How do I use XPath?

XPath is a way to express a path to elements and parts of an XML document. Taverna provides a service for this purpose, located under Local Services -> xml > XPath From Text in Taverna 2.x and Local Services -> Local Java widgets -> xml > XPath From Text in Taverna 1.7.x.

There is an XPath tutorial available online that might be helpful. Here we will 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, i.e. 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 elements. 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. The xpath:

//user

would select all user elements, 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 XPath (and consequently the XPath service in Taverna) has an issue with XML documents with namespaces. A workaround is possible, see TAV-492.

How do I use XPath?

Documentation