Less is more: inventory of XPath subsets

Many specifications that manipulate XML content have taken the step to create their own subset of XPath. Typically, they need an XML query/pointer language but full XPath (or XPointer) is overkill for their purpose (I am talking about XPath 1.0 here, XPath 2.0 is usually over-over-kill-and-then-some). Defining a subset of XPath rather than inventing a query language from scratch is attractive because:

  • people are relatively familiar with XPath (at least the most common parts)
  • it is already specified so you can leverage the W3C spec-writing work
  • implementers who have access to an XPath engine get an implementation of the subset “for free” since the XPath engine will process statements from any XPath subset

Here is a quick inventory of spec-defined XPath subsets that I am aware of.

XML Schema

Section 3.11.6 of the XML Schema specification (part 1) defines a subset of XPath used to point to a set of elements that are the target of an identity constraint. Here is the BNF of the abbreviated form of the subset (you can also use the functionally equivalent full-length notation):

Selector   ::=    Path ( '|' Path )*
Path       ::=    ('.//')? Step ( '/' Step )*
Step       ::=    '.' | NameTest
NameTest   ::=    QName | '*' | NCName ':' '*'

Actually, there is a second subset defined, to point to the identifying key from the identified element. It is very similar to the previous one but it allows attribute nodes to be selected, via this modification:

Path       ::=    ('.//')? ( Step '/' )* ( Step | '@' NameTest )

According to the specification, these subsets were defined “in order to reduce the burden on implementers, in particular implementers of streaming processors”. As this article points out, stream-friendliness of an XPath subset is a relative notion. But the subsets above seems, indeed, to fit the bill. They also include many simplifications that “reduce the burden on implementers” but have nothing to do with streaming, such as removing functions and all predicates (rather than simply restricting the content of predicates).

WS-Management and WS-ResourceTransfer

WS-Managment also defines two subsets (it calls them dialects) of XPath. I won’t copy the BNF definitions here, as they are pretty long. You can find them in Appendix D of the specification. They are called “XPath level 1″ and “XPath level 2″.

The main use cases driving these dialects had to do with implementing WS-Management in resource-constrained environments, e.g. the board management controller of a server.

WS-ResourceTransfer took this idea from WS-Management and it too defines an “XPath level 1″ dialect (see Appendix I of the specification)

Windows EventLog Remoting Protocol v6

Version 6.0  of the Microsoft EventLog Remoting Protocol, new in Vista, adds a filter mechanism (to select events in the log) that is based on a subset of XPath. Streaming appears to be a concern there too (“evaluation of each event MUST be restricted to forward-only, in-order, depth-first traversal of the XML”). And, being Microsoft, they also add some extensions to XPath.

CMDBf (coming soon)

CMDBf also defines a subset of XPath. It is not defined via a restricted syntax but via a limitation of the type of objects returned. There is no BNF provided. You can write your XPath any way you want as long as it only returns objects of the right types (e.g. nodesets containing comment nodes are out of luck). Think of it as “management by objectives” rather than micromanagement. The main driver here is not support for  streaming. It’s that since XPath nodeset serialization is a pain we only want to do it where there is a compelling use case. There is no point creating interoperability challenges for no practical benefit.

Others?

Except for XSD, all the examples above come from the IT management world, because that’s where I live. There are probably plenty of specifications in other domains which took similar steps, such as the Digital Talking Book ANSI standard (see this section).

And those are only XPath subsets defined as part of specifications. There are also XPath subsets defined by implementations (e.g. ElementTree’s limited XPath support). And others defined for research purposes (e.g. “Univariate XPath”, from this ACM article, that is quickly described in the previously-mentioned post about stream-friendly XPath subsets).

If you know of other interesting XPath subsets, please leave a comment.

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