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From Markus Thömmes <markusthoem...@apache.org>
Subject Re: Kafka and Proposal on a future architecture of OpenWhisk
Date Tue, 21 Aug 2018 08:16:36 GMT
Hi Tyson,

if we take the concerns apart as I proposed above, timeouts should only
ever be triggered after a request is scheduled as you say, that is: As soon
as it's crossing the user-container mark. With the concern separation, it
is plausible that blocking invocations are never buffered anywhere, which
makes a lot of sense, because you cannot persist the open HTTP connection
to the client anyway.

To make the distinction clear: A blocking request can still be put onto the
work-stealing queue to be balanced between different ContainerRouters.

A blocking request though would never be written to a persistent buffer
that's used to be able to efficiently handle async invocations and
backpressuring them. That buffer should be entirely separate and could
possibly be placed outside of the execution system to make the distinction
more explicit. The execution system itself would then only deal with
request-response style invocations and asynchronous invocations are done by
having a seperate queue and a consumer that creates HTTP requests to the
execution system.

Cheers,
Markus

Am Mo., 20. Aug. 2018 um 23:30 Uhr schrieb Tyson Norris
<tnorris@adobe.com.invalid>:

> Thanks for summarizing Markus.
>
> Yes this is confusing in context of current system, which stores in kafka,
> but not to indefinitely wait, since timeout begins immediately
> So, I think the problem of buffering/queueing is: when does the timeout
> begin? If not everything is buffered the same, their timeout should not
> begin until processing begins.
>
> Maybe it would make sense to:
> * always buffer (indefinitely) to queue for async, never for sync
> * timeout for async not started till read from queue - which may be
> delayed from time of trigger or http request
> * this should also come with some system monitoring to indicate the queue
> processing is not keeping up with some configurable max delay threshold ("I
> can’t tolerate delays of > 5 minutes", etc)
> * ContainerRouters can only pull from async queue when
>         * increasing the number of pending activations won’t exceed some
> threshold (prevent excessive load of async on ContainerRouters)
>         * ContainerManager is not overloaded (can still create containers,
> or has some configurable way to indicate the cluster is healthy enough to
> cope with extra processing)
>
> We could of course make this configurable so that operators can choose to:
> * treat async/sync activations the same for sync/async (the overloaded
> system fails when either ContainerManager or ContainerRouters are max
> capacity)
> * treat async/sync with preference for:
>         * sync - where async is buffered for unknown period before
> processing, incoming sync traffic (or lack of)
>         * async - where sync is sent to the queue, to be processed in
> order of receipt interleaved with async traffic (similar to today, I think)
>
> I think the impact here (aside from technical) is the timing difference if
> we introduce latency in side affects based on the activation being sync vs
> async.
>
> I’m also not sure prioritizing message processing between sync/async
> internally in ContainerRouter is better than just have some dedicated
> ContainerRouters that receive all async activations, and others that
> receive all sync activations, but the end result is the same, I think.
>
>
> > On Aug 19, 2018, at 4:29 AM, Markus Thömmes <markusthoemmes@apache.org>
> wrote:
> >
> > Hi Tyson, Carlos,
> >
> > FWIW I should change that to no longer say "Kafka" but "buffer" or
> "message
> > queue".
> >
> > I see two use-cases for a queue here:
> > 1. What you two are alluding to: Buffering asynchronous requests because
> of
> > a different notion of "latency sensitivity" if the system is in an
> overload
> > scenario.
> > 2. As a work-stealing type balancing layer between the ContainerRouters.
> If
> > we assume round-robin/least-connected (essentially random) scheduling
> > between ContainerRouters, we will get load discrepancies between them. To
> > smoothen those out, a ContainerRouter can put the work on a queue to be
> > stolen by a Router that actually has space for that work (for example:
> > Router1 requests a new container, puts the work on the queue while it
> waits
> > for that container, Router2 already has a free container and executes the
> > action by stealing it from the queue). This does has the added complexity
> > of breaking a streaming communication between User and Container (to
> > support essentially unbounded payloads). A nasty wrinkle that might
> render
> > this design alternative invalid! We could come up with something smarter
> > here, i.e. only putting a reference to the work on the queue and the
> > stealer connects to the initial owner directly which then streams the
> > payload through to the stealer, rather than persisting it somewhere.
> >
> > It is important to note, that in this design, blocking invokes could
> > potentially gain the ability to have unbounded entities, where
> > trigger/non-blocking invokes might need to be subject to a bound here to
> be
> > able to support eventual execution efficiently.
> >
> > Personally, I'm much more torn to the work-stealing type case. It
> implies a
> > wholy different notion of using the queue though and doesn't have much to
> > do with the way we use it today, which might be confusing. It could also
> > well be the case, that work-stealing type algorithms are easier to back
> on
> > a proper MQ vs. trying to make it work on Kafka.
> >
> > It might also be important to note that those two use-cases might require
> > different technologies (buffering vs. queue-backend for work-stealing)
> and
> > could well be seperated in the design as well. For instance, buffering
> > triggers fires etc. does not necessarily need to be done on the execution
> > layer but could instead be pushed to another layer. Having the notion of
> > "async" vs "sync" in the execution layer could be benefitial for
> > loadbalancing itself though. Something worth exploring imho.
> >
> > Sorry for the wall of text, I hope this clarifies things!
> >
> > Cheers,
> > Markus
> >
> > Am Sa., 18. Aug. 2018 um 02:36 Uhr schrieb Carlos Santana <
> > csantana23@gmail.com>:
> >
> >> triggers get responded right away (202) with an activation is and then
> >> sent to the queue to be processed async same as async action invokes.
> >>
> >> I think we would keep same contract as today for this type of
> activations
> >> that are eventually process different from blocking invokes including we
> >> Actions were the http client hold a connection waiting for the result
> back.
> >>
> >> - Carlos Santana
> >> @csantanapr
> >>
> >>> On Aug 17, 2018, at 6:14 PM, Tyson Norris <tnorris@adobe.com.INVALID>
> >> wrote:
> >>>
> >>> Hi -
> >>> Separate thread regarding the proposal: what is considered for routing
> >> activations as overload and destined for kafka?
> >>>
> >>> In general, if kafka is not on the blocking activation path, why would
> >> it be used at all, if the timeouts and processing expectations of
> blocking
> >> and non-blocking are the same?
> >>>
> >>> One case I can imagine: triggers + non-blocking invokes, but only in
> the
> >> case where those have some different timeout characteristics. e.g. if a
> >> trigger fires an action, is there any case where the activation should
> be
> >> buffered to kafka if it will timeout same as a blocking activation?
> >>>
> >>> Sorry if I’m missing something obvious.
> >>>
> >>> Thanks
> >>> Tyson
> >>>
> >>>
> >>
>
>

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