Gateway and aggregator - CorrelationStrategy error - spring-boot

Question

I have to access an external service using spring integration. The process is (1) I had to pass an id to get basic information (2) using basic info from step 1, I need to access to more services and merge the information into a singlr object.
integration-graph:
input: Channel1 outputs to : Channel1Out
I have recipient list router that puts message to the 2 channels Channel2 and Channel3.
Channel2 and Channel3's output channels use a xml xpath-transformer
and output to Channel 4
<int:aggregator id="aggregatorChannel"
correlation-strategy-expression="headers['jms_messageId']"
release-strategy-expression="size() == 2" method="mergeVO"
input-channel="channel4" output-channel="dest-channel">
<bean class="n.b.lbr.eai.vo.PojoAggregator"></bean>
</int:aggregator>
This is giving error.
java.lang.IllegalStateException: Null correlation not allowed. Maybe the CorrelationStrategy is failing?
at org.springframework.util.Assert.state(Assert.java:70) ~[spring-core-4.3.8.RELEASE.jar:4.3.8.RELEASE]
at org.springframework.integration.aggregator.AbstractCorrelatingMessageHandler.handleMessageInternal(AbstractCorrelatingMessageHandler.java:385) ~[spring-integration-core-4.3.9.RELEASE.jar:4.3.9.RELEASE]
I did see some posts on this topic, but I do not understand how to solve the below error
{
"timestamp": 1533137160301,
"status": 500,
"error": "Internal Server Error",
"exception": "java.lang.IllegalStateException",
"message": "Null correlation not allowed. Maybe the CorrelationStrategy is failing?",
"path": "/w/b/search/11223"
}
please suggest, if this is a design issue or how to solve this problem
EDIT1:
Is the below valid scatter gather?
<bean id="messageStore" class="org.springframework.integration.store.SimpleMessageStore"/>
<int:scatter-gather id="scatterGather2" input-channel="drBInputChannel" gather-channel="gatherChannel" gather-timeout="5000">
<int:scatterer id="myScatterer" apply-sequence="true">
<int:recipient channel="bserviceInputChannel"/>
<int:recipient channel="aserviceInputChannel"/>
</int:scatterer>
<int:gatherer id="myGatherer"
**??**
message-store="messageStore"
correlation-strategy=**??**
release-strategy-expression="size() == 2"
>
<bean class="nd.wbr.eai.vo.PojoAggregator"></bean>
</int:gatherer>
</int:scatter-gather>
I need help to convert to xml and use in the above
#Bean
public MessageHandler gatherer() {
return new AggregatingMessageHandler(
***new ExpressionEvaluatingMessageGroupProcessor("^[payload gt 5] ?:-1D"),***
new SimpleMessageStore(),
***new HeaderAttributeCorrelationStrategy(
IntegrationMessageHeaderAccessor.CORRELATION_ID),***
new ExpressionEvaluatingReleaseStrategy("size() == 2"));
}

Answer 1

The "java.lang.IllegalStateException: Null correlation not allowed. Maybe the CorrelationStrategy is failing?" exception means that your correlation-strategy-expression="headers['jms_messageId']" doesn't produce anything meaningful. To be precise there is just no jms_messageId header in the message.
Not sure why you did such a choice for the correlation key, but there is definitely not going to be such a headers when you perform HTTP request. You may emulate it though, but it might be better to choose some other correlation strategy.
On the other hand, looking to your original task description, I would say that you need to take a look into the Scatter-Gather pattern and stop to worry about correlation key altogether!