Usage
In a typical message-queueing implementation, a system administrator installs and configures off-the-shelf message-queueing software (a queue manager or broker), and defines a named message queue. Or they register with a Message Queuing Service.
An application then registers a software routine that "listens" for messages placed onto the queue.
Second and subsequent applications may connect to the queue and transfer a message onto it.
The queue-manager software stores the messages until a receiving application connects and then calls the registered software routine. The receiving application then processes the message in an appropriate manner.
There are often numerous options as to the exact semantics of message passing, including:
- Durability - messages may be kept in memory, written to disk, or even committed to a DBMS if the need for reliability indicates a more resource-intensive solution.
- Security policies - which applications should have access to these messages?
- Message purging policies - queues or messages may have a Time to live
- Message filtering - some systems support filtering data so that a subscriber may only see messages matching some pre-specified criteria of interest
- Delivery policies - do we need to guarantee that a message is delivered at least once, or no more than once?
- Routing policies - in a system with many queue servers, what servers should receive a message or a queue's messages?
- Batching policies - should messages be delivered immediately? Or should the system wait a bit and try to deliver many messages at once?
- Queuing criteria - when should a message be considered "enqueued"? When one queue has it? Or when it has been forwarded to at least one remote queue? Or to all queues?
- Receipt notification - A publisher may need to know when some or all subscribers have received a message.
These are all considerations that can have substantial effects on transaction semantics, system reliability, and system efficiency.
Read more about this topic: Message Queue
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