On 12/20/2013 11:18 AM, Herndon, John Luke wrote:
On Dec 20, 2013, at 10:47 AM, Julien Danjou <jul...@danjou.info> wrote:

On Fri, Dec 20 2013, Herndon, John Luke wrote:

Yeah, I like this idea. As far as I can tell, AMQP doesn't support grabbing
more than a single message at a time, but we could definitely have the
broker store up the batch before sending it along. Other protocols may
support bulk consumption. My one concern with this approach is error
handling. Currently the executors treat each notification individually. So
let's say the broker hands 100 messages at a time. When client is done
processing the messages, the broker needs to know if message 25 had an error
or not. We would somehow need to communicate back to the broker which
messages failed. I think this may take some refactoring of
executors/dispatchers. What do you think?
Yeah, it definitely needs to change the messaging API a bit to handle
such a case. But in the end that will be a good thing to support such a
case, it being natively supported by the broker or not.

For brokers where it's not possible, it may be simple enough to have a
"get_one_notification_nb()" method that would either return a
notification or None if there's none to read, and would that
consequently have to be _non-blocking_.

So if the transport is smart we write:

  # Return up to max_number_of_notifications_to_read
  notifications =
      transport.get_notificatations(conf.max_number_of_notifications_to_read)
  storage.record(notifications)

Otherwise we do:

  for i in range(conf.max_number_of_notifications_to_read):
      notification = transport.get_one_notification_nb():
      if notification:
          notifications.append(notification)
      else:
          break
   storage.record(notifications)

So it's just about having the right primitive in oslo.messaging, we can
then build on top of that wherever that is.

I think this will work. I was considering putting in a timeout so the broker 
would not send off all of the messages immediately, and implement using 
blocking calls. If the consumer consumes faster than the publishers are 
publishing, this just becomes single-notification batches. So it may be 
beneficial to wait for more messages to arrive before sending off the batch. If 
the batch is full before the timeout is reached, then the batch would be sent 
off.

--
Julien Danjou
/* Free Software hacker * independent consultant
   http://julien.danjou.info */
-----------------
John Herndon
HP Cloud
john.hern...@hp.com





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A couple of things that I think need to be emphasized here:
1. the mechanism needs to be configurable, so if you are more worried about reliability than performance you would be able to turn off bulk loading 2. the caching size should also be configurable, so that we can limit your exposure to lost messages 3. while you can have the message queue hold the messages until you acknowledge them, it seems like this adds a lot of complexity to the interaction. you will need to be able to propagate this information all the way back from the storage driver. 4. any integration that is depdendent on a specific configuration on the rabbit server is brittle, since we have seen a lot of variation between services on this. I would prefer to control the behavior on the collection side.

So in general, I would prefer a mechanism that pulls the data in a default manner, caches on the collection side based on configuration that allows you to determine your own risk level and then manager retries in the storage driver or at the cache controller level.

Dan Dyer
HP cloud
dan.d...@hp.com

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