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Re: [Pulp-list] Messaging Questions

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Sorry I'm so late getting back to this.

>> Synchronous messages will fail immediately if the agent is unavailable
>> so let's assume for the moment, we're only talking about asynchronous
>> messages (RMI). 

Makes sense.

>>I believe that all asynchronous messages to the agent
>> should be dispatched through the Tasking framework. That way *all*
>> policy around asynchronous operations will be in one place.  

So in this rationale, all message bus invocations are synchronous, they
just get their asynchronous-ness from our task framework?

I like that, it limits the amount of places we need to address these
ugly cases.

>> The
>> lifecycle of the asynchronous message should be tied to (and implemented
>> by) the Task.  So long as the task lives, the message should also live. 
>> If the task times out, the message should be dequeued.

Does our tasking framework support time outs yet?

>>  So, the
>> messaging framework need to support message dequeuing.  It can do this
>> by sending a cancellation message with higher priority if dequeuing not
>> directly supported by qpid.
>> This leaves orphaned queues for consumer un-registration.  Seems like
>> the ConsumerApi could be responsible for this by doing something like:
> from pulp.agent import Agent
> Agent.purge('foo')
>> which would remove the associated queue.

That covers the case where an agent knowingly is going away, but what
about when the consumer just full on disappears? For instance, the box
is reprovisioned, goes up in flames, or whatever other reason and the
admin doesn't think to unregister it?

I think we still need some sort of reaper/ping on agents to make sure
they are still alive. That'll get us into the questions on what happens
if an agent is temporarily down, but I think those are better than the
alternative of dead queues floating around.

>> The messaging framework (pmf) ensures that messages are processed
>> (dispatched) before they are acknowledged (taken from the queue).  This
>> prevents against cases where the agent consumes a message then dies and
>> thus never processes it.  Due to guaranteed message delivery, the agent
>> will always reply unless it's dead.  In which case, see above.

I see what you're saying, but I'm thinking of a different case. Maybe
I'm viewing this wrong. I thought the flow looked like:

- - Server sends message to agent
- - Agent acknowledges and says it'll start processing the request
- - Server makes note somewhere that the requested action is "in progress"
- - Later, when it's finished, the agent sends a message to the server
that the operation has completed and its status. Looking at the wiki,
this looks like its sent to the server queue.

If that's the case, then my question is about what happens when that
last bullet point doesn't happen (for instance, zombie attack caused the
power to go out and the machine died). Won't there still be something in
the server that says "I sent a message to the agent that was accepted,
but he never sent me a message back. I'm sad."

If that's not the case, can you clear up how that flow looks for me?

>> Yes.  All requests (messages) have unique serial numbers which are
>> placed in the reply and matched by the message framework.  Agent B, will
>> never see request 1234.  This behaviour is standardized and enforced by
>> the messaging framework.

I'm gonna punt on my follow up question until I'm clear on the above
flow so I don't make us discuss something that's potentially not relevant.

>> Assuming that it cannot re-register with the same ID, it would be
>> considered a new consumer.   The previous registration, will orphan many
>> resources in pulp - including the queue.  Orphans need to be addressed
>> across the board.  See comment above for queue clean up.
>> If not, when does that queue get deleted? What happens if that
> re-registration happens while the agent is doing a task before it
> replies, will it confuse the server that the reply came from a
> "different" consumer?
> - Are replies back to the server guaranteed delivery as well?
>> Yes.
>> I'm
> thinking of the situation where the server is offline when the agent
> finishes doing its business.
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Jason Dobies
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