Following is how you can read query params from your rest service when using Enterprise Integrator.
There are two ways to read your query params.
1. Assigning value to a property within a URI template. (get-property('uri.var.qtest')).
If you look at the following synapse config I'm assigning the required query param to
<api xmlns="http://ws.apache.org/ns/synapse" name="test" context="/qtest">
<resource methods="GET" uri-template="/search?q={qtest}"...
