Pattern: Correlation Identifier

Pattern: Correlation Identifier 

How does a requestor that has received a reply know which request this is the reply for?

But Messaging is asynchronous, so from the caller’s point of view, it makes the call, then sometime later a result appears. The caller may not even remember making the request, or may have made so many requests that it no longer knows which one this is the result for.

Each reply message should contain a Correlation Identifier, a unique identifier that indicates which request message this reply is for. 

how a Correlation Identifier works

When the requestor creates a request message, it assigns the request a request ID—an identifier that is different from those for all other currently outstanding requests (e.g., requests that do not yet have replies). When the replier processes the request, it saves the request ID and adds that ID to the reply as a correlation ID. When the requestor processes the reply, it uses the correlation ID to know which request the reply is for. This is called a correlation identifier because of the way the caller uses the identifier to correlate (e.g., match; show the relationship) each reply to the request that caused it.

Example: JMS Correlation-ID Property

JMS messages have a predefined property for correlation identifiers, JMSCorrelationID, which is

typically used in conjunction with another predefined property, JMSMessageID.

Message requestMessage = // Get the request message
Message replyMessage = // Create the reply message
String requestID = requestMessage.getJMSMessageID();
replyMessage.setJMSCorrelationID(requestID);

Example: Web Services Request/Response

use message-id and response-to fields in the SOAP header to correlate a response to its request. This is the request/response example: 

Example: SOAP request message containing a message identifier

<?xml version="1.0" ?>
<env:Envelope xmlns:env="http://www.w3.org/2002/06/soap-envelope">
<env:Header>
<n:MsgHeader xmlns:n="http://example.org/requestresponse">
<n:MessageId>uuid:09233523-345b-4351-b623-5dsf35sgs5d6</n:MessageId>
</n:MsgHeader>
</env:Header>
<env:Body>
........
</env:Body>
</env:Envelope>

Example: SOAP response message containing correlation to original request

<?xml version="1.0" ?>
<env:Envelope xmlns:env="http://www.w3.org/2002/06/soap-envelope">
<env:Header>
<n:MsgHeader xmlns:n="http://example.org/requestresponse">
<n:MessageId>uuid:09233523-567b-2891-b623-9dke28yod7m9</n:MessageId>
<n:ResponseTo>uuid:09233523-345b-4351-b623-5dsf35sgs5d6</n:ResponseTo>
</n:MsgHeader>
</env:Header>
<env:Body>
........
</env:Body>
</env:Envelope>