Open Stack

On Mon, 2013-04-29 at 09:39 -0400, Doug Hellmann wrote:
> 
> 
> 
> On Mon, Apr 29, 2013 at 7:00 AM, Mark McLoughlin <markmc at redhat.com> wrote:
>         On Fri, 2013-04-26 at 15:18 -0400, Doug Hellmann wrote:
>         
>         > We've gone around a few times with ideas for having better driver-parity in
>         > the rpc library, so maybe the best thing to do is start by making sure we
>         > have all of the requirements lined up. Here's a list of some that I came up
>         > with based on existing features and my understanding of the shortcomings
>         > (numbered for reference, but in no particular order):
>         
>         
>         Thanks for doing this. We definitely need to be stepping back and
>         thinking about this at a high level. I've attempted to step a little
>         further back in my writeup:
>         
>           https://wiki.openstack.org/wiki/Oslo/Messaging
>         
>         > 1. Application code using RPC connections must not be required to know to
>         > pass different arguments to establish the connection (i.e., the app
>         > shouldn't have to care if it is using Rabbit or ZMQ).
>         
>         
>         Yes.
>         
>         > 2. An application must be able to direct a message to a specific peer
>         > (i.e., call() with a host specified).
>         
>         
>         s/direct a message to/invoke a method on/
> 
> 
> RPC and notification aren't the only usage patterns for messaging. If
> we're going to design a general purpose library, we should not limit
> ourselves to RPC semantics. RPC can be built on top of messaging, but
> it has been painful to try to make it work the other way around.

I don't think the goal is to design a general purpose library, it's to
clean up the APIs we already have to support our current usage patterns.
The library can grow new APIs to support new usage patterns over time.

I really don't want to design generic APIs in a vacuum when we have the
much more pressing concern of our current usage patterns. I also don't
want to unnecessarily restrict what messaging systems could be used to
support our current patterns.

>         But, yes.
>         
>         > 3. An application must be able to direct a message to a pool of peers
>         > (i.e., cast()).
>         
>         
>         ... and for it to be delivered to one of those peers, yes.
>         
>         > 4. An application must be able to direct a message to a specific peer (or
>         > unknown? or both?) using a different rpc endpoint than the originating
>         > app's default (i.e., calling into cells).
>         
>         
>         I've tried to separate the notion of a transport from the target. The
>         properties of the target is what's known to the application code, the
>         properties of the transport are target specific and come from
>         configuration.
>         
>         So, I'd say "an application must be able to invoke a method on a
>         specific target using a supplied transport configuration".
> 
> 
> Something has to know how to map the target to the configuration. What
> does that, and how much does that code know about the transport?

Ok, on the client side:

  https://wiki.openstack.org/wiki/Oslo/Messaging#Client_Side_API

for the simple case, you'd have something like:

  rpc_driver = rpc.get_transport_driver()

  base_rpcapi = BaseAPIClient(rpc_driver)

  base_rpcapi.ping(context, 'foo')

for the more complex case, you'd have:

  class MeteringAPIClient(rpc.RPCClient):

      target = rpc.Target(exchange='ceilometer',
                          topic='metering',
                          version='1.0')

      def __init__(self, driver):
          # FIXME: need some way to override with exchange from URL
  ---
          
  rpc_driver = rpc.get_transport_driver(url='kombu://broker//ceilometer')

  metering_rpcapi = MeteringAPIClient(rpc_driver)
  metering_rpc_api.record_metering_data(...)

The annoying bit here is the application code should know what the
default exchange is, but there is a use case for it to be overridden by
configuration.

>         > 5. An application must be able to listen for messages without interfering
>         > with others receiving those same messages (i.e., join_consumer_pool()).
>         
>         
>         For notifications, yes - and we should have an API for consuming
>         notifications.
>         
>         But for RPC (i.e. create_worker()), I don't really see it. See here:
>         
>           https://wiki.openstack.org/wiki/Oslo/Messaging#Ceilometer_Metering_Messages
>         
>         Should ceilometer be using notifications instead of record_metering_data()
> 
> 
> Probably.

Hmm, ok :)

Cheers,
Mark.