Error Handling in WSO2 ESB

When a Proxy Service or a Sequence which mediate messages, is considered, error handling would be a key concern. Depending on the error type, we might need to send a fault message, custom response etc.

WSO2 ESB makes our life easier by allowing us to specify a custom fault sequence for error handling. i.e. It gives the flexibility to use mediator and do various tasks even when an error occurred.

Following is a sample sequence which logs the error details and sends a custom message to the client when an error occurs. Further, if you need to send a SOAP Fault, then Fault mediator can be used instead of Payload Factory mediator.

<sequence name="myErrorSequence">
    <header name="To" action="remove"/>
    <property name="RESPONSE" value="true"/>
    <property name="NO_ENTITY_BODY" scope="axis2" action="remove"/>

    <log level="custom">
        <property name="error-message" expression="get-property('ERROR_MESSAGE')"/>
        <property name="error-code" expression="get-property('ERROR_CODE')"/>
        <property name="error-detail" expression="get-property('ERROR_DETAIL')"/>
        <property name="error-exception" expression="get-property('ERROR_EXCEPTION')"/>
    </log>

    <payloadFactory>
        <format>
            <ns:MyResponse xmlns:ns="http://services.samples">
                <ns:Error/>
            </ns:MyResponse>
        </format>
    </payloadFactory>
    <send/>
</sequence> 

As response might be sending even without going to the outSequence, we need to remove the To header and specify the other two properties named RESPONSE and NO_ENTITY_BODY.

You can also get the details of the error occurred using properties named ERROR_MESSAGE, ERROR_CODE, ERROR_DETAIL and ERROR_EXCEPTION.

After you added the above sequence into the configuration, you can refer it as below from a Proxy or a Sequence.

For a Sequence :

<sequence name="mySequence" onerror="myErrorSequence">
.......
</sequence>

For a Proxy Service :

<proxy name="MyProxy" xmlns="http://ws.apache.org/ns/synapse">
    <target faultSequence="myErrorSequence">
    ......
    </target>
</proxy>