WSO2 EI: Can I use mediator to request another API and pass its response to the body request? - dynamic

Question

In my case, I want to add a dynamic value ("Bearer" + {access-token}) to the header mediator .
So before the header mediator, I want to invoke a get-token API and extract {access-token} element from its response. How can I get that ? Thank you so much.

Answer 1

You can achieve such requirements with mediation sequences. You can refer to this blog for more detailed instructions on how to develop a sequence for your requirement. The blog is written for the API Manager product, but nevertheless, you can follow the same to get it done in the EI.
A sample mediation sequence will be as following
<?xml version="1.0" encoding="UTF-8"?>
<sequence name="oauth2-sequence" xmlns="http://ws.apache.org/ns/synapse">
<!-- token generation to the oauth server's token endpoint -->
<!-- add the base64 encoded credentials -->
<property name="client-authorization-header" scope="default" type="STRING" value="MDZsZ3BTMnh0enRhOXBsaXZGUzliMnk4aEZFYTpmdE4yWTdLcnE2SWRsenBmZ1RuTVU1bkxjUFFh" />
<property name="request-body" expression="json-eval($)" scope="default" type="STRING" />
<property name="resource" expression="get-property('axis2', 'REST_URL_POSTFIX')" scope="default" type="STRING" />
<!-- creating a request payload for client_credentials -->
<payloadFactory media-type="xml">
<format>
<soapenv:Envelope xmlns:soapenv="http://schemas.xmlsoap.org/soap/envelope/">
<soapenv:Body>
<root xmlns="">
<grant_type>client_credentials</grant_type>
</root>
</soapenv:Body>
</soapenv:Envelope>
</format>
<args></args>
</payloadFactory>
<!-- set related headers to call the token endpoint -->
<header name="Authorization" expression="fn:concat('Basic ', get-property('client-authorization-header'))" scope="transport" />
<header name="Content-Type" value="application/x-www-form-urlencoded" scope="transport" />
<property name="messageType" value="application/x-www-form-urlencoded" scope="axis2" type="STRING" />
<property name="REST_URL_POSTFIX" value="" scope="axis2" type="STRING" />
<!-- change the token endpoint -->
<call blocking="true">
<endpoint>
<http method="POST" uri-template="https://localhost:9443/oauth2/token" />
</endpoint>
</call>
<!-- append the acquired access token and make the call to the backend service -->
<property name="bearer-token" expression="json-eval($.access_token)" scope="default" type="STRING" />
<property name="REST_URL_POSTFIX" expression="get-property('resource')" scope="axis2" type="STRING" />
<header name="Authorization" expression="fn:concat('Bearer ', get-property('bearer-token'))" scope="transport" />
<payloadFactory media-type="json">
<format>$1</format>
<args>
<arg evaluator="xml" expression="get-property('request-body')" />
</args>
</payloadFactory>
<!-- perform a send or call to complete the execution of the backend service call in EI -->
</sequence>
Hope this helps you to start with implementation.