Review of the CMDBf specification version 1.0

Having read the recently released CMDBf 1.0 specification over the weekend, I see several improvements since 0.95, including:

• the introduction of depthLimit
• the lastModified metadata element
• the ability to specify more than one instanceId in a template
• the ability to advertise what parts of the specification you implement
• the definition of faults

But while 1.0 is more complete than 0.95, I think it makes it harder to achieve interoperability. Here are the main friction points for interop:

New role for XPath

The xpathExpression element (which replaces xpath1Selector) changes in two very important ways. First, rather than being limited to XPath 1.0, it now also allows XPath 2.0. Support for this is a lot harder to achieve for people who don’t use XML as the backend format for their data. Considering the current state of adoption of XPath 2.0 and the low level of XML complexity exposed by most CMDB models, I don’t think it was opportune to bring this into CMDBf yet. And my guess is that most implementations will stay away from this. But there is a second change, less obvious but even more problematic. XPath is not just another constraint mechanism for a CMDBf template anymore, one that returns a boolean result indicating whether the instance meets the constraint or not, as it used to be in 0.95. It is now an alternative selection and filtering mechanism that lives in parallel to all the other elements in a template (and can’t mix with them). Overall, I think this change goes too far in the direction of turning a shared agreement to exchange data in XML into an assumption that the internal data models are all based on XML. And the killer with regards to interoperability is that the specification says nothing about how the resulting node sets are serialized in the response. There may be a serialization for the XPath 2.0 model, but there is no such thing for XPath 1.0 and I don’t see in the current state of the specification how two implementations have any chance to interoperate when using this feature.

Introduction of linkDepth

As I mentioned earlier, linkDepth is a very useful addition (even though it pales in comparison to the inferencing capabilities that could have been derived from basing CMDBf on RDF). But it is also a complicated feature. The intermediateItemTemplate attribute is a good re-use of the existing plumbing, but it needs at least a detailed example. I trust that the group will generate one once they’ve caught their breath from putting out the specification.

Service capability metadata

There is a new section (#6) to provide ways to describe what CMDBf features an implementation supports. But it is a very granular representation. Basically, for every feature you can describe if you support it or not. So someone may describe that they support everything inside propertyValue, except for the “like” operator. And someone else might support all the operators but not the caseSensitive modifier. That might be ok for human consumptions, but automated scenarios rely on pre-programmed queries and that is made very hard by all the possible combinations of options. What we need is a few well-defined profiles that people implement fully. Starting of course with a profile that rules out xpathExpression.

Record metadata

This new version introduces metadata on records. While recordId and lastModified are probably well understood and interoperably usable I am a bit more dubious about whether baselineId and snapshotId are going to be interoperable across vendors based on their limited description in the specification. The nice thing is that this metadata can not only be returned but also searched on. Well, at least that’s the intent. But this goes through the recordMetadata attribute on propertyValue which, while present in the pseudo-schema, is missing in the XSD…

The contentSelector element

This new element is more flexible that the propertySubsetDirective element that it replaces. In addition to specifying what properties you want returned it also allows you to specify that you only want certain record types and/or that you only want the record(s) that were used to satisfy constraints in the template. Those are nice additions, but the way the second part is implemented (through the use of the matchedRecords attribute) seems to assume that only one record in the instance was used to match all the constraints in the template. This is not necessarily the case, an instance can be selected by having different records match the different constraints in the template as long as it has at least one matching record per constraint (line 765 says “the item satisfies all the constraints”, not “a record of the item satisfies all the constraints” and you can also see this in the example in section 4.2 where the records mentioned on lines 637 and 639 don’t have to be the same). So do you return all records that have a role in matching the template, or only those (if there is any) that matches all the constraints on their own as the text seems to imply? And if several record combinations inside an instance can be used to match the constraints in a template, do I return all of them or can I just pick any subset that matches? Also, how can I say that I want all records that established the template match, independently of their type? There doesn’t seem to be a way to do this, or is it by putting a contentSelector element with no child element and the matchedRecords attribute set to false? There won’t be much interoperability on this feature until all this is clarified.

Relationships as items

A major change between 0.95 and 1.0 is that now a relationship can match an itemTemplate. For example, if you ask for all items that were modified during the last 24 hours you will get all the items and all the relationships that meet that criteria while in the previous version you’d have to explicitly request the relationships with a relationshipTemplate if you wanted to get them too). There is a good case to be made for either view and the one that works best largely depends on your backend implementation technology (RDF, objects, SQL, CIM…). But the important thing is for the spec to be clear and on this point I think the change wasn’t made explicit enough in the query section of the specification. If Van hadn’t called my attention to this on his blog, I would have missed this important change.

Security boilerplate

There is a person at IBM (probably located in a well-stoked underground bunker in upstate NY) who has instilled the fear of god in all IBM employees (at least all those who author publicly available specifications) and forces them to include a boilerplate “security considerations” section everywhere. I have co-authored several documents with IBM employees and it never fails, even thought it doesn’t add anything useful to the specification. You should see the look of fear on the face of the IBM employees when someone else suggests doing without it. We somehow managed to sneak one such slimmer specification past the IBMers with CMDBf 0.95 but I see that this has been “corrected” in 1.0. I hope that whatever painful punishment Scott, Jacob, Andrew and Mark (or their families and pets) were subjected to in the process by the IBM security ogre wasn’t too cruel. Sure, this doesn’t really impact interoperability, but now that I don’t work for a company that makes money from ink anymore, I have even less patience for this bloating.

OK, that’s enough back seat driving for now. Hopefully the standards group that will take over the specification will address all these questions. In the context of the entire specification, these are pretty small issues and mostly easy to fix. And the CMDBf group can go on to address the hard issues of federation (including security-related issues that abound in this field if one really wants to tackle them). The current specification is a useful graph-oriented query language that is a good match for CMDB data. But it’s really just a query language (plus a simple registration system).

[UPDATE: while updating the CMDBf query algorithm, I noticed another small error: maxIntermediateItems is an attribute in the pseudo-schema but an element in the schema. Something else to fix in the next version.]