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From "Pradeep A. Dalvi" <p...@apache.org>
Subject Re: [DISCUSSION] Custom Control Tuples
Date Wed, 02 Nov 2016 20:28:01 GMT
As a rule of thumb in any real time operating system, control tuples should
always be handled using Priority Queues.

We may try to control priorities by defining levels. And shall not
be delivered at window boundaries.

In short, control tuples shall never be treated as any other tuples in real
time systems.

On Thursday, November 3, 2016, David Yan <david@datatorrent.com> wrote:

> Hi all,
>
> I would like to renew the discussion of control tuples.
>
> Last time, we were in a debate about whether:
>
> 1) the platform should enforce that control tuples are delivered at window
> boundaries only
>
> or:
>
> 2) the platform should deliver control tuples just as other tuples and it's
> the operator developers' choice whether to handle the control tuples as
> they arrive or delay the processing till the next window boundary.
>
> To summarize the pros and cons:
>
> Approach 1: If processing control tuples results in changes of the behavior
> of the operator, if idempotency needs to be preserved, the processing must
> be done at window boundaries. This approach will save the operator
> developers headache to ensure that. However, this will take away the
> choices from the operator developer if they just need to process the
> control tuples as soon as possible.
>
> Approach 2: The operator has a chance to immediately process control
> tuples. This would be useful if latency is more valued than correctness.
> However, if this would open the possibility for operator developers to
> shoot themselves in the foot. This is especially true if there are multiple
> input ports. as there is no easy way to guarantee processing order for
> multiple input ports.
>
> We would like to arrive to a consensus and close this discussion soon this
> time so we can start the work on this important feature.
>
> Thanks!
>
> David
>
> On Tue, Jun 28, 2016 at 10:04 AM, Vlad Rozov <v.rozov@datatorrent.com
> <javascript:;>>
> wrote:
>
> > It is not clear how operator will emit custom control tuple at window
> > boundaries. One way is to cache/accumulate control tuples in the operator
> > output port till window closes (END_WINDOW is inserted into the output
> > sink) or only allow an operator to emit control tuples inside the
> > endWindow(). The later is a slight variation of the operator output port
> > caching behavior with the only difference that now the operator itself is
> > responsible for caching/accumulating control tuples. Note that in many
> > cases it will be necessary to postpone emitting payload tuples that
> > logically come after the custom control tuple till the next window
> begins.
> >
> > IMO, that too restrictive and in a case where input operator uses a push
> > instead of a poll (for example, it provides an end point where remote
> > agents may connect and publish/push data), control tuples may be used for
> > connect/disconnect/watermark broadcast to (partitioned) downstream
> > operators. In this case the platform just need to guarantee order barrier
> > (any tuple emitted prior to a control tuple needs to be delivered prior
> to
> > the control tuple).
> >
> > Thank you,
> >
> > Vlad
> >
> >
> >
> > On 6/27/16 19:36, Amol Kekre wrote:
> >
> >> I agree with David. Allowing control tuples within a window (along with
> >> data tuples) creates very dangerous situation where guarantees are
> >> impacted. It is much safer to enable control tuples (send/receive) at
> >> window boundaries (after END_WINDOW of window N, and before BEGIN_WINDOW
> >> for window N+1). My take on David's list is
> >>
> >> 1. -> window boundaries -> Strong +1; there will be a big issue with
> >> guarantees for operators with multiple ports. (see Thomas's response)
> >> 2. -> All downstream windows -> +1, but there are situations; a caveat
> >> could be "only to operators that implement control tuple
> >> interface/listeners", which could effectively translates to "all
> >> interested
> >> downstream operators"
> >> 3. Only Input operator can create control tuples -> -1; is restrictive
> >> even
> >> though most likely 95% of the time it will be input operators
> >>
> >> Thks,
> >> Amol
> >>
> >>
> >> On Mon, Jun 27, 2016 at 4:37 PM, Thomas Weise <thomas@datatorrent.com
> <javascript:;>>
> >> wrote:
> >>
> >> The windowing we discuss here is in general event time based, arrival
> time
> >>> is a special case of it.
> >>>
> >>> I don't think state changes can be made independent of the streaming
> >>> window
> >>> boundary as it would prevent idempotent processing and transitively
> >>> exactly
> >>> once. For that to work, tuples need to be presented to the operator in
> a
> >>> guaranteed order *within* the streaming window, which is not possible
> >>> with
> >>> multiple ports (and partitions).
> >>>
> >>> Thomas
> >>>
> >>> On Mon, Jun 27, 2016 at 2:53 PM, David Yan <david@datatorrent.com
> <javascript:;>>
> >>> wrote:
> >>>
> >>> I think for session tracking, if the session boundaries are allowed to
> be
> >>>> not aligned with the streaming window boundaries, the user will have
a
> >>>>
> >>> much
> >>>
> >>>> bigger problem with idempotency. And in most cases, session tracking
> is
> >>>> event time based, not ingression time or processing time based, so
> this
> >>>>
> >>> may
> >>>
> >>>> never be a problem. But if that ever happens, the user can always
> alter
> >>>>
> >>> the
> >>>
> >>>> default 500ms width.
> >>>>
> >>>> David
> >>>>
> >>>> On Mon, Jun 27, 2016 at 2:35 PM, Vlad Rozov <v.rozov@datatorrent.com
> <javascript:;>>
> >>>> wrote:
> >>>>
> >>>> Ability to send custom control tuples within window may be useful, for
> >>>>> example, for sessions tracking, where session boundaries are not
> >>>>>
> >>>> aligned
> >>>
> >>>> with window boundaries and 500 ms latency is not acceptable for an
> >>>>> application.
> >>>>>
> >>>>> Thank you,
> >>>>>
> >>>>> Vlad
> >>>>>
> >>>>>
> >>>>> On 6/25/16 10:52, Thomas Weise wrote:
> >>>>>
> >>>>> It should not matter from where the control tuple is triggered.
It
> >>>>>>
> >>>>> will
> >>>
> >>>> be
> >>>>
> >>>>> good to have a generic mechanism to propagate it and other things
can
> >>>>>>
> >>>>> be
> >>>
> >>>> accomplished outside the engine. For example, the new comprehensive
> >>>>>> support
> >>>>>> for windowing will all be in Malhar, nothing that the engine
needs
> to
> >>>>>>
> >>>>> know
> >>>>
> >>>>> about it except that we need the control tuple for watermark
> >>>>>>
> >>>>> propagation
> >>>
> >>>> and idempotent processing.
> >>>>>>
> >>>>>> I also think the main difference to other tuples is the need
to send
> >>>>>>
> >>>>> it
> >>>
> >>>> to
> >>>>
> >>>>> all partitions. Which is similar to checkpoint window tuples, but
not
> >>>>>>
> >>>>> the
> >>>>
> >>>>> same. Here, we probably also need the ability for the user to control
> >>>>>> whether such tuple should traverse the entire DAG or not. For
a
> batch
> >>>>>>
> >>>>> use
> >>>>
> >>>>> case, for example, we may want to send the end of file to the next
> >>>>>> operator, but not beyond, if the operator has asynchronous
> processing
> >>>>>> logic
> >>>>>> in it.
> >>>>>>
> >>>>>> For any logic to be idempotent, the control tuple needs to be
> >>>>>>
> >>>>> processed
> >>>
> >>>> at
> >>>>
> >>>>> a window boundary. Receiving the control tuple in the window callback
> >>>>>> would
> >>>>>> avoid having to track extra state in the operator. I don't think
> >>>>>>
> >>>>> that's
> >>>
> >>>> a
> >>>>
> >>>>> major issue, but what is the use case for processing a control tuple
> >>>>>> within
> >>>>>> the window?
> >>>>>>
> >>>>>> Thomas
> >>>>>>
> >>>>>>
> >>>>>>
> >>>>>> On Sat, Jun 25, 2016 at 6:19 AM, Pramod Immaneni <
> >>>>>>
> >>>>> pramod@datatorrent.com <javascript:;>>
> >>>>
> >>>>> wrote:
> >>>>>>
> >>>>>> For the use cases you mentioned, I think 1) and 2) are more
likely
> to
> >>>>>>
> >>>>>>> be controlled directly by the application, 3) and 4) are
more
> likely
> >>>>>>> going to be triggered externally and directly handled by
the engine
> >>>>>>> and 3) is already being implemented that way (apexcore-163).
> >>>>>>>
> >>>>>>> The control tuples emitted by an operator would be sent
to all
> >>>>>>> downstream partitions isn't it and that would be the chief
> >>>>>>>
> >>>>>> distinction
> >>>
> >>>> compared to data (apart from the payload) which would get partitioned
> >>>>>>> under normal circumstances. It would also be guaranteed
that
> >>>>>>> downstream partitions will receive control tuples only after
the
> data
> >>>>>>> that was sent before it so we could send it immediately
when it is
> >>>>>>> emitted as opposed to window boundaries.
> >>>>>>>
> >>>>>>> However during unification it is important to know if these
control
> >>>>>>> tuples have been received from all upstream partitions before
> >>>>>>> proceeding with a control operation. One could wait till
end of the
> >>>>>>> window but that introduces a delay however small, I would
like to
> add
> >>>>>>> to the proposal that the platform only hand over the control
tuple
> to
> >>>>>>> the unifier when it has been received from all upstream
partitions
> >>>>>>> much like how end window is processed but not wait till
the actual
> >>>>>>>
> >>>>>> end
> >>>
> >>>> of the window.
> >>>>>>>
> >>>>>>> Regd your concern about idempotency, we typically care about
> >>>>>>> idempotency at a window level and doing the above will still
allow
> >>>>>>>
> >>>>>> the
> >>>
> >>>> operators to preserve that easily.
> >>>>>>>
> >>>>>>> Thanks
> >>>>>>>
> >>>>>>> On Jun 24, 2016, at 11:22 AM, David Yan <david@datatorrent.com
> <javascript:;>>
> >>>>>>>
> >>>>>> wrote:
> >>>
> >>>> Hi all,
> >>>>>>>>
> >>>>>>>> I would like to propose a new feature to the Apex core
engine --
> the
> >>>>>>>> support of custom control tuples. Currently, we have
control
> tuples
> >>>>>>>>
> >>>>>>> such
> >>>>
> >>>>> as
> >>>>>>>
> >>>>>>> BEGIN_WINDOW, END_WINDOW, CHECKPOINT, and so on, but we
don't have
> >>>>>>>>
> >>>>>>> the
> >>>
> >>>> support for applications to insert their own control tuples. The way
> >>>>>>>> currently to get around this is to use data tuples and
have a
> >>>>>>>>
> >>>>>>> separate
> >>>
> >>>> port
> >>>>>>>
> >>>>>>> for such tuples that sends tuples to all partitions of the
> >>>>>>>>
> >>>>>>> downstream
> >>>
> >>>> operators, which is not exactly developer friendly.
> >>>>>>>>
> >>>>>>>> We have already seen a number of use cases that can
use this
> >>>>>>>>
> >>>>>>> feature:
> >>>
> >>>> 1) Batch support: We need to tell all operators of the physical DAG
> >>>>>>>>
> >>>>>>> when
> >>>>
> >>>>> a
> >>>>>>>
> >>>>>>> batch starts and ends, so the operators can do whatever
that is
> >>>>>>>>
> >>>>>>> needed
> >>>
> >>>> upon
> >>>>>>>
> >>>>>>> the start or the end of a batch.
> >>>>>>>>
> >>>>>>>> 2) Watermark: To support the concepts of event time
windowing, the
> >>>>>>>> watermark control tuple is needed to tell which windows
should be
> >>>>>>>> considered late.
> >>>>>>>>
> >>>>>>>> 3) Changing operator properties: We do have the support
of
> changing
> >>>>>>>> operator properties on the fly, but with a custom control
tuple,
> the
> >>>>>>>> command to change operator properties can be window
aligned for
> all
> >>>>>>>> partitions and also across the DAG.
> >>>>>>>>
> >>>>>>>> 4) Recording tuples: Like changing operator properties,
we do have
> >>>>>>>>
> >>>>>>> this
> >>>>
> >>>>> support now but only at the individual physical operator level,
and
> >>>>>>>>
> >>>>>>>> without
> >>>>>>>
> >>>>>>> control of which window to record tuples for. With a custom
control
> >>>>>>>>
> >>>>>>>> tuple,
> >>>>>>>
> >>>>>>> because a control tuple must belong to a window, all operators
in
> >>>>>>>>
> >>>>>>> the
> >>>
> >>>> DAG
> >>>>>>>> can start (and stop) recording for the same windows.
> >>>>>>>>
> >>>>>>>> I can think of two options to achieve this:
> >>>>>>>>
> >>>>>>>> 1) new custom control tuple type that takes user's serializable
> >>>>>>>>
> >>>>>>> object.
> >>>>
> >>>>> 2) piggy back the current BEGIN_WINDOW and END_WINDOW control
> >>>>>>>>
> >>>>>>> tuples.
> >>>
> >>>> Please provide your feedback. Thank you.
> >>>>>>>>
> >>>>>>>> David
> >>>>>>>>
> >>>>>>>>
> >
>

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