Call mediator end point type(XPath) - wso2-api-manager

Question

In call mediator we can form endpoint using XPath (from the response).But the problem is endpoint is not calling.
<?xml version="1.0" encoding="UTF-8"?>
<api context="/xpath" name="call"
xmlns="http://ws.apache.org/ns/synapse">
<resource methods="GET" protocol="http">
<inSequence>
<call>
<endpoint key-expression="/root/a"/>
</call>
<respond/>
</inSequence>
<outSequence/>
<faultSequence/>
</resource>
</api>
following response is from postman.
<root>
<a>http://www.mocky.io/v2/5ca6db71340000132f76b192</a>
</root>
Expected response :
<root>
<name>abcd</name>
<no>82382832</no>
</root>

Answer 1

Firstly, when using the resolving endpoint (using key-expression), we cannot give the URL directly [1]. We have to have the endpoint defined beforehand and should give only the key of the endpoint in the payload.
Secondly, for the key-expression to parse the xpath, the message should be built beforehand. Since call mediator is content-unaware, it will not build the message. Therefore, we should use a content aware mediator to build the message.
Following is the sample inSequence that'd work.
<inSequence>
<log level="full"/>
<call>
<endpoint key-expression="//a"/>
</call>
<respond/>
</inSequence>
Now the payload should be like
<root>
<a>testEndpoint</a>
</root>
Edit:
The endpoint named 'testEndpoint' should be defined with the backend url.
You need to define an endpoint [2]. For example, I'm using an address endpoint.
<endpoint xmlns="http://ws.apache.org/ns/synapse" name="testEndpoint">
<address uri="http://www.mocky.io/v2/5ca6db71340000132f76b192">
<suspendOnFailure>
<progressionFactor>1.0</progressionFactor>
</suspendOnFailure>
<markForSuspension>
<retriesBeforeSuspension>0</retriesBeforeSuspension>
<retryDelay>0</retryDelay>
</markForSuspension>
</address>
</endpoint>

Answer 2

Try this. I receieved needed response in Postman, but you should use POST method as your request has a Body.
<api xmlns="http://ws.apache.org/ns/synapse" name="call" context="/xpath">
<resource methods="POST">
<inSequence>
<property name="uri.var.httpendpointurl" expression="$body/root/a" scope="default" type="STRING"/>
<call>
<endpoint>
<http uri-template="{uri.var.httpendpointurl}"/>
</endpoint>
</call>
<respond/>
</inSequence>
<outSequence/>
<faultSequence/>
</resource>
</api>