error-channel and reply-channel vanishes during header aggregation - spring-integration

Question

I have the following workflow.
inbound-channel
splitter
task executor for split channels - all the threads execute the same workflow.
3.a. construct the request
3.b. service activator wrapper for a gateway message endpoint.
3.c. gateway wrapper over the http-outbound-gateway with the configuration of error-channel (to handle exceptions while invoking http-outbound-gateway)
3.d. http-outbound-gateway
aggregator
response out of spring integration workflow.
If an exception occurs in 3.d, the control goes to the service-activator configured for the gateway error channel.
I copy just the following from the failed message to the new header to the header passed to the error channel.
a. correlationId
b. sequenceNumber
c. sequenceSize
But while aggregating the splitter response, the DefaultAggregatingMessageGroupProcessor.java removes the conflicting headers and by that it removes the error-channel and reply-channel before providing the control to aggregator.
So once the aggregator completes it's operation it is unable to find the reply or error channel and it results in an exception.
I'm using spring-integration-core version 2.2.1 and I'm not sure why the reply-channel and error-channel is being removed during header aggregation.
Any input on resolving this issue will be of great help.
Thank You :)
EDIT 1:
Thank You very much Gary for helping me out with this scenario. I'm sharing my current configuration
<!-- SPLITTER -->
<int:splitter id="dentalSplitter" ref="dentalServiceSplitter"
method="getDentalServiceList" input-channel="dentalServiceSplitterChannel"
output-channel="dentalSplitterTaskChannel" />
<int:channel id="dentalSplitterTaskChannel">
<int:dispatcher task-executor="dentalTaskExecutor" />
</int:channel>
<int:chain input-channel="dentalSplitterTaskChannel" output-channel="dentalGatewayChannel">
<int:header-enricher>
<int:header name="CHAIN_START_TIME" expression="T(System).currentTimeMillis()" overwrite="true" />
<int:object-to-json-transformer content-type="application/json"/>
</int:chain>
<int:service-activator input-channel="dentalGatewayChannel" ref="dentalGatewayWrapper" output-channel="dentalReplyChannel" />
<int:gateway id="dentalGatewayWrapper" default-request-channel="dentalCostEstimateRequestChannel" error-channel="dentalErrorChannel"/>
<int-http:outbound-gateway id="dentalGateway"
url-expression="#urlBuilder.build('${service.endpoint}')"
http-method="POST" request-factory="clientHttpRequestFactory"
request-channel="dentalCostEstimateRequestChannel" extract-request-payload="true"
expected-response-type="com.dental.test.DentalResponse">
<int-http:request-handler-advice-chain>
<ref bean="logChainTimeInterceptor" />
</int-http:request-handler-advice-chain>
</int-http:outbound-gateway>
<!-- EXCEPTION -->
<int:chain input-channel="dentalErrorChannel" output-channel="dentalAggregatorChannel">
<int:transformer ref="commonErrorTransformer" method="dentalGracefulReturn"/>
</int:chain>
<!-- SUCCESS -->
<int:chain input-channel="dentalReplyChannel" output-channel="dentalAggregatorChannel">
<int:filter discard-channel="dentalErrorChannel"
expression="T(com.dental.util.InvocationOutcomeHelper).isOutcomeSuccess(payload?.metadata?.outcome?.code,payload?.metadata?.outcome?.message)" />
</int:chain>
<!-- AGGREGATION -->
<int:chain input-channel="dentalAggregatorChannel" output-channel="wsDentalServiceOutputChannel" >
<int:aggregator ref="dentalServiceAggregator" />
<int:service-activator ref="dentalResponseServiceActivator" />
</int:chain>
What I noticed was this, every split channel when passing through the gateway creates a new temporary channel for error and reply and after getting the response back from the gateway, it retains the preserved (original inbound) error and reply channel header. And as you had mentioned, after the control gets to the error transformer that flow of retaining the preserved headers gets broken and the aggregating message group processor receives three different instances of temporary channel and hence removes them.
I was planning to have a custom message group processor and modify the conflict resolution strategy for aggregating the header and came up with this config.
<bean id="channelPreservingAggregatingMessageHandler" class="org.springframework.integration.aggregator.AggregatingMessageHandler">
<constructor-arg name="processor" ref="channelPreservingMessageGroupProcessor"/>
</bean>
I'm yet to test this out though. But based on this discussion, this does not look like a viable solution.
And looks like my configuration for error handling in gateway is incorrect.
However, I'm confused on this statement of yours "Instead of forwarding the message directly, simply handle the error on your error flow and return the result normally to the gateway "wrapper"". If I remove the error channel how will I get the control back when an exception occurs? May be I'm missing to understand something here. Can you elaborate more on this please?

Answer 1

When asking questions about scenarios such as this, you generally need to show your configuration. However, I suspect you are forwarding the message from the error flow directly to the aggregator.
This is like doing a GOTO in code and breaks the scoping.
It won't work because the replyChannel header in the error message is for the gateway "wrapper", not the original upstream inbound gateway. When the aggregator gets conflicting headers, it has no choice but to drop the headers (you will see a DEBUG log message to that effect).
Instead of forwarding the message directly, simply handle the error on your error flow and return the result normally to the gateway "wrapper" (simply omit the error channel on the last element on the error flow).
The gateway will then fix up the reply so it is consistent with other messages (good and bad) and forward it to the aggregator.
You don't need to mess with headers in your error flow, just return the value you want to be aggregated along with the good results.
You should really update to a current release, or at least the latest in the 2.2.x line (2.2.6).