Spring Integration Multiplexing correlation - spring-integration

Question

I am new to SI. I am using the code from the SI TCP Multiplexing example as a starting point for an app server I am writing. The caller of the service already exists and will be sending the payload prefixed by a byte length header. I am having a bit of trouble with the correlation of the response. As you can see below, I changed the Multiplexing example to first add a correlation id header to the incoming request before pushing on to the publish-subscribe-channel. The rest of the code is pretty much the same as the example.
So, the problem. The correlation id header is not available on the call to MessageController from the TcpSendingMessageHandler which serializes the message payload and sends it. Should I enrich the payload to include the correlation id (no correlation header) or is there a simpler way of doing all of this? Any guidance would be greatly appreciated.
<gateway id="gw"
service-interface="is.point.tokens.server.MessageGateway"
default-request-channel="input">
</gateway>
<ip:tcp-connection-factory id="client"
type="client"
host="${tcpClientServer.address}"
port="${tcpClientServer.port}"
single-use="false"
serializer="bigEndianFormatSerializer"
deserializer="bigEndianFormatSerializer"
so-timeout="10000"/>
<channel id="input" datatype="java.lang.String"/>
<header-enricher input-channel="input" output-channel="enriched.input">
<correlation-id expression="headers['id']"/>
</header-enricher>
<publish-subscribe-channel id="enriched.input"/>
<ip:tcp-outbound-channel-adapter id="outAdapter.client"
order="2"
channel="enriched.input"
connection-factory="client"/>
<!-- Collaborator -->
<!-- Also send a copy to the custom aggregator for correlation and
so this message's replyChannel will be transferred to the
aggregated message. The order ensures this gets to the aggregator first -->
<bridge input-channel="enriched.input" output-channel="toAggregator.client" order="1"/>
<!-- Asynchronously receive reply. -->
<ip:tcp-inbound-channel-adapter id="inAdapter.client"
channel="toAggregator.client"
connection-factory="client"/>
<!-- Collaborator -->
<channel id="toAggregator.client" datatype="java.lang.String"/>
<aggregator input-channel="toAggregator.client"
output-channel="toTransformer.client"
correlation-strategy-expression="headers.get('correlationId')"
release-strategy-expression="size() == 2">
</aggregator>
<!-- The response is always second -->
<transformer input-channel="toTransformer.client" expression="payload.get(1)"/>
<!-- Server side -->
<ip:tcp-connection-factory id="server"
type="server"
port="${tcpClientServer.port}"
using-nio="true"
serializer="bigEndianFormatSerializer"
deserializer="bigEndianFormatSerializer"/>
<ip:tcp-inbound-channel-adapter id="inAdapter.server"
channel="toSA"
connection-factory="server" />
<channel id="toSA" datatype="java.lang.String"/>
<service-activator input-channel="toSA"
output-channel="toObAdapter"
ref="messageController"
method="handleMessage"/>
<beans:bean id="messageController"
class="example.server.MessageController"/>
<channel id="toObAdapter"/>
<ip:tcp-outbound-channel-adapter id="outAdapter.server"
channel="toObAdapter"
connection-factory="server"/>

Answer 1

Yes, you need something in the data so the reply can be correlated.
But I am confused. If you are the server side for an existing application, then you can simply use an inbound gateway.
If you really are providing the client and server side, we did add a mechanism in 3.0 to selectively add headers (to the tcp message), for example, using JSON.
EDIT:
From your comments, you only need the server side. All you need is this...
<int-ip:tcp-connection-factory id="server"
type="server"
serializer="serializer"
deserializer="serializer"
port="${port}"/>
<int-ip:tcp-inbound-gateway id="gateway"
connection-factory="crLfServer"
request-channel="toSA"
error-channel="errorChannel"/>
<int:channel id="toSA" />
<int:service-activator input-channel="toSA"
ref="messageController"
method="handleMessage"/>
<bean id="serializer" class="org.springframework.integration.ip.tcp.serializer.ByteArrayLengthHeaderSerializer" />
The fact that the service-activator has no output-channel means the framework will send the reply to the gateway automatically.
This assumes the service can handle a byte[]; if you need a String, you'll need a transformer like in the sample. Either way, the payload will not include the length header; it is stripped off (inbound) and added (outbound).
This also assumes the length header is 4 bytes (default); the serializer takes a size in a constructor...
<bean id="serializer" class="org.springframework.integration.ip.tcp.serializer.ByteArrayLengthHeaderSerializer">
<constructor-arg value="2" />
</bean>