openstack team mailing list archive

["Day, Phil" <philip.day@xxxxxx>]

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@xxxxxxxxxxxxxxxxxxx [mailto:openstack-bounces+philip.day=hp.com@xxxxxxxxxxxxxxxxxxx] On Behalf Of David Ripton
Sent: 09 October 2012 20:59
To: openstack@xxxxxxxxxxxxxxxxxxx
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@xxxxxxxxxx

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