Folks,
Recently I submitted a fix on trunk that addressed a problem where QMF v2
object update notifications were being dropped by the python QMF console
(qmf.console). The fix merely re-enabled the update notifications.
As pointed out by both Fraser Adams and Bill, this may cause problems with
existing clients. As described below (and elsewhere in this thread), now both
QMFv1 updates and QMFv2 updates will be delivered via the qmf.console.Console
callback interface.
Sadly, the way this is done is confusing at best. For V1 updates, there are
separate callbacks for statistics updates (via objectStats()) and property
updates (via objectProps). The confusion comes from the fact that V2 handles
these updates in a different way: it is optimized to generate only one update
that combines both statistics and properties. However that update arrives via
the objectProps() callback. For QMFv2, the objectStats() callback is ignored.
This forces the application's objectProps to have to deal with different update
formats - v1 and v2 - and handle the different use cases (updating statistics
in the v2 case, not for v1).
AFAIK - this bug has prevented v2 updates from arriving on that objectProps
callback since prior to 0.18. Its likely that apps written using the console
haven't ever received v2 style updates.
I propose we fix this in the manner suggested by Bill and Fraser. Specifically:
1) Add a new interface to the qmf.console.Console object:
def objectUpdate( self, broker, record )
This callback will be invoked only for QMFv2 object updates. The parameters
are analogous to objectProps and objectStats
2) The existing objectProps and objectStats callbacks are invoked only for the
v1 style updates, as was their behavior prior to my fix. v2 object updates
will never arrive on these callbacks.
Any objections?
----- Original Message -----
> From: "Bill Freeman" <[email protected]>
> To: [email protected]
> Sent: Friday, April 5, 2013 4:20:37 PM
> Subject: Re: Questions from a novice
>
> On Fri, Apr 5, 2013 at 3:38 PM, Ken Giusti <[email protected]> wrote:
>
> > ----- Original Message -----
> > > From: "Bill Freeman" <[email protected]>
> >
> > QMF V2 has been available for some time now, and we've cut all the project
> > tools and libraries over to speaking it. If we start down the road to
> > deprecating V1, turning off the broker updates is about the least risky
> > first step I can think of. It would not remove v1 at all - but it would
> > require manual intervention to restore the old behavior. Having to
> > explicitly take that action would be a (hopefully gentle) nudge to assess a
> > deployment's V1 dependency.
> >
> >
> > I think the on line documentation may have something to answer for here.
> When I started this project, since we are using MRG, I started with
> RedHat's documentation, but didn't find any advice about coding QMF. Then
> Google got me to the Apache Qpid pages, link to QMF, and eventually
> dribbled down to an example that became the base of my current design.
> That was based on qmf.console. There was no insight there about selecting
> V1 versus V2. There were also several pages, at least, that seemed to
> imply that V1 was now, and everything would support it, while V2 was new
> and for the future, and not universally supported (and that turned out to
> be correct until your patch).
>
> I think that the concept that consoles, for some time yet to come, should
> endeavor to support both versions, is a good one. They should perhaps log
> a deprecation warning if they discover a V1 only broker (aren't V2 brokers
> supposed to translate for V1 only agents?), but they should work anyway.
>
> One thing that makes it tough for consoles to do so, at least if they are
> based on qmf.console, is the fact that the objectProps callback is shared
> between versions, but has a different meaning. One callback carrying
> deltas is a good design, but it seems that either it should use a
> completely separate callback, or it should call both objectProps and
> objectStats (in that order), omitting objectProps if it has no properties,
> and omitting objectStats if it has no statistics. A V2 specific callback
> would be my preference, but only by a little. Since the other stuff has
> never been called for V2 updates before, this is the time to make that
> change.
>
> And I guess that's also a potential problem with making the fix from your
> patch: Code that has been working for years using qmf.console has never
> received a V2 objectProps call before may not work with it (mine certainly
> didn't). A separate callback for V2 updates fixes this issue.
>
> Bill
>
--
-K
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