I want to extract the value from request header. For example session key from cookie. I was wondering how to do it?
You can't extract a value from a request because it does not make sense. Your session key is probably coming from a previous server response. Have you checked using the Flag feature?
It could be useful to read a value from request and insert corresponding variable into the body of the same request.
For example:
<GWTRequest>
<service>myPackage.myService</service>
<method>myMethod</method>
<strongId>${StrongIdsCSV.${myMethod}}</strongId>
<encoded>false</encoded>
<parameters/>
</GWTRequest>
where "StrongIdsCSV" is a file having full list of all StrongIds of your war file as an hash map key/value.
In this way script will be understand which service is called in a request and parametrize it with right StrongId.