[servicemix-dev] JMSFlow architecture questions

[Peter Smith]

Geez...this is probably going to cause somebody a headache. I'm sorry about that - I am just trying to understand how the architecture works This email is specifically about the JMSFlow NMR flow type. I am looking at the ServiceMix 2.0.1 JMSFlow code revision 749 which is latest AFAIK. Unfortunately I am not in a position to be running this yet, which means all this email is based just on me staring at the code for a couple of hrs... If somebody who knows the implementation details could go through this and confirm my understanding of JMSFlow  it would be a big help. Here goes.... My understanding of JMSFlow ====================== There is a main broadcast Topic and a number of Queues (container specific Queues & Queues for each component) Topic is for broadcasting changes to remote containers Also for getting advisories if remote container activated/deactivated Queue (per component) is for routing messages to... Queue (container) is also for routing messages... The JMSFlow instance seems to be the sole JMS MessageListener for all these Queues/Topics The JMSFlow is also an ActiveMQ ConsumerAdvisoryEventListener meaning it is notified of the start/stop of Consumers for the broadcast Topic - ie when some other container runs the JMSFlow init/shutdown onMessage - JMS MessageListener for Queues/Topic ObjectMessage might be a MessageExchangeImpl (for message routing) OR a ComponentPacketEvent (notifying some change in a remote component) If ComponentPacketEvent add/update/remove local knowledge of the remote component depending on the ComponentPacketEvent status If MessageExchangeImpl AbstractFlow.doRouting does message routing for the incoming message to some local component onEvent(ConsumerAdvisoryEvent event). I think this is called when some remote Consumer (ie container/JMSFlow instance) of the Topic starts/stops If remote consumer is stopping trash all knowledge of the container components for the remote container that is leaving If remote consumer is starting call onEvent(ComponentPacketEvent) for ALL of the LOCAL container components. I think the main purpose of this is to make use of the side effect to tell the remote container all the components that are available to it from this local container. Presumably it there were a dozen container then remote container 'X' is started then all the other dozen would be advised that 'X' had started and they would madly broadcast their respective components to the Topic so container 'X' would learn about them. onEvent(ComponentPacketEvent event) I think this is called when a LOCAL component is activated/deactivated Each component has a Queue associated with it and the JMSFlow instance is the message listener for this Queue [see onMessage] Activation makes queue & consumer Deactivation closes the consumer for that component Queue doSend/doRouting - routing for outgoing messages... ObjectMessage send to either Queue for container or Queue for specific component QUESTIONS ========== In the init() method, how come the connection.start is called prior to setting up the message listeners. Maybe it makes no difference - just curious. I am a bit confused by the terminology "remote" used throughout JMSFlow? Does it make any difference at all to JMSFlow if the container (ie JMSFlow instance) is on another machine versus just another container instance on the same machine. I think there is no distinction - is that true? If I understand the onEvent(ConsumerAdvisoryEvent event) method correctly then that will mean that if a lot of containers get started, then the same ComponentPacketEvents will be sent to the broadcast Topic a number of times. That would mean most of the (already started) containers will be told again and again about remote components that they already know about. Is that true? How is the addRemotePacket method is accounting for this scenario (???) The use of createQueue JMS API for the component name is something I haven't seen much before. The spec - http://java.sun.com/j2ee/1.4/docs/api/javax/jms/Session.html#createQueue(java.lang.String) - says that this doesn't actually create any physical queue, so what exactly is it doing or are all the queues already configured elsewhere? Or is this doing something that is ActiveMQ specific? If onEvent(ConsumerAdvisoryEvent event) is called whenever some remote container starts, but because it in turn calls onEvent(ComponentPacketEvent event) pretending to ACTIVATE local components won't it just end up creating the same queues for the local components over and over and over again (???) I don't really understand the distinction of routing the message to Queue (destination = INBOUND_PREFIX + componentName;) or Queue (destination = INBOUND_PREFIX + id.getContainerName();). Can you explain why the difference? It seems to be something like the "request" going to the remote container Queue, and the "response" is going to the component specific Queue - but why and what difference does it make since either way the remote MessageListener is the same isn't it? Congratulations if you managed to read this far ;-) And thanks so much for any help in my understanding of this flow type. Cheers, Peter.