On 10/20/2016 05:08 AM, Joshua Harlow wrote:
Matt Riedemann wrote:
There are a lot of specs up for review in ocata related to adding new
versioned notifications for operations that we didn't have notifications
on before, like CRUD operations on resources like flavors and server
groups.
We've got a lot of legacy notifications for server actions, like
server.pause.start and server.pause.end. Those are pretty simple.
The thing that has me concerned about the CRUD operation notifications
on resources is the extra DB query overhead to create the payloads which
might not even get sent out.
For example, I was reviewing this spec about adding notifications for
CRUD ops on server groups:
https://review.openstack.org/#/c/375316/
Looking at the code for InstanceGroup, when a member is added to or
removed from the group, the hosts field implicitly changes, but to
calculate the hosts field we have to get all of the instances (members)
in the group and then built the list of instance.host values.
This is probably less of an issue if the server group object in scope
already has the hosts field set, but if it doesn't and we're
constructing it just for the notification, that's extra DB and RPC
overhead - and notifications might not even be setup.
I was thinking about it like logging details at debug level. If I need
to build some large object or get some data for debugging something
that's not in scope, I'd wrap that in a conditional:
if LOG.isEnabledFor(logging.DEBUG):
LOG.debug('gimme da deets: %s', self.build_da_deets())
But do we have anything like that for notifications? Basically, tell me
if I should even bother building payloads for notifications.
Also at what point is the notification system actually the equivalent of
the database transaction log (just something to ask/think about).
At no point, really. :) One is a serialized write-ahead log of commits.
The other is an asynchronous, fire-and-forget, pub-sub system.
If every CRUD operation is doing notifications, it sorta feels like
it is nearly just the same...
No, not really. You wouldn't compare the initial creation of a Tweet
with the notifications (email, mobile app, AJAX callback, etc) about
that Tweet that get sent out to all subscribers. Same thing here.
Best,
-jay
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