Mediation flow of WSO2 ESB

WSO2 ESB is one of the most important products in the WSO2 product stacks which enables users to do all sorts of transformations. Instead of having to make each of your applications communicate directly with each other in all their various formats, each application simply communicates with the WSO2 ESB, which handles transforming and routing the messages to their appropriate destinations. While working with this product, we believe it is important to understand how the messages flow through ESB. So when a message comes inside ESB, it goes through the transport layer and then the Axis2 engine which converts the incoming message into a SOAP envelope.

Once converted, it is then handed over to the Mediation engine, which is considered as the backbone of the product. This is where all the mediation work happens. In this post, I will be explaining in detail what happens to a message which comes inside ESB and the path it takes until a response is delivered back to the client.

To explain the scenario, I will use the below Proxy Service which talks to a simple back-end and logs a message in the inSequence as well as the outSequence.

<proxy name="DebugProxy" startOnLoad="true" transports="https http">
        <description/>
        <target>
            <endpoint>
                <address uri="http://localhost:9000/services/SimpleStockQuoteService"/>
            </endpoint>
            <inSequence>
                <log level="full">
                    <property name="IN_SEQ" value="Executing In Sequence"/>
                </log>
            </inSequence>
            <outSequence>
                <log level="full">
                    <property name="OUT_SEQ" value="Inside the out sequence"/>
                </log>
                <send/>
            </outSequence>
            <faultSequence>
                <log level="full">
                    <property name="FAULT_SEQ" value="Inside Fault Sequence"/>
                </log>
            </faultSequence>
        </target>
    </proxy>   
           
The entry point to the mediation engine for a message which comes in to a Proxy Service, is the receive method of the ProxyServiceMessageReceiver while for the message out-flow, the entry point is SynapseCallbackReceiver. Below I've listed down each method that is being called in each class inside the Mediation engine.

In-flow

Out-flow