...
- 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
...
Concrete destinations in a Collection
The following sections documents the destinations used in the reference system together with alternatives. Note all destinations in the reference system are local to a given collection, but the protocol supports cross-collection destinations.
The Collection topic
LISTENERS: Every component participating in a Collection.
MESSAGES: IdentifyPillarFor<Op>Request, other messages MAY be sent on this topic.
NAMING: "type:<collection-id>"
DISCUSSION: This topic is the abstraction layer, that makes it possible to use the protocol with no knowledge of other participants. The destination is used to send the initial 'Identify...' requests used to start a conversation. See Message flow documentation for details.
IMPLEMENTATION: The topic is defined in the Collection settings.
GENERATION ALTERNATIVES: This topic is generated by the message bus administrator when a new Collection is setupThe collection topic might be used for other message types. It might in fact be used for all the messages in a collection, if the separation recommended here isn't necessary.
The per-pillar topic
...
LISTENERS: One specific pillar
MESSAGES: <Op>Request
NAMING: "type:<collection-id>.<pillar-id>"
DISCUSSION: This topic is the interface for requesting operations on a pillar.
IMPLEMENTATION: Pillar topics are identified through the IdentifyPillarFor<Op>Responses returned as a result of a IdentifyPillarFor<Op>Request.
GENERATION: These topics are generated for each pillar by the message bus administrator when a new Collection is setup.
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 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 per-client temporary queue
LISTENERS: Clients
MESSAGE: IdentifyPillarFor<Op>Response, <Op>Response, <Op>Complete
NAMING: "type:<collection-id>.client-name.unique-client-identifier"
DISCUSSION: This durable topic queue 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 addedThe reference maintains a conversation state while running, which isen't shared between client, nor persisted. This means replies sent to other clients or previous instances of a client are irrelevant. Thus a client is only interested in replies to it's own responses.
IMPLEMENTATION: A client initiates a temporary queue on startup with a unique identifier.
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(durable shared) topic, 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 operationIn case of stateless client, clients with persisted state or shared state this might be relevant.
Alarms
Alarms are probably best modelled in a separate destination.
...
LISTENERS: One specific client
MESSAGE: Alarms
NAMING: "type:<collection-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 alarm destination 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 Collection topic could be used instead.