Messages are sent over the message bus. For optimal performance, messages may be sent over dedicated queues and topics. This page describes a suggested use of queues and topics.
Types of destinations
The destinations are split into different categories, inspired by Java JMS. There are four types of possible JMS destinations:
Topics
Properties:
- Messages are not persistent on a topic, messages will only be received if someone listens at the time of the message.
- Topics can have multiple listeners that will receive a message.
Queues
Properties:
- 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.
Persistent 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. (Danger of flooding the queue if a listener never returns to acknowledge messages)
Temporary queues
Properties:
- Only one listener receives the message, like a queue
- Messages are not persistent.
- Queue is generated by a client.
Topics
Topics are used for initiating operations (Operations ending in ...Request). This seems appropriate for initiating an operation. Topic semantics are required for the Identify...Request operations. Followup messages MAY be sent to a topic, but in this case replies are not persistent, so queues are suggested.
The General Topic
LISTENERS: Everybody.
MESSAGES: None defined, but all can be sent to this topic
NAMING: "GENERAL"
DISCUSSION: This topic is a fallback, if for any reason the proper communication channels are not available.
IMPLEMENTATION: A participant in the protocol is given the topic name ("GENERAL") in configuration, and all listeners connect to this topic.
GENERATION: This topic is generated by the message bus administrator when first setting up the system.
The per-SLA topic
LISTENERS: Every client or pillar recognising this SLA
MESSAGES: IdentifyPillarFor<Op>Request, probably alarms, other messages MAY be sent on this topic
NAMING: "<sla-id>"
DISCUSSION: This topic is the abstraction layer, that makes it possible to use the protocol with no knowledge of other players.
IMPLEMENTATION: A participant in the protocol is given a list of all SLA's it should recognise in configuration, and all listeners connect to this topic.
GENERATION: This topic is generated by the message bus administrator when a new SLA is added.
The per-pillar topic
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.
Queues
Queues are used for following up on operations. (Operations ending in ...Response or ...Complete)
Individual for one client/pillar. Only one listener should be allowed per queue. It seems appropriate for followup operations that they are picked up when the listener regains contact to the JMS server.
The dedicated client queue:
LISTENERS: One specific client
MESSAGE: IdentifyPillarFor<Op>Response, <Op>Response, <Op>Complete
NAMING: "<sla-id>.<client-id>"
DISCUSSION: This queue is intended to receive replies to requests sent to the pillars.
IMPLEMENTATION: A client in the protocol is given it's id in configuration, and for each SLA listeners connect to this queue.
GENERATION: These queues are generated for each client 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 persistent topic or a temporary queue, and the naming is quite optional. A queue seems appropriate, since it will ensure that replies are received and processed. However, it might be reasonable to choose a persistent topic, to allow multiple listeners, or it may be considered desirable to use temporary queues, so the queue name will never be reused in a later operation.