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- Messages are persistent on the queue, meaning that if a listener looses connection or is restarted, it can pick up hte messages when it comes back.
- Only one listener will receive a message if there are multiple listeners.
- Queue are globally created and managed
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Durable topics
Properties:
- Multiple listeners can all receive a message, like a topic
- Messages are persistent, that is they are remembered until all known listeners have acknowledged them, even if the listener is not currently available. (Danger of flooding the queue if a listener never returns to acknowledge messages)
- Persistent Durable topics are globally created and managed
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LISTENERS: One specific pillar
MESSAGES: <Op>Request
NAMING: "<sla-id>.<pillar-id>"
DISCUSSION: This topic is the interface for operations on a pillar.
IMPLEMENTATION: A pillar in the protocol is given it's id in configuration, and for each SLA listeners connect to this topic.
GENERATION: These topics are generated for each pillar by the message bus administrator when a new SLA is added.
ALTERNATIVES: It is protocol-safe to make a different choice for making pillar destinations on a per-pillar basis. This can be replaced with a queue or a temporary queue, and the naming is quite optional. A topic seems appropriate, since it will not have persistent messages lying around if the pillar is not available at the time of request. However, there could also be advantages of a queue, since you could replicate you pillar instances and only one would receive the message. It may be considered desirable to use temporary queues, so the queue name will never be reused in a later operation.
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Durable Topics
Persistent Durable topics are used for following up on operations. (Operations ending in ...Response or ...Complete)
Individual for one client. It seems appropriate for followup operations that they are persistent, so they are picked up when the listener regains contact to the JMS server.
The dedicated client
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durable topic
LISTENERS: One specific client
MESSAGE: IdentifyPillarFor<Op>Response, <Op>Response, <Op>Complete
NAMING: "<sla-id>.client"
DISCUSSION: This persistent durable topic is intended to receive replies to requests sent to the clients.
IMPLEMENTATION: A client in the protocol is given the SLA id in it's configuration, and each listener connects to this persistent durable topic.
GENERATION: This persistent durable topic is generated for each SLA by the message bus administrator when a new SLA is added.
ALTERNATIVES: It is protocol-safe to make a different choice for making client destinations on a per-client basis. This can be replaced with a queue or a temporary queue, and the naming is quite optional. A persistent destination seems appropriate, since it will ensure that replies are received and processed, even if the listener is not available when the message was sent. However, it may be considered desirable to use temporary queues, so the queue name will never be reused in a later operation.
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Alarms are probably best modelled in separate destinations. Currently it is expected that two destinations for alarms are reasonable:
The general alarm
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durable topic
LISTENERS: An alarm service
MESSAGE: Alarms
NAMING: "ALARMS"
DISCUSSION: This topic is intended to receive alarms that are general for the system.
IMPLEMENTATION: All participants in the protocol are given the topic name ("ALARMS") in configuration, and send general alarms to this topic. The ALARM service also listens to this topic.
GENERATION: This topic is generated by the message bus administrator when first setting up the system.
ALTERNATIVES: The GENERAL topic could be used instead, or all known SLA topic destinations.
The SLA alarm
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durable topic
LISTENERS: One specific client
MESSAGE: Alarms
NAMING: "<sla-id>.alarms"
DISCUSSION: This queue is intended to receive alarms that are reported during a SLA-specific conversation.
IMPLEMENTATION: All participants in the protocol are given the SLA id in it's configuration, and send SLA-specific alarms to this topic.
GENERATION: This persistent durable topic is generated for each SLA by the message bus administrator when a new SLA is added.
ALTERNATIVES: The SLA topic could be used instead.