If user have a WordCount StatefulDoFn, the result of counts is always changing before the expiration of window. Maybe the user want a signal to know the count is the final value and then archive the value to the timing database or somewhere else. best, JingsongLee ------------------------------------------------------------------From:Kenneth Knowles <k...@google.com.INVALID>Time:2017 Mar 29 (Wed) 09:07To:dev <dev@beam.apache.org>Subject:Re: [PROPOSAL] @OnWindowExpiration On Tue, Mar 28, 2017 at 2:47 PM, Eugene Kirpichov < kirpic...@google.com.invalid> wrote:
> Kenn, can you quote some use cases for this, to make it more clear what are > the consequences of having this API in this form? > > I recall that one of the main use cases was batching DoFn, right? > I believe every stateful DoFn where the data stored in state represents some accumulation of the input and/or buffering of output requires this. So, yes: - batching DoFn and the many variants that may spring up - combine-like stateful DoFns that require state, like blended accumulation modes or selective composed combines - trigger-like stateful DoFns that output based on some complex user-defined criteria The stateful DoFns that do not require such a timer are those where the stored data is a phase transition or side-input-like enrichment, and I think also common join algorithms. I don't have a sense of which of these will be more prevalent. Both categories represent common user needs. Kenn > On Tue, Mar 28, 2017 at 1:37 PM Kenneth Knowles <k...@google.com.invalid> > wrote: > > > On Tue, Mar 28, 2017 at 1:32 PM, Robert Bradshaw < > > rober...@google.com.invalid> wrote: > > > > > Another alternative is to be able to set special timers, e.g. end of > > window > > > and expiration of window. That at least addresses (2). > > > > > > > Potentially a tangent, but that would perhaps fit in with the idea of > > removing TimeDomain from user APIs ( > > https://issues.apache.org/jira/browse/BEAM-1308) and instead having > > TimerSpecs.eventTimeTimer(), TimerSpecs.processingTimeTimer(), > > TimerSpecs.windowExpirationTimer() that each yield distinct sorts of > > parameters in @ProcessElement. > > > > A bit more heavyweight, syntactically. > > > > Kenn > > > > > > > > > > On Tue, Mar 28, 2017 at 1:27 PM, Kenneth Knowles > <k...@google.com.invalid > > > > > > wrote: > > > > > > > Hi all, > > > > > > > > I have a little extension to the stateful DoFn annotations to > circulate > > > for > > > > feedback: Allow a method to be annotated with @OnWindowExpiration to > > > > automatically get a callback at some point after the window has > > expired, > > > > but before the state for the window has been cleared. > > > > > > > > Today, a user can pretty easily get the same effect by setting a > timer > > > for > > > > the end of the window + allowed lateness in their @ProcessElement > > calls. > > > > But having just one annotation for it has a couple nice benefits: > > > > > > > > 1. Some users assume a naive implementation so they are concerned > that > > > > setting a timer repeatedly is costly. This eliminates the cause for > > user > > > > alarm and allows a runner to do a better job in case it didn't > already > > do > > > > it efficiently. > > > > > > > > 2. Getting the allowed lateness to be available to your > @ProcessElement > > > is > > > > a little crufty. > > > > > > > > 3. Often, if you don't have @OnWindowExpiration, you are leaving > behind > > > > state that might contain data that is otherwise lost. So I would even > > > > consider making it mandatory (with some way of indicating state you > > don't > > > > care about dropping) though that could be annoying. > > > > > > > > Another interesting moment in a window's lifecycle is @EndOfWindow. > > This > > > is > > > > not critical for correctness, though. > > > > > > > > Thoughts? > > > > > > > > Kenn > > > > > > > > > >
