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Excerpts from Joshua Harlow's message of 2016-06-03 09:14:05 -0700:
> Deja, Dawid wrote:
> > On Thu, 2016-05-05 at 11:08 +0700, Renat Akhmerov wrote:
> >>
> >>> On 05 May 2016, at 01:49, Mehdi Abaakouk <sileht at sileht.net
> >>> <mailto:sileht at sileht.net>> wrote:
> >>>
> >>>
> >>> Le 2016-05-04 10:04, Renat Akhmerov a écrit :
> >>>> No problem. Let’s not call it RPC (btw, I completely agree with that).
> >>>> But it’s one of the messaging patterns and hence should be under
> >>>> oslo.messaging I guess, no?
> >>>
> >>> Yes and no, we currently have two APIs (rpc and notification). And
> >>> personally I regret to have the notification part in oslo.messaging.
> >>>
> >>> RPC and Notification are different beasts, and both are today limited
> >>> in terms of feature because they share the same driver implementation.
> >>>
> >>> Our RPC errors handling is really poor, for example Nova just put
> >>> instance in ERROR when something bad occurs in oslo.messaging layer.
> >>> This enforces deployer/user to fix the issue manually.
> >>>
> >>> Our Notification system doesn't allow fine grain routing of message,
> >>> everything goes into one configured topic/queue.
> >>>
> >>> And now we want to add a new one... I'm not against this idea,
> >>> but I'm not a huge fan.
> >>>
> >>>>>>> Thoughts from folks (mistral and oslo)?
> >>>>> Also, I was not at the Summit, should I conclude the Tooz+taskflow
> >>>>> approach (that ensure the idempotent of the application within the
> >>>>> library API) have not been accepted by mistral folks ?
> >>>> Speaking about idempotency, IMO it’s not a central question that we
> >>>> should be discussing here. Mistral users should have a choice: if they
> >>>> manage to make their actions idempotent it’s excellent, in many cases
> >>>> idempotency is certainly possible, btw. If no, then they know about
> >>>> potential consequences.
> >>>
> >>> You shouldn't mix the idempotency of the user task and the idempotency
> >>> of a Mistral action (that will at the end run the user task).
> >>> You can have your Mistral task runner implementation idempotent and just
> >>> make the workflow to use configurable in case the user task is
> >>> interrupted or badly finished even if the user task is idempotent or not.
> >>> This makes the thing very predictable. You will know for example:
> >>> * if the user task has started or not,
> >>> * if the error is due to a node power cut when the user task runs,
> >>> * if you can safely retry a not idempotent user task on an other node,
> >>> * you will not be impacted by rabbitmq restart or TCP connection issues,
> >>> * ...
> >>>
> >>> With the oslo.messaging approach, everything will just end up in a
> >>> generic MessageTimeout error.
> >>>
> >>> The RPC API already have this kind of issue. Applications have
> >>> unfortunately
> >>> dealt with that (and I think they want something better now).
> >>> I'm just not convinced we should add a new "working queue" API in
> >>> oslo.messaging for tasks scheduling that have the same issue we already
> >>> have with RPC.
> >>>
> >>> Anyway, that's your choice, if you want rely on this poor structure,
> >>> I will
> >>> not be against, I'm not involved in Mistral. I just want everybody is
> >>> aware
> >>> of this.
> >>>
> >>>> And even in this case there’s usually a number
> >>>> of measures that can be taken to mitigate those consequences (reruning
> >>>> workflows from certain points after manually fixing problems, rollback
> >>>> scenarios etc.).
> >>>
> >>> taskflow allows to describe and automate this kind of workflow really
> >>> easily.
> >>>
> >>>> What I’m saying is: let’s not make that crucial decision now about
> >>>> what a messaging framework should support or not, let’s make it more
> >>>> flexible to account for variety of different usage scenarios.
> >>>
> >>> I think the confusion is in the "messaging" keyword, currently
> >>> oslo.messaging
> >>> is a "RPC" framework and a "Notification" framework on top of 'messaging'
> >>> frameworks.
> >>>
> >>> Messaging framework we uses are 'kombu', 'pika', 'zmq' and 'pingus'.
> >>>
> >>>> It’s normal for frameworks to give more rather than less.
> >>>
> >>> I disagree, here we mix different concepts into one library, all concepts
> >>> have to be implemented by different 'messaging framework',
> >>> So we fortunately give less to make thing just works in the same way
> >>> with all
> >>> drivers for all APIs.
> >>>
> >>>> One more thing, at the summit we were discussing the possibility to
> >>>> define at-most-once/at-least-once individually for Mistral tasks. This
> >>>> is demanded because there cases where we need to do it, advanced users
> >>>> may choose one or another depending on a task/action semantics.
> >>>> However, it won’t be possible to implement w/o changes in the
> >>>> underlying messaging framework.
> >>>
> >>> If we goes that way, oslo.messaging users and Mistral users have to
> >>> be aware
> >>> that their job/task/action/whatever will perhaps not be called
> >>> (at-most-once)
> >>> or perhaps called twice (at-least-once).
> >>>
> >>> The oslo.messaging/Mistral API and docs must be clear about this
> >>> behavior to
> >>> not having bugs open against oslo.messaging because script written
> >>> via Mistral
> >>> API is not executed as expected "sometimes".
> >>> "sometimes" == when deployers have trouble with its rabbitmq (or
> >>> whatever)
> >>> broker and even just when a deployer restart a broker node or when a TCP
> >>> issue occurs. At this end the backtrace in theses cases always trows only
> >>> oslo.messaging trace (the well known MessageTimeout...).
> >>>
> >>>
> >>> Also oslo.messaging is already a fragile brick used by everybody that
> >>> a very small subset of people maintain (thanks to them).
> >>>
> >>> I'm afraid that adding such new API will increase the needed
> >>> maintenance for this lib while currently not many people care about
> >>> (the whole lib not the new API).
> >>>
> >>> I also wonder if other project have the same needs (that always help
> >>> to design a new API).
> >>
> >> Mehdi,
> >>
> >> What are you proposing? Can you confirm that we should be just dealing
> >> with this problem on our own in Mistral? If so, that works well for
> >> us. Initially we didn’t want to switch to oslo.messaging from direct
> >> access to RabbitMQ for this and also other reasons. But we got a
> >> strong feedback from the community that said “you guys need to reuse
> >> technologies from the community and hence switch to oslo.messaging”.
> >> So we did, assuming that we would fix all needed issues in
> >> oslo.messaging relatively soon. Now it’s been ~2 years since then and
> >> we keep struggling with all that stuff.
> >>
> >> When I see these discussions again and again where people try to
> >> convince that at-least-one delivery is a bad thing I can’t participate
> >> in them anymore. We spent a lot of time thinking about it and
> >> experimenting with it and know all pros and cons.
> >>
> >> Renat Akhmerov
> >> @Nokia
> >
> > Maybe this could be resolved in oslo.messaging by following one of
> > Python slogans /we are all responsible users here/ [1].
> >
> > What I'm proposing is to let the consumer of the message decide when to
> > send ACK, because it knows best when to do so. I can think of scenarios
> > when it is required to send ACK in a middle of message process e.g.
> > after receiving message I want to store it in the DB before sending an
> > ACK and send it when message is safely stored. Having that we could
> > implement whatever delivery model we want in mistral on top of
> > oslo.messaging.
> 
>  From my understanding (and some of the oslo.messaging folks can correct 
> me if I am wrong); but they (the oslo.messaging maintainers) don't feel 
> comfortable allowing such a option to be made possible because of how 
> doing such a thing alters the principles of oslo.messaging and increases 
> the complexity of the code-base (and subsequent testing, bug reports, 
> feature support that come along with enabling such a thing).
> 
> Thus why I think the preference was to have this model (which isn't 
> really the `rpc` kind of model that oslo.messaging has been targeting at 
> that point, but is more like a work-queue) be in another library with a 
> clear API that explicitly is targeted at this kind of model. Thus 
> instead of having a multi-personality codebase with hidden features like 
> this (say in oslo.messaging) instead it gets its own codebase and API 
> that is 'just right' (or more close to being 'right') for it's concept 
> (vs trying to stuff it into oslo.messaging).

What happened to the idea of adding new functions at the level of the
call & cast functions we have now, that work with at-least-once instead
of at-most-once semantics? Yes this is a different sort of use case, but
it's still "messaging".

Doug

> 
> >
> > [1] https://en.wikipedia.org/wiki/Python_syntax_and_semantics#Objects
> >
> > Thanks,
> > Dawid Deja
> >
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