Open Stack

If we want to have a notification system that could handle messages with
different payloads and different versions, we have two options:

1) detect the version of the payload in the notification message
2) add a version number in the notification message

Option 1 sounds to me like something hard to maintain. Option 2 seems to be
correct way to do it in the long term.

+1 for a version number in the notification message

Cheers
Diego
 --
Diego Parrilla
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*www.stackops.com | * diego.parrilla at stackops.com** | +34 649 94 43 29 |
skype:diegoparrilla*
* <http://www.stackops.com/>
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On Wed, Oct 10, 2012 at 9:27 AM, Day, Phil <philip.day at hp.com> wrote:

> Hi All,
>
> I guess I may have mis-stated the problem a tad in talking about version
> numbering.  The notification system is an outbound interface, and my
> interest is in being able to write consumers with some guarantee that they
> won't be broken as the notification message format evolves.
>
> Having a version number gives the client a way to know that it may now be
> broken, but that's not really the same as having an interface with some
> degree of guaranteed compatibility,
>
> Phil
>
> -----Original Message-----
> From: openstack-bounces+philip.day=hp.com at lists.launchpad.net [mailto:
> openstack-bounces+philip.day=hp.com at lists.launchpad.net] On Behalf Of
> David Ripton
> Sent: 09 October 2012 20:59
> To: openstack at lists.launchpad.net
> Subject: Re: [Openstack] Versioning for notification messages
>
> On 10/09/2012 01:07 PM, Day, Phil wrote:
>
> > What do people think about adding a version number to the notification
> > systems, so that consumers of notification messages are protected to
> > some extent from changes in the message contents ?
> >
> > For example, would it be enough to add a version number to the
> > messages - or should we have the version number as part of the topic
> > itself (so that the notification system can provide both a 1.0 and 1.1
> feed), etc ?
>
> Putting a version number in the messages is easy, and should work fine.
>   Of course it only really helps if someone writes clients that can deal
> with multiple versions, or at least give helpful error messages when they
> get an unexpected version.
>
> I think using separate topics for each version would be inefficient and
> error-prone.
>
> Inefficient because you'd have to send out multiples of each message, some
> of which would probably never be read.  Obviously, if you're sending out N
> copies of each message then you expect only 1/N the queue performance.
>  Worse, if you're sending out N copies of each message but only 1 of them
> is being consumed, your queue server is using a lot more memory than it
> needs to, to hold onto old messages that nobody needs.
> (If you properly configure a high-water mark or timeout, then the old
> messages will eventually be thrown away.  If you don't, then your queue
> server will eventually consume way too much memory and start swapping, your
> cloud will break, and someone will get paged at 2 a.m.)
>
> Error-prone because someone would end up reusing the notification queue
> code for less idempotent/safe uses of queues, like internal API calls.
> And then client A would pick up the message from topic_v1, and client B
> would pick up the same message from topic_v2, and they'd both perform the
> same API operation, resulting in wasted resources in the best case and data
> corruption in the worst case.
>
> --
> David Ripton   Red Hat   dripton at redhat.com
>
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