Ahhh I see. Thank you very much for this additional info. Really helpful! I think after considering further, its probably more appropriate and less risky in my current scenario to try to use the Session combiner. I did really like the Stateful ParDo way of doing things tho, if it were simpler to get correct and as performant as Windows (I understand that Flink has some special optimizations for windowing that go all the way down into the rocks db code) I might have liked to see this method work out.
Thanks again! From: Reuven Lax <re...@google.com> Reply-To: "dev@beam.apache.org" <dev@beam.apache.org> Date: Sunday, August 9, 2020 at 11:25 PM To: dev <dev@beam.apache.org> Subject: Re: Stateful Pardo Question Notice: This email is from an external sender. Lookin at the code in the repo, it seems to assume that context.timestamp() is the "watermark" time. It is not - context.timestamp() is the time of the current element being processed. Generally the watermark will always be smaller than the timestamp of the current element, as the watermark is a lower bound on element timestamps (so you can't really check context.timestamp() to determine if a timer is eligible to fire). It's also worth mentioning that Beam provides no ordering guarantees on the input elements (unless you are using TestStream in a unit test). In theory they could arrive in reverse timestamp order. In the real world that degree of disorder is probably unlikely (and would be inefficient, as the watermark would then not advance until all elements were processed), however the model makes no guarantees about order. The fact that inputs can arrive in any order means that the sessions code you are trying to implement would need some more complexity if you wanted it to be correct. The problem is that you may have buffered elements from multiple different sessions in your bag, and you may see those elements out of order. Resetting the timer to event.getTimestamp().plus(SESSION_TIMEOUT) will cause you to potentially create a timer that is too early. There are various ways to solve this (e.g. storing an interval tree in a separate state tag so you can keep track of which sessions are in flight). The upcoming TimestampOrderedList state type will also help to make this sort of use case easier and more effficient. Reuven On Sun, Aug 9, 2020 at 5:05 PM Reza Ardeshir Rokni <raro...@gmail.com<mailto:raro...@gmail.com>> wrote: +1 on having the behavior clearly documented, would also be great to try and add more stat and timer patterns to the Beam docs patterns page https://beam.apache.org/documentation/patterns/overview/. I think it might be worth thinking about describing these kind of patterns with an emphasis on the OnTimer being where the work happens. One thing that would make all of this a lot easier in reducing the boiler plate code that would need to be written is a sorted map state. ( a topic of discussion on a few threads). On Mon, 10 Aug 2020 at 01:16, Reuven Lax <re...@google.com<mailto:re...@google.com>> wrote: Timers in Beam are considered "eligible to fire" once the watermark has advanced. This is not the same as saying that they will fire immediately. You should not assume ordering between the elements and the timers. This is one reason (among many) that Beam does not provide a "read watermark" primitive, as it leads to confusions such as this. Since there is no read-watermark operator, the only way for a user's ParDo to view that the watermark has been set is to set a timer and wait for it to expire. Watermarks on their own can act in very non-intuitive ways (due to asynchronous advancement), so generally we encourage people to reason about timers and windowing in their code instead. Reuven On Sun, Aug 9, 2020 at 9:39 AM jmac...@godaddy.com<mailto:jmac...@godaddy.com> <jmac...@godaddy.com<mailto:jmac...@godaddy.com>> wrote: I understand that watermarks are concurrently advanced, and that they are estimates and not precise. but I’m not sure this is relevant in this case. In this repro code we are in processElement() and the watermark HAS advanced but the timer has not been called even though we asked the runtime to do that. In this case we are in a per-key stateful operating mode and our timer should not be shared with any other runners (is that correct?) so it seems to me that we should be able to operate in a manner that is locally consistent from the point of view of the DoFn we are writing. That is to say, _before_ we enter processElement we check any local timers first. I would argue that this would be far more sensible from the authors perspective. From: Reuven Lax <re...@google.com<mailto:re...@google.com>> Reply-To: "dev@beam.apache.org<mailto:dev@beam.apache.org>" <dev@beam.apache.org<mailto:dev@beam.apache.org>> Date: Thursday, August 6, 2020 at 11:57 PM To: dev <dev@beam.apache.org<mailto:dev@beam.apache.org>> Subject: Re: Stateful Pardo Question Notice: This email is from an external sender. On Tue, Aug 4, 2020 at 1:08 PM jmac...@godaddy.com<mailto:jmac...@godaddy.com> <jmac...@godaddy.com<mailto:jmac...@godaddy.com>> wrote: So, after some additional digging, it appears that Beam does not consistently check for timer expiry before calling process. The result is that it may be the case that the watermark has moved beyond your timer expiry, and if youre counting on the timer callback happening at the time you set it for, that simply may NOT have happened when you are in DoFn.process(). You can “fix” the behavior by simply checking the watermark manually in process() and doing what you would normally do for timestamp exipry before proceeding. See my latest updated code reproducing the issue and showing the fix at https://github.com/randomsamples/pardo_repro. I would argue that users of this API will naturally expect that timer callback semantics will guarantee that when they are in process(), if the current watermark is past a timers expiry that the timer callback in question will have been called. Is there any reason why this isn’t happening? Am I misunderstanding something? Timers do not expire synchronously with the watermark advancing. So if you have a timer set for 12pm and the watermark advances past 12pm, that timer is now eligible to fire, but might not fire immediately. Some other elements may process before that timer fires. There are multiple reasons for this, but one is that Beam does not guarantee that watermark advancement is synchronous with element processing. The watermark might advance suddenly while in the middle processing an element, or at any other time. This makes it impossible (or at least, exceedingly difficult) to really provide the guarantee you expected. Reuven From: "jmac...@godaddy.com<mailto:jmac...@godaddy.com>" <jmac...@godaddy.com<mailto:jmac...@godaddy.com>> Reply-To: "dev@beam.apache.org<mailto:dev@beam.apache.org>" <dev@beam.apache.org<mailto:dev@beam.apache.org>> Date: Monday, August 3, 2020 at 10:51 AM To: "dev@beam.apache.org<mailto:dev@beam.apache.org>" <dev@beam.apache.org<mailto:dev@beam.apache.org>> Subject: Re: Stateful Pardo Question Notice: This email is from an external sender. Yeah, unless I am misunderstanding something. The output from my repro code shows event timestamp and the context timestamp every time we process an event. Receiving event at: 2000-01-01T00:00:00.000Z Resetting timer to : 2000-01-01T00:15:00.000Z Receiving event at: 2000-01-01T00:05:00.000Z Resetting timer to : 2000-01-01T00:20:00.000Z <-- Shouldn’t the timer have fired before we processed the next event? Receiving event at: 2000-01-01T00:40:00.000Z Why didnt the timer fire? Resetting timer to : 2000-01-01T00:55:00.000Z Receiving event at: 2000-01-01T00:45:00.000Z Resetting timer to : 2000-01-01T01:00:00.000Z Receiving event at: 2000-01-01T00:50:00.000Z Resetting timer to : 2000-01-01T01:05:00.000Z Timer firing at: 2000-01-01T01:05:00.000Z From: Reuven Lax <re...@google.com<mailto:re...@google.com>> Reply-To: "dev@beam.apache.org<mailto:dev@beam.apache.org>" <dev@beam.apache.org<mailto:dev@beam.apache.org>> Date: Monday, August 3, 2020 at 10:02 AM To: dev <dev@beam.apache.org<mailto:dev@beam.apache.org>> Subject: Re: Stateful Pardo Question Notice: This email is from an external sender. Are you sure that there is a 15 minute gap in your data? On Mon, Aug 3, 2020 at 6:20 AM jmac...@godaddy.com<mailto:jmac...@godaddy.com> <jmac...@godaddy.com<mailto:jmac...@godaddy.com>> wrote: I am confused about the behavior of timers on a simple stateful pardo. I have put together a little repro here: https://github.com/randomsamples/pardo_repro I basically want to build something like a session window, accumulating events until quiescence of the stream for a given key and gap time, then output results. But it appears that the timer is not firing when the watermark is passed it expiration time, so the event stream is not being split as I would have expected. Would love some help getting this work, the behavior is for a project I’m working on.
