Expression analysis

Availability

This is an Orbeon Forms PE feature.

Rationale

Whenever a user makes a change to a form, for example by entering a new value in a form field, the XForms engine must process that change, including:

storing the user data into an XML document (XForms instance)
re-evaluate validity constraints and other properties on the data
run recalculations
update the controls that bind to the data so that they reflect the latest data

Often, this results in a very large number of XPath evaluations, which can be costly.

The XPath analysis feature of Orbeon Forms PE allows the XForms engine, when loading a page, to go through XPath expressions to analyze them and understand dependencies between XML data, constraints, and controls.

This enables the XForms engine to process user interactions faster.

Enabling XPath analysis

NOTE: At the XForms level, this feature is disabled by default, but it is enabled for forms created by Form Builder by default.

You enable XPath analysis with the following property:

<property 
    as="xs:boolean" 
    name="oxf.xforms.xpath-analysis" 
    value="true"/>

You can also turn on this property specifically for a given form by adding xxf:xpath-analysis="true", on the first model:

<xf:model xxf:xpath-analysis="true">

How XPath analysis works

Consider this instance in a people-model model:

<xf:instance id="people">
  <people>
    <person>
      <name>Mary</name>
      <age>20</age>
    </person
    <person>
      <name>Bob</name>
      <age>27</age>
    </person
  </people>
</xf:instance>

And consider a group at the top-level:

<xf:group id="my-group" ref="person[age ge 21]">

The binding with ref is relative to the root element of the instance('people') instance. The expression person [age ge 21] is relative to that root element. It is analyzed as if the form author had written directly:

instance('people')/`person[age ge 21]

Now by analyzing this expression, the XForms engine can know that the expression:

depends on the value of an element called <age> child of an element <person> child of instance('people')
returns an element <person> child of instance('people')
only depends on the people instance in the people-model model

The XForms engine then uses this information to determine when the ref binding on the group needs to be re-evaluated.

So what can cause the binding for my-group to require an update?

A structural change to the instance, such as a new <person> element replacing the current <person> element.
Currently, any structural change in a model invalidates all bindings and values touching that model, and it is as if XPath analysis had been turned off. (This can be improved in the future.)
A change to the value of any element matching instance('people')/person/age.
If the value of such an element changes between two refreshes, then the binding needs to be reevaluated.
Otherwise, no evaluation is needed!
For example, if the value of instance('people')/person/name changes, the binding need not change.

So the XForms engine can simply follow this algorithm:

Was there a structural change since the last refresh? If so, re-evaluate the binding.
Was there a change to the value of any <age> element matching instance('people')/person/age? If so, re-evaluate the binding.
Otherwise, don't re-evaluate the binding.

The same idea applies to:

control bindings
control values
itemsets
values of label, help, hint, and help elements
<xf:bind> bindings
values of model item properties

XPath expressions supported

Not all XPath expressions are currently analyzed fully. If an XPath expression is not analyzable, it will simply be re-evaluated whenever needed.

These expression are not analyzed:

Expression containing the following functions:

index() / xxf:index()
xxf:case()
all functions depending on external data, like the request, session, or xxf:call-xpl()
Java functions

For debugging purposes, you can log the result of the XPath analysis to the servlet container's standard output by enabling this property:

<property 
    as="xs:boolean" 
    name="oxf.xforms.debug.log-xpath-analysis" 
    value="true">

This lists all the expressions considered, mark whether they were analyzed or not, and list all aspects of the analysis.

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