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 <http://www.stackops.com/>*CEO* *www.stackops.com | * diego.parri...@stackops.com** | +34 649 94 43 29 | skype:diegoparrilla* * <http://www.stackops.com/> * * On Wed, Oct 10, 2012 at 9:27 AM, Day, Phil <philip....@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....@lists.launchpad.net [mailto: > openstack-bounces+philip.day=hp....@lists.launchpad.net] On Behalf Of > David Ripton > Sent: 09 October 2012 20:59 > To: openstack@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 drip...@redhat.com > > _______________________________________________ > Mailing list: https://launchpad.net/~openstack > Post to : openstack@lists.launchpad.net > Unsubscribe : https://launchpad.net/~openstack > More help : https://help.launchpad.net/ListHelp > > _______________________________________________ > Mailing list: https://launchpad.net/~openstack > Post to : openstack@lists.launchpad.net > Unsubscribe : https://launchpad.net/~openstack > More help : https://help.launchpad.net/ListHelp >
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