With the IBM/Microsoft/Intel/HP WSDM/WS-Management convergence now implicitly (if not yet officially) dead, it will be interesting to see what IBM is going to do with WSRF. WSRF is being used today, rarely explicitly but rather in an embedded fashion. People who use WSDM use it, people who use CDDLM use it, people who use the Globus Toolkit use it, etc. IBM could write off the convergence work (WS-ResourceTransfer, which was published as a draft, and WS-ResourceEnumeration and WS-EventNotification which were never published) and stick to using the existing WSRF specifications when they need the corresponding functionality. That’s what I hope they do.
Alternatively, they could decide to get the forceps out of the drawer. They can create a new, IBM-friendly (e.g. Fujitsu, CA, Cisco…) private consortium to take over the unfinished drafts (if the IBM/Microsoft/Intel/HP legal agreement allows this) or start new ones. Or they could go directly to W3C, OASIS or OGF and push for a new working group to do the work in the open (and since no-one else would really care about this work IBM should have relatively free hands there, the way Microsoft did in DMTF when IBM chose to boycott WS-Management). Why W3C would care and why OASIS or OGF would want to start commitees to obsolete their existing work is a separate question.
While I hope that IBM doesn’t try to push another pile of WS-* resouce management specifications on an industry that already has too many, if they do I hope that at least they’ll do it right. And that means doing away with the approach embedded in WS-ResourceTransfer. Having personally been involved in many iterations on this problem, I hope to have some insight to contribute.
Along the lines of the age-old parental advice “don’t do it but if you are going to do it then use a condom”, here is my advice to anyone thinking of doing another iteration on the WSRF question: don’t do it but if you are going to do it then be specific about what problem you are addressing.
First, let’s separate three scenarios.
WS-ResourceTransfer should not be seen as a way to query an XML database. Use XQuery for this.
While architecturally it should be possible to build RESTful applications on top of WS-Transfer‘s operations, this is simply not what is happening. WS-Transfer is being used either by CIM people (who get to it via WS-Management) or by big-SOA people (who get is as part of the whole WS-* stack) and neither of them is doing anything remotely RESTful. So just leave that aside and don’t see WS-ResourceTransfer as a way to do “fine-grained REST”. No REST user is loosing sleep over WS-ResourceTransfer being in limbo.
A flexible way to interact with a complex system
This is the use case that you should focus on. You have a system made up of many parts (e.g. a composite application or a server that is made of many components) that you can represent as an XML document. The XML repesentation contains some important information about the system, but it isn’t the system. There are identified resources within the system that have lifecycles, management capabilities and internal parameters. Not everything relevant is captured in the XML model. This is why it is different from an XML database.
In general, I don’t think that XML is the best way to represent complex IT systems. It has plenty of complications that are not relevant to IT management and it doesn’t elegantly support the representation of graphs, often the most natural way to represent such a system (more on this here). CMDBf, with its graph-oriented approach, is a better choice in general. But there are plenty of areas (especially smaller, well-defined, sub-systems) in which XML formats have been defined to represent systems. SCA and SML for example.
In the case where you are dealing with such an XML-described system, then there is value in standard ways to simplify interactions with the system and its parts. But here too, we need to distinguished different patterns rather than trying to handle them all in the same way.
Filtering/sequencing of returned data
Complex IT systems can generate a lot of configuration and/or monitoring data and often you only care for a small subset. For example, an asset record has dozens of elements (lease terms, owner, assigned user…) but you may only care to retrieve the date the lease expires. When you do a GET on the record, you want to qualify it by specifying that only that date needs to be returned. That’s what WS-RP, WS-RT and the WS-Management wsman:TransferFragment header allow. In a variation of this, you want all the data but you don’t want it in one go, you want to pull it piece by piece. That’s what WS-Enumeration gives you. The problem with all these specifications is that they only offer that feature when you are retrieving the resource representation (a WS-Transfer GET or equivalent), not for other operations. But how is this different from invoking an AirlineBooking operation and saying that you only want to be sent the confirmation code, not the full itinerary, equipment type, assigned seat, etc? Bundling this inside WS-RT (or equivalent) is not helpful. A generic SOAP header that can go on any message would be more appropriate (the definition of this header would need to pay special attention to security considerations, especially if the response is signed, because it could be abused to trick the server into sending, and signing, specifically-crafted messages).
Interacting with a sub-element of the system
If you have a handle to a computer system resource and you know that it has one CPU and that this CPU is represented by the /comp:CPU element of the system, why would you need to use some out-of-band discovery mechanism to interact with that CPU? It’s right there, you can see it, you can point to it. Surely there must be a way to address operations to it directly, right? WS-Management tries to do it with its wsman:Selector mechanism, but the selectors are not tied to the model and require, effectively, a separate out-of-band agreement for addressing. There shouldn’t be a need for such an additional agreement once an agreement has already been reached on the model.
What is needed is a way, for systems that have a known XML model, to address message to subpart by using the model itself to support that addressing. Call it SOAPy mashup if you want to feel like you are part of the cool kids. I described such a mechanism a while ago. In effect, it is an improvement on wsman:Selector that an eventual new iteration of WSRF should at least consider.
In some cases, namely when the operation is a WS-Transfer GET, this capability overlaps with the “filtering of returned data” capability. One way to look at it is that you are doing a GET at the level of the overall computer system and filtering the results down to the part that represents the CPU. Another way to look at it is that you are pinpointing the message to a subset of the model (the CPU part) and doing an unmodified GET on it. It doesn’t matter how you choose to think about it. In my proposal, these two ways produce the same message. Like the wave view and particle view of a photon, that in the end, describe the same physical entity with each being the best representation for a set of situations.
The problem with WS-RT and its predecessors is that it doesn’t recognise that this is just the intersection of two orthogonal concerns (filering of output versus addressing of sub-elements) and only handles that intersection.
Interacting with a set of resources as a set
The same kind of expression (typically XPath) that lets you point at a sub-element inside of a system also lets you point at a set of such sub-elements. But even though from an XPath perspective there isn’t much of a different (the first one just happens to return a nodeset that contains only one node), from an architectural perspective it is a very different use case. If you want to support such a use case then you have handle it as such and define all the associated semantics (sequential/parallel execution, fault handling, partial completion, resource-specific permissions…). You can’t just cross your fingers and assume that you get such features “for free” just because XPath can return a nodeset.
I know that this post illustrates a way of giving free advice that virtually ensures that it gets ignored. Similar (if you’ll allow the big stretch) to the way Chirac and Villepin were arguing againt an Iraq invasion in ways that probably reinforced the Bush administration’s determination to do it. When will the world finally learn to appreciate the oh-so-slightly obnoxious undertone that is inherently French (because, let me tell you, we’re not about to loose it)? At least, when my grandchildren ask me “where were you when IBM invented WS-ManagementHammer?” I can point to this post and say “I tried to stop it, I tried”.
[UPDATED 2008/5/15: How timely! Just after publishing this I find, via Coté, what looks like another example of French abrasiveness in the systems management world: the attitude, name and the way Jeff ends with a French-language quote make it quite likely that the "Jacques" person discounting the fact that his company's SNMP agent is broken is indeed a compatriot. French obnoxiousness aside, and despite my respect for standards, my advice to Jeff is that if a given SNMP agent works with HP, IBM, BMC and CA you will probably save yourself time in the long run by finding a way to support it (even if it is not spec-compliant) rather than getting the vendor to change. There are lots of sites out there that work fine with Firefox and IE but are not compliant with Web standards. Good luck getting them all fixed.]
[UPDATED 2008/7/14: I don't really plan to turn this post into a ongoing set of updates about "French attitude" but since today is Bastille Day I'll point to this map of the world as seen from Paris. If I wasn't on strike right now, I'd explain why the commenter is wrong to assert that "French self-deprecating humour" is rare.]