I will vote for approach 1. First of all that one sounds easier to do to me. And I think idempotency is important. It may run at the cost of higher latency but I think it is ok
And in addition, when in the future if users do need realtime control tuple processing, we can always add the option on top of it. So I vote for 1 Thanks, Siyuan On Wed, Nov 2, 2016 at 1:28 PM, Pradeep A. Dalvi <p...@apache.org> wrote: > 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 <da...@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.ro...@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 <tho...@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 <da...@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.ro...@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 < > > >>>>>> > > >>>>> pra...@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 <da...@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 > > >>>>>>>> > > >>>>>>>> > > > > > >
