important to avoid validation failures (because it’s really hard to write an XSD that doesn’t care about order even though it is often not meaningful to the domain being modeled).

In early 2007, I wrote an implementation of WS-RT and in the process I found many gaps in the specification, especially in the PUT operation. It is not the ideal answer in any way. If one was to try to fix it, a good place to start might be to make the specification a bit less flexible (e.g. restricting the change granularity to the level of an element, not an attribute and not a text node). There is plenty of room to find the simplicity/flexibility sweetspot for IT management scenarios between what WS-RT tries to offer and what Joe’s proposal offers.

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