On 15/09/14 13:28, Clint Byrum wrote:
Excerpts from Flavio Percoco's message of 2014-09-15 00:57:05 -0700:
On 09/12/2014 07:13 PM, Clint Byrum wrote:
Excerpts from Thierry Carrez's message of 2014-09-12 02:16:42 -0700:
Clint Byrum wrote:
Excerpts from Flavio Percoco's message of 2014-09-11 04:14:30 -0700:
Is Zaqar being optimized as a *queuing* service? I'd say no. Our goal is
to optimize Zaqar for delivering messages and supporting different
messaging patterns.
Awesome! Just please don't expect people to get excited about it for
the lighter weight queueing workloads that you've claimed as use cases.
I totally see Horizon using it to keep events for users. I see Heat
using it for stack events as well. I would bet that Trove would benefit
from being able to communicate messages to users.
But I think in between Zaqar and the backends will likely be a lighter
weight queue-only service that the users can just subscribe to when they
don't want an inbox. And I think that lighter weight queue service is
far more important for OpenStack than the full blown random access
inbox.
I think the reason such a thing has not appeared is because we were all
sort of running into "but Zaqar is already incubated". Now that we've
fleshed out the difference, I think those of us that need a lightweight
multi-tenant queue service should add it to OpenStack. Separately. I hope
that doesn't offend you and the rest of the excellent Zaqar developers. It
is just a different thing.
Should we remove all the semantics that allow people to use Zaqar as a
queue service? I don't think so either. Again, the semantics are there
because Zaqar is using them to do its job. Whether other folks may/may
not use Zaqar as a queue service is out of our control.
This doesn't mean the project is broken.
No, definitely not broken. It just isn't actually necessary for many of
the stated use cases.
Clint,
If I read you correctly, you're basically saying the Zaqar is overkill
for a lot of people who only want a multi-tenant queue service. It's
doing A+B. Why does that prevent people who only need A from using it ?
Is it that it's actually not doing A well, from a user perspective ?
Like the performance sucks, or it's missing a key primitive ?
Is it that it's unnecessarily complex to deploy, from a deployer
perspective, and that something only doing A would be simpler, while
covering most of the use cases?
Is it something else ?
I want to make sure I understand your objection. In the "user
perspective" it might make sense to pursue both options as separate
projects. In the "deployer perspective" case, having a project doing A+B
and a project doing A doesn't solve anything. So this affects the
decision we have to take next Tuesday...
I believe that Zaqar does two things, inbox semantics, and queue
semantics. I believe the queueing is a side-effect of needing some kind
of queue to enable users to store and subscribe to messages in the
inbox.
What I'd rather see is an API for queueing, and an API for inboxes
which integrates well with the queueing API. For instance, if a user
says "give me an inbox" I think Zaqar should return a queue handle for
sending into the inbox the same way Nova gives you a Neutron port if
you don't give it one. You might also ask for a queue to receive push
messages from the inbox. Point being, the queues are not the inbox,
and the inbox is not the queues.
However, if I just want a queue, just give me a queue. Don't store my
messages in a randomly addressable space, and don't saddle the deployer
with the burden of such storage. Put the queue API in front of a scalable
message queue and give me a nice simple HTTP API. Users would likely be
thrilled. Heat, Nova, Ceilometer, probably Trove and Sahara, could all
make use of just this. Only Horizon seems to need a place to keep the
messages around while users inspect them.
Whether that is two projects, or one, separation between the two API's,
and thus two very different types of backends, is something I think
will lead to more deployers wanting to deploy both, so that they can
bill usage appropriately and so that their users can choose wisely.
This is one of the use-cases we designed flavors for. One of the mail
ideas behind flavors is giving the user the choice of where they want
their messages to be stored. This certainly requires the deployer to
have installed stores that are good for each job. For example, based on
the current existing drivers, a deployer could have configured a
high-throughput flavor on top of a redis node that has been configured
to perform for this job. Alongside to this flavor, the deployer could've
configured a flavor that features durability on top of mongodb or redis.
When the user creates the queue/bucket/inbox/whatever they want to put
their messages into, they'll be able to choose where those messages
should be stored into based on their needs.
I do understand your objection is not against Zaqar being able to do
this now or not but whether an integrate API for both kind of semantics
makes sense or not. I still fail to see why they need to be separated,
though. Billing can still happen properly based on the resource usage,
flavors and other metrics.
As long as it can be built into the API that certain flavors don't allow
pagination of messages, I can see it working.
If the v2 API removes random access to messages, is there any reason for
it to allow listing of messages at all? AFAICT the v2 API could
eliminate any access to or knowledge about anything but the first
unclaimed message in the queue while still satisfying all of the target
use cases.
Also maybe some flavors
are auto-ACK and thus can be backed entirely by AMQP/gearman/etc. without
any special gymnastics to support sessions.
That doesn't make a lot of sense to me - if you have an extremely high
volume of messages and you don't care about losing some, just spin up
your own AMQP broker on a Nova server. Zaqar offers guaranteed delivery
of messages, which at very small scales can only be economically
provided by the cloud operator. Users who don't care about either of
those things have a host of other good, cheap options already. If you
just want someone else to administer that for you, then you get the
Trove-style queue-provisioning service that was mooted at various points
in this thread.
However, I tend to think that if one thing is a superset of the other,
they can also be separate, and typically in a scale-out context, if
something can be separate, it should be.
AIUI the Zaqar API processes are essentially stateless and defer the
storage to some other system encapsulated behind a storage plugin. Given
that the storage parts already scale out separately, it doesn't follow
at all that they should have separate APIs for scale-out reasons.
cheers,
Zane.
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