Question

This would seem like a pretty common thing to do, but I am having trouble figuring out how to make it work. I wrote a custom filter for a request-response flow that has an inbound http endpoint. The custom filter checks if certain required query parameters are present and contain valid values. If the filter rejects a message, I would like to return an HTTP error status code, but I cannot figure out how to send a different response.

The Mule docs say:

If the inbound endpoint defined on a Flow has a request-response
exchange-pattern and there are no response blocks in your flow then
the response used is simply the result from the last Message Processor
in the flow, which will be null.

I am not sure if this is entirely accurate. My flow does have a <response> block, but it is not getting called if my custom filter returns false.

What is the best way to return a different response if the filter rejects a message?

Here is an example of my flow:

<flow name="Incoming_requests">
    <http:inbound-endpoint exchange-pattern="request-response" host="localhost" port="8081" path="incoming" />

    <http:body-to-parameter-map-transformer />

    <custom-filter class="com.mycompany.MyQueryParamFilter"/>

    <!--  Process valid messages here -->

    <response>
       <!--  Return a successful response  -->
    </response>
</flow>

EDIT: Here is are the docs that inferred that <response> blocks would be executed.
http://www.mulesoft.org/documentation/display/MULE3USER/Using+Filters#UsingFilters-UsinginFlows

Answer 1

Using a choice router would be better in your case:

<choice>
  <when expression="...validation expression...">
    <!--  Process valid messages here -->
    <!--  Return a successful response  -->
  </when>
  <otherwise>
    <!--  Return a failure response  -->
  </otherwise>
</choice>    

No response elements are necessary if the choice is at the end of the flow.