How do I use xpath to extract information from XML?

XPath is a way to express a path to elements and parts of an XML document. Taverna provides a processor for this purpose, located under Local Services -> Local Java widgets -> xml -> XPath From Text.

There is an XPath tutorial available online that might be helpful, here we'll just mention a simple example. Consider a service returns this XML:

  <result>
    <users>
       <user>Katy</user>
       <user>Stian</user>
    </users>
    <groups>
      <group id="mygrid">
        <user>Katy</user>
        <user>Carole</user>
      </group>
    </groups>
  </result>

Say we are only interested in getting the list {Katy, Stian} out of this document, ie. the content of the user elements below users, but not the ones below group.

XPath allows you to express this in a path, similar to a path on a filesystem. The xpath:

  /result/users/user

would select those two user element. In this case we specified the full path, which is normally the most precise, and avoids potential unrelated results such as the two other user tags belows. Another path:

 //user

would select all user element, and thus result in the list {Katy, Stian, Katy, Carole} In many cases this short-form (the blank node between // means 'below anything') can be good enough, but as shown in this example, can be quite volatile if additional information is added to the XML at a later time.

Note that currently the processor in Taverna has an issue with XML documents with namespaces. A workaround is suggested, see TAV-492