I don't think that would work well. It's entirely possible that the input watermark will already have passed the timestamp of the hold, in which case an event-time timer would fire immediately. You could make it a looping timer, but the new timer would also fire immediately after being set, and a tight timer loop isn't a good idea.
A Timer.get() is one solution, though I think the only way we have to implement it is to store the timer's timestamp in a ValueState; doing this for every timer would add a lot of cost to pipelines. Another option is a Timer.getCurrentTime() method, that would return the current base time that setRelative is based off of. It seems like a strange function to add to Timer though. Another option is to use TimerMap to bucket timers. Every minute we round the current processing time to the nearest minute and set a timer with an expiration of that minute (and with the minute timestamp as its tag as well). This way we would have a continuous sequence of expiring timers, and we wouldn't have to set just the first one. The biggest problem with this approach is that we would also have to use MapState to store the desired watermark hold per processing-time bucket. MapState is not supported by many runners yet, so I don't want to use it in a basic transform like GroupIntoBatches (furthermore - a transform that works on most runners today). Reuven On Thu, May 20, 2021 at 2:11 AM Jan Lukavský <je...@seznam.cz> wrote: > Sounds like you could solve that using second event time timer, that would > be actually used only to hold the output timestamp (watermark hold). > Something like > > > eventTimer.withOutputTimestamp(currentBufferMinimalStamp).offset(Duration.ZERO).setRelative() > > when the timer fires, you would only reset the minimum. > > It is sort of ugly, though. It would be cool to have a way to get the > current timestamp a timer is set to (if any). > > Jan > On 5/20/21 3:12 AM, Reuven Lax wrote: > > 100% - the contract should not change because things are in a bundle. IIRC > there are some open bugs in Beam around this that really should be fixed. > > My issue with GroupIntoBatches is different. This transform works as > follows: > > if (this is the first element in batch - checked by reading a count stored > in a ValueState) > timer.offset(bufferingDuration).setRelative() > > This makes it tricky to use setTimer.withOutputTimestamp. Inputs are not > guaranteed to be in order, so simply adding a withOutputTimestamp would set > the timestamp to be whatever the first element happened to be; it really > should be the minimum timestamp of all elements in the buffer. If we > started setting the timer on every element, then timer.offset.setRelative > would keep bumping the (processing-time) timer into the future and it would > never expire. > > One solution would be to store the timer timestamp in a ValueState, and > use Timer.set to set the timer to an absolute timestamp. This would allow > us to always reset the timer to the same expiration target, just modifying > the output timestamp each time. However, this will break DirectRunner > tests. The DirectRunner allows the user to control the advancement of > processing time when using TestStream, but this facility doesn't work well > if the transform sets the processing-time timer using absolute set() calls. > > I'm not sure how to solve this using the existing Timer API. > > On Wed, May 19, 2021 at 4:39 PM Robert Bradshaw <rober...@google.com> > wrote: > >> +1. It was my understanding as well that consensus was that timers >> must be delivered in timestamp order, and "within bundle" >> resetting/clearing of timers should be respected (as if each timer was >> in its own bundle). >> >> On Wed, May 19, 2021 at 3:01 PM Kenneth Knowles <k...@apache.org> wrote: >> > >> > Reading over the other thread, there was consensus to implement. >> > >> > Reading commentary on the PR, there were good questions raised about >> the semantics. Questions which I feel able to have an opinion about :-) >> > >> > The questions surrounded bundling and timers in the same bundling >> clearing each other. Actually the same questions apply to timers re-setting >> later timers and +Jan Lukavský has raised this already (among other people) >> so we kind of know the answer now, and I think +Boyuan Zhang code was good >> (from my quick read). What has changed is that we have a better idea of the >> contract with the runner. I'm not sure if portability makes this more >> complex. I will share all my thoughts on this: >> > >> > I think one key to the Beam model is that bundles are for performance >> and also intended as the unit of commitment (until FinishBundle is called, >> there may be unfinished work). They can affect *behavior* (what the program >> does - including what can be observed) ... but not *semantics* (what the >> output means). >> > >> > So, for example, bundling affects how many files are written but you >> are expected to read all the files and the number or ordering you must not >> depend on. The behavior is different, but the semantics are the same. >> > >> > When it comes to timers, behavior and semantics are very tightly >> coupled; timers are like a self loop. The firing of a timer is a behavior >> w.r.t. the output of the stateful ParDo but it is semantics of the timer >> itself (my definitions don't actually apply so simply so don't dig too hard >> :-). So to get bundling-invariant semantics, we should try for >> bundling-invariant behavior. When some clock moves from T to T+D between >> bundles then all the timers in the range [T, T+D) may fire so they are >> delivered in the bundle. I believe we have in the prior re-setting issue >> agreed that timers should always be called back in timestamp order. >> Bundling-invariance then implies that earlier timers can clear later timers >> just like they can re-set them. So even though a timer is delivered in a >> bundle, the local state of the timer wins. This matches how state works as >> well; no matter how things are bundled, the state you read is always >> whatever was written last. >> > >> > Kenn >> > >> > On Tue, May 11, 2021 at 1:24 PM Siyuan Chen <syc...@google.com> wrote: >> >> >> >> >> >> >> >> On Tue, May 11, 2021 at 11:08 AM Reuven Lax <re...@google.com> wrote: >> >>> >> >>> >> >>> >> >>> On Tue, May 11, 2021 at 9:01 AM Kenneth Knowles <k...@apache.org> >> wrote: >> >>>> >> >>>> >> >>>> >> >>>> On Mon, May 10, 2021 at 7:40 PM Reuven Lax <re...@google.com> wrote: >> >>>>> >> >>>>> Hi, >> >>>>> >> >>>>> I've been looking at the implementation of GroupIntoBatches (hoping >> to add support to group based on byte size), and I have a few questions >> about the current implementation. >> >>>>> >> >>>>> 1. I noticed that the transform does not preserve input timestamps. >> The output timestamp is not explicitly set, so it will be whatever the >> default output timestamp. Confusingly this will depend on which path is >> taken. If the batch is output while processing an element, then the >> timestamp of tha element will be used. If the batch is output from the >> timer, then the processing-time value of the timer will be used. >> >>>> >> >>>> >> >>>> Seems like bugs. >> >>>> >> >>>>> >> >>>>> - Should I start setting outputTimestamp explicitly - ideally >> to the minimum timestamp in the current batch of elements? >> >>>> >> >>>> >> >>>> That's a sensible default. Could pass a TimestampCombiner if that >> leads to watermark delays. Given the nature of the transform, the simple >> approach will probably be fine. >> >>> >> >>> >> >>> This is a bit tricky to do. in order to update the output timestamp, >> we need to call setTimer again. However since we are calling >> timer.offset().setRelative(), this will keep bumping the timer into the >> future and it will never fire. >> >>> >> >>> One solution would be for GIB to just store the current timer ts in >> state, and make sure that keep setting the same target until the timer >> fires. However that precludes us from using setRelative (since that method >> does not allow the DoFn to see what target time is being set). I think that >> this approach won't play nice with TestStream.advanceProcessingTimeTo. >> >>> >> >>> We could also add a way to update _just_ the output time for a timer >> without resetting the entire timer. >> >>> >> >>>> >> >>>> >> >>>>> - Should we have the option to preserve all element >> timestamps? this could look something like this: >> >>>>> >> >>>>> PColllection<KV<K, TimestampedValue<V>>> batches = >> >>>>> >> input.apply(GroupIntoBatches.ofSize(N).withTimestampedValues()); >> >>>> >> >>>> >> >>>> This seems useful. >> >>>> >> >>>>> >> >>>>> 2. flushBatch always resets the timer, even after the batch is >> processed. The only reason I can think of for doing so is to update the >> watermark hold. TimerInternals contains a deleteTimer method - is there any >> reason we shouldn't simply implement Timer.clear and hook it up to the >> existing deleteTimer? >> >>>> >> >>>> >> >>>> Makes sense to me. Prior thread on this seemed to have lightweight >> consensus: >> https://lists.apache.org/thread.html/r91af7dff0070b80b275082ca0cff7486dc5dfdfc113f35f560600792%40%3Cdev.beam.apache.org%3E >> >> >> >> There was another thread about adding Timer.clear in Java: >> https://lists.apache.org/thread.html/r47f5f14ba9729a2800222440425543ef40bea12eef1f2739d42e75db%40%3Cdev.beam.apache.org%3E >> >> and a previous attempt in https://github.com/apache/beam/pull/12836. >> Boyuan might have more insights into this +Boyuan Zhang >> >>>> >> >>>> >> >>>>> >> >>>>> 3. This transform was implemented before OnWindowExpiration was >> implemented. I think we should add a window-expiration callback, and stop >> setting the end-of window timer. >> >>>> >> >>>> >> >>>> +1 >> >>>> >> >>>> Kenn >> >
