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 <sil...@sileht.net > >>> <mailto:sil...@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 > > > > __________________________________________________________________________ > > OpenStack Development Mailing List (not for usage questions) > > Unsubscribe: openstack-dev-requ...@lists.openstack.org?subject:unsubscribe > > http://lists.openstack.org/cgi-bin/mailman/listinfo/openstack-dev > __________________________________________________________________________ OpenStack Development Mailing List (not for usage questions) Unsubscribe: openstack-dev-requ...@lists.openstack.org?subject:unsubscribe http://lists.openstack.org/cgi-bin/mailman/listinfo/openstack-dev
