On 12/10/13 at 11:09am, Flavio Percoco wrote:
On 09/12/13 17:37 -0500, Russell Bryant wrote:
On 12/09/2013 05:16 PM, Gordon Sim wrote:
On 12/09/2013 07:15 PM, Russell Bryant wrote:
[...]
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.
Indeed. We haven't tried to do that anywhere, but it would be an
improvement for some cases.
Agreed, this is something that worths experimenting.
[...]
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).
Soudns awesome. :-)
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.
I think the lack of store-and-forward is OK.
Right now, all of the Nova code is written to assume that the messaging
is unreliable and that any message could get lost. It may result in an
operation failing, but it should fail gracefully. Doing end-to-end
acknowledgement may actually be an improvement.
This is interesting and a very important point. I wonder what the
reliability expectations of other services w.r.t OpenStack messaging
are.
I agree on the fact that p2p acknowledgement could be an improvement
but I'm also wondering how this (if ever) will affect projects - in
terms of requiring changes. One of the goals of this new driver is to
not require any changes on the existing projects.
Also, a bit different but related topic, are there cases where tasks
are re-scheduled in nova? If so, what does nova do in this case? Are
those task sent back to `nova-scheduler` for re-scheduling?
Yes, there are certain build failures that can occur which will cause a
re-schedule. That's currently accomplished by the compute node sending
a message back to the scheduler so it can pick a new host. I'm trying
to shift that a bit so we're messaging the conductor rather than the
scheduler, but the basic structure of it is going to remain the same for
now.
If you mean in progress operations being restarted after a service is
restarted, then no. We're working towards making that possible but at
the moment it doesn't exist.
Cheers,
FF
--
@flaper87
Flavio Percoco
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