processing multiple http outbound gateways - spring-integration

Question

I am working on the spring integration http. Wanted to make multiple http calls and collect the response to one common java object. I am facing an issues which says no output/reply channel at the end of the aggregator.
This is the xml definition
<int:channel id="intermediateWChannel">
<int:interceptors>
<int:wire-tap channel="intermediateWLogger" />
</int:interceptors>
</int:channel>
<int:channel id="intermediateSChannel">
<int:interceptors>
<int:wire-tap channel="intermediateSLogger" />
</int:interceptors>
</int:channel>
<int:logging-channel-adapter id="intermediateWLogger" expression="'Fetch Wtms Trip : '.concat(payload)" level="INFO" />
<int:logging-channel-adapter id="intermediateSLogger" level="INFO" />
<int:chain input-channel="intermediateSChannel" output-channel="publishsubscribechannel">
<int-http:outbound-gateway
id="outboundGateway"
url="{url2}"
http-method="GET"
request-factory="requestFactory"
charset="UTF-8"
mapped-request-headers="Accept:application/json"
expected-response-type="java.lang.String"></int-http:outbound-gateway>
<int:object-to-json-transformer/>
</int:chain>
<int:chain id="chain2" input-channel="publishsubscribechannel" output-channel="aggregatorChannel">
<int:transformer ref="fetchTransformer" method="process" />
</int:chain>
<int:chain id="request-chain" input-channel="publishsubscribechannel" output-channel="aggregatorChannel">
<int-http:outbound-gateway
id="strideOutboundGateway1"
url="url3"
http-method="GET"
request-factory="requestFactory"
charset="UTF-8"
mapped-request-headers="Accept:application/json"
expected-response-type="java.lang.String"></int-http:outbound-gateway>
<int:transformer ref="fetchTransformer" method="process1" />
</int:chain>
<int:chain id="chain3" input-channel="aggregatorChannel" output-channel="outputChannel">
<int:aggregator id="tAggregator"
ref="tDataAggregator"
method="processAggregator"
correlation-strategy-expression="headers['id']"
release-strategy="aggregatorReleaseStrategy"
expire-groups-upon-completion="true"/>
</int:chain>
But, I have specified the output channel in the aggregator.
Below is the exception and even if I add the reply-channel in headers, application is going for stackoverflow.
org.springframework.messaging.core.DestinationResolutionException: no output-channel or replyChannel header available
at org.springframework.integration.handler.AbstractMessageProducingHandler.sendOutput(AbstractMessageProducingHandler.java:452) ~[spring-integration-core-5.5.13.jar:5.5.13]
at org.springframework.integration.handler.AbstractMessageProducingHandler.doProduceOutput(AbstractMessageProducingHandler.java:325) ~[spring-integration-core-5.5.13.jar:5.5.13]
at org.springframework.integration.handler.AbstractMessageProducingHandler.produceOutput(AbstractMessageProducingHandler.java:268) ~[spring-integration-core-5.5.13.jar:5.5.13]
at org.springframework.integration.handler.AbstractMessageProducingHandler.sendOutputs(AbstractMessageProducingHandler.java:232) ~[spring-integration-core-5.5.13.jar:5.5.13]

Answer 1

The transformer is different from other endpoints that it does not modify a Message we return from its method call. It is counted as transformer has a full control over the reply message creation. So, if there is a need to preserve some request message headers, it is recommended to use a MessageBuilder.copyHeadersIfAbsent() API. This way in request-reply scenario, the necessary replyChannel header is present in the reply message.
We probably have to mention shouldCopyRequestHeaders() behavior for the transformer in docs: https://docs.spring.io/spring-integration/docs/current/reference/html/message-transformation.html#messaging-transformation-chapter. Feel free to raise a GH issue.