> On 04 Jun 2016, at 04:16, Doug Hellmann <d...@doughellmann.com> wrote: > > 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".
The idea I think is dead. Joshua essentially told the reasons in the previous message. Renat Akhmerov @Nokia
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