MQ publish/subscribe

WebSphere MQ publish/subscribe is used on MQ queues that employ a broker process to perform subscription resolution. In its simplest form:

• A publisher is the application that is sending the message.

• A subscriber is the application that is receiving the message.

• The following queues are involved:

  • Control queue – where publishers and subscribers send directives such as subscriber registration and cancellation to the pub/sub broker.

  • Stream queue – where the publisher sends its messages directly. The pub/sub broker reads the messages from the stream queue and distributes them to the appropriate subscriber’s queue.

  • Subscriber queue – where the subscriber reads its messages directly.

  More queues may be involved, depending on the type of publications.

• The pub/sub broker responds to MQRFH messages sent to the control queue. These command messages control how the pub/sub broker processes messages that arrive on the stream queue. For instance, a subscriber can register an interest in a particular topic.

• The publisher sends messages directly to the stream queue.

• The pub/sub broker reads messages from the stream queue and determines the subscriber queue to which to copy the message. This depends on topics that the subscribers have registered interest in.

• The subscriber reads messages directly from the subscriber queue.

Subscribers register “subscriptions,” which means it is interested in one or more “topics.”

This example, which shows the MQ pub/sub process, uses these variables:

declare @BROKER    varchar(100)
declare @STREAM    varchar(100)
declare @SUBQ      varchar(100)
declare @QM        varchar(100)
select @QM         = 'ibm_mq:channel1/tcp/host1(9876)?qmgr=QM'
select @BROKER     = 'SYSTEM.BROKER.CONTROL.QUEUE'
select @STREAM     = 'ANIMALS'
select @SUBQ       = 'MY_ANIMALS'

1. Publisher registers to send publications to ANIMALS with topics on fish:

   select msgsend(NULL,
            @QM + ',queue=' + @BROKER
            option 'rfhCommand=registerPublisher'
            message header 'topics=fish,streamName=' + @STREAM)
   

2. Subscriber registers to receive publications published to ANIMALS with topics on fish. The subscriber receives the publications on MY_ANIMALS:

   select msgsend(NULL,
            @QM + ',queue=' + @BROKER
            option 'rfhCommand=registerSubscriber'
            message header 'topics=fish'
                            + ',streamName=' + @STREAM
                            + ',queueName=' + @SUBQ’)
   
   

3. Publisher publishes publication to ANIMALS about fish. The MQ pub/sub broker automatically forwards the publication to MY_ANIMALS:

   select msgsend('something about fish',
            @QM + ',queue=' + @STREAM
            option 'rfhCommand=publish'
            message header 'topics=fish')
   
   

4. Subscriber reads the forwarded message from MY_ANIMALS:

   select msgrecv(@QM + ',queue=' + @SUBQ option 'timeout=30ss')
   
   

A message can have one or more topics. The WebSphere MQ pub/sub model recommends that topics use a hierarchical naming convention as in the examples show below. Subscribers can specify wildcards (such as * and ?) when specifying topics of interest. These are examples of topics:

Sport
Sport/Soccer
Sport/Tennis

These are examples of how subscribers can specify topics of interest:

     Sport/*           - Any topic about sports.
     */Soccer          - Any topics about soccer.
     */Soccer/Trades   - Any topics about soccer where a 'trade' is involved.

A retained publication is a type of publication where the MQ pub/sub broker maintains a copy of a message even after it has delivered it to all subscribers. Normally, a publication is deleted after a copy has been delivered to all subscribers. A retained publication allows a subscriber to asynchronously request the retained publication instead of relying on it being delivered by the MQ pub/sub broker. These types of messages normally contain state information, and are also referred to as state publications.