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.
- Topic are globally created and managed
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.
- Queue are globally created and managed
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)
- Durable topics are globally created and managed
Temporary queues
Properties:
- Only one listener receives the message, like a queue
- Messages are not persistent (since the queue disappears with the client).
- 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 per-SLA topic
LISTENERS: Every client or pillar recognising this SLA
MESSAGES: IdentifyPillarFor<Op>Request, 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 (note: See "ALTERNATIVES" for other choices than a 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.
Durable Topics
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 durable topic
LISTENERS: One specific client
MESSAGE: IdentifyPillarFor<Op>Response, <Op>Response, <Op>Complete
NAMING: "<sla-id>.client"
DISCUSSION: This 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 durable topic.
GENERATION: This 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.
Alarms
Alarms are probably best modelled in separate destinations. Currently it is expected that two destinations for alarms are reasonable:
The general alarm 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 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 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.
Other proposed destinations
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. Currently, we try to do without it, since it seems a dangerous practice to communicate on a shared channel that everybody listens to.
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.