Thursday, December 01, 2016

Implementing a Request Response with Messaging (JMS)

Quite often, we need to implement a request/response paradigm on JMS - e.g. calling a backend function on a mainframe or a third-party interface that has an AMQP endpoint. To implement this, we have the following 3 options:

Option 1: The client creates a temporary queue and embeds the name-address of this queue in the message header (JMSReplyTo message header) before sending it to the server. The server processes the request message and writes the response message on to the temp queue mentioned in the JMSReplyTo header.
The advantage of this approach is that the server does not need to know the destination of the client response message in advance.
An important point to remember is that a temporary queue is only valid till the connection object is open. Temporary queues are generally light-weight, but it depends on the implementation of the MOM (message-oriented middleware).

 Option 2: Create a permanent queue for response messages from the server. The client sets a correlation ID on the message before it is sent to the server. The client then listens on the response queue using a JMS selector - using the correlation ID header value as the selector property. This ensures that only the appropriate message from the response queue is delivered to the appropriate client.

Option 3: Use a combination of Option 1 and Option 2.  Each client creates a temporary queue and JMS Consumer on startup. We need to use this option if the client does not block for each request. So the client can keep on sending messages and listen to multiple response messages on the temp queue and then match the req/res using the correlation ID.

Spring has implemented this design pattern in the JMS Outbound Gateway class -

The JMS spec also contains an API for basic req/res using temporary queues - QueueRequestor and TopicRequestor. So if your MOM supports JMS, then you can use this basic implementation if it suffices your needs.

The following links would serve as a good read on this topic.