more than one listener for the queue manager
Can there more than one listener to a queue manager ? I have used one listener/queue manager combination so far and wonder if this possible. This is because we have 2 applications connecting to same queue manager and seems to have problem with that.
Answer
There are a couple meanings for the term listener in an MQ context. Let's see if we can clear up some confusion over the terminology and then answer the question as it relates to each.
As defined in the spec, a JMS listener is an object that implements a callback mechanism. It listens on destinations for messages and calls onMessage when they arrive. The destinations may be queues or topics hosted by any JMS-compliant transport provider.
In IBM MQ terms, a listener is a process (runmqlsr) that handles inbound connection requests on a server. Although these can handle a variety of protocols, in practice they are almost exclusively TCP listeners that bind a port (1414 by default) and negotiate connection requests on sockets.
TCP Ports
Tim's answer applies to the second of these contexts. MQ can listen for sockets on multiple ports and indeed it is quite common to do so. Each listener listens on one and only one port. It may listen on that port across all network interfaces or can be bound to a specific network interface. No two listeners can bind to the same combination of interface and port though.
In a B2B context the best practice is to run a dedicated listener for each external business partner to isolate each of their connections across dedicated access paths. Internally I usually recommend separate ports for QMgr-to-QMgr, app-to-QMgr and interactive user connections.
In that sense it is possible to run multiple listeners on a given QMgr. Each of those listeners can accept many connections. Their job is to negotiate the connection then hand the socket off to a Message Channel Agent which talks to the QMgr on behalf of the remotely connected client or QMgr.
JMS Listeners
Based on comments, Ulab refers to JMS listeners. These objects establish a connection to a queue manager and then wait in GET mode for new messages arriving on a destination. On arrival of a message, they call the onMessage method which is an asynchronous callback routine.
As to the question "can there more than one (JMS) listener to a queue manager?" the answer is definitely yes. A multi-threaded application can have multiple listeners connected, multiple application instances can connect at the same time, and many thousands of application connections can be handled by a single queue manager with sufficient memory, disk and CPU available.
Of course, each of these applications is ultimately connected to one or more queues so then the question becomes one of whether they can connect to the same queue.
- Many listeners can listen on the same queue so long as they do not get exclusive access to it. Each will receive a portion of the messages arriving.
- Listeners on QMgr-managed subscriptions are exclusively attached to a dynamic queue but multiple instances on the same topic will all receive the same messages.
- If the queue is clustered and there is more than one instance of it multiple listeners will be required to get all the messages since they will normally be distributed by MQ workload distribution across those instances.