On 12/09/2013 07:15 PM, Russell Bryant wrote:
On 12/09/2013 12:56 PM, Gordon Sim wrote:
In the case of Nova (and others that followed Nova's messaging
patterns), I firmly believe that for scaling reasons, we need to move
toward it becoming the norm to use peer-to-peer messaging for most
things. For example, the API and conductor services should be talking
directly to compute nodes instead of through a broker.
Is scale the only reason for preferring direct communication? I don't
think an intermediary based solution _necessarily_ scales less
effectively (providing it is distributed in nature, which for example is
one of the central aims of the dispatch router in Qpid).
That's not to argue that peer-to-peer shouldn't be used, just trying to
understand all the factors.
Scale is the primary one. If the intermediary based solution is easily
distributed to handle our scaling needs, that would probably be fine,
too. That just hasn't been our experience so far with both RabbitMQ and
Qpid.
Understood. The Dispatch Router was indeed created from an understanding
of the limitations and drawbacks of the 'federation' feature of qpidd
(which was the primary mechanism for scaling beyond one broker) as well
learning lessons around the difficulties of message replication and storage.
One other pattern that can benefit from intermediated message flow is in
load balancing. If the processing entities are effectively 'pulling'
messages, this can more naturally balance the load according to capacity
than when the producer of the workload is trying to determine the best
balance.
Yes, that's another factor. Today, we rely on the message broker's
behavior to equally distribute messages to a set of consumers.
Sometimes you even _want_ message distribution to be 'unequal', if the
load varies by message or the capacity by consumer. E.g. If one consumer
is particularly slow (or is given a particularly arduous task), it may
not be optimal for it to receive the same portion of subsequent messages
as other less heavily loaded or more powerful consumers.
One example is how Nova components talk to the nova-scheduler service.
All instances of the nova-scheduler service are reading off a single
'scheduler' queue, so messages hit them round-robin.
In the case of the zeromq driver, this logic is embedded in the client.
It has to know about all consumers and handles choosing where each
message goes itself. See references to the 'matchmaker' code for this.
Honestly, using a distributed more lightweight router like Dispatch
sounds *much* nicer.
The exception
to that is cases where we use a publish-subscribe model, and a broker
serves that really well. Notifications and notification consumers
(such as Ceilometer) are the prime example.
The 'fanout' RPC cast would perhaps be another?
Good point.
In Nova we have been working to get rid of the usage of this pattern.
In the latest code the only place it's used AFAIK is in some code we
expect to mark deprecated (nova-network).
Interesting. Is that because of problems in scaling the messaging
solution or for other reasons?
[...]
I'm very interested in diving deeper into how Dispatch would fit into
the various ways OpenStack is using messaging today. I'd like to get
a better handle on how the use of Dispatch as an intermediary would
scale out for a deployment that consists of 10s of thousands of
compute nodes, for example.
Is it roughly just that you can have a network of N Dispatch routers
that route messages from point A to point B, and for notifications we
would use a traditional message broker (qpidd or rabbitmq) ?
For scaling the basic idea is that not all connections are made to the
same process and therefore not all messages need to travel through a
single intermediary process.
So for N different routers, each have a portion of the total number of
publishers and consumers connected to them. Though client can
communicate even if they are not connected to the same router, each
router only needs to handle the messages sent by the publishers directly
attached, or sent to the consumer directly attached. It never needs to
see messages between publishers and consumer that are not directly attached.
To address your example, the 10s of thousands of compute nodes would be
spread across N routers. Assuming these were all interconnected, a
message from the scheduler would only travel through at most two of
these N routers (the one the scheduler was connected to and the one the
receiving compute node was connected to). No process needs to be able to
handle 10s of thousands of connections itself (as contrasted with full
direct, non-intermediated communication, where the scheduler would need
to manage connections to each of the compute nodes).
This basic pattern is the same as networks of brokers, but Dispatch
router has been designed from the start to simply focus on that problem
(and not deal with all other broker related features, such as
transactions, durability, specialised queueing etc).
The other difference is that Dispatch Router does not accept
responsibility for messages, i.e. it does not offer any
store-and-forward behaviour. Any acknowledgement is end-to-end. This
avoids it having to replicate messages. On failure they can if needed by
replayed by the original sender.
The Dispatch Router can work for pub-sub patterns as well, though not
store and forward directly. In theory, for flows where store-and-forward
is needed, that can be supplied by an additional service e.g. a more
traditional broker, which would take responsibility for replaying over
from the publisher in order that subscribers could if needed have
message replayed even after the original publisher had exited.
--Gordon.
_______________________________________________
OpenStack-dev mailing list
OpenStack-dev@lists.openstack.org
http://lists.openstack.org/cgi-bin/mailman/listinfo/openstack-dev
