+1 on the metadata use case. For performance reasons the Timer API does not support a read() operation, which for the vast majority of use cases is not a required feature. In the small set of use cases where it is needed, for example when you need to set a Timer in EventTime based on the smallest timestamp seen in the elements within a DoFn, we can make use of a ValueState object to keep track of the value.
On Fri, 26 Apr 2019 at 00:38, Reuven Lax <re...@google.com> wrote: > I see examples of people using ValueState that I think are not captured > CombiningState. For example, one common one is users who set a timer and > then record the timestamp of that timer in a ValueState. In general when > you store state that is metadata about other state you store, then > ValueState will usually make more sense than CombiningState. > > On Thu, Apr 25, 2019 at 9:32 AM Brian Hulette <bhule...@google.com> wrote: > >> Currently the Python SDK does not make ValueState available to users. My >> initial inclination was to go ahead and implement it there to be consistent >> with Java, but Robert brings up a great point here that ValueState has an >> inherent race condition for out of order data, and a lot of it's use cases >> can actually be implemented with a CombiningState instead. >> >> It seems to me that at the very least we should discourage the use of >> ValueState by noting the danger in the documentation and preferring >> CombiningState in examples, and perhaps we should go further and deprecate >> it in Java and not implement it in python. Either way I think we should be >> consistent between Java and Python. >> >> I'm curious what people think about this, are there use cases that we >> really need to keep ValueState around for? >> >> Brian >> >> ---------- Forwarded message --------- >> From: Robert Bradshaw <rober...@google.com> >> Date: Thu, Apr 25, 2019, 08:31 >> Subject: Re: [docs] Python State & Timers >> To: dev <dev@beam.apache.org> >> >> >> >> >> On Thu, Apr 25, 2019, 5:26 PM Maximilian Michels <m...@apache.org> wrote: >> >>> Completely agree that CombiningState is nicer in this example. Users may >>> still want to use ValueState when there is nothing to combine. >> >> >> I've always had trouble coming up with any good examples of this. >> >> Also, >>> users already know ValueState from the Java SDK. >>> >> >> Maybe we should deprecate that :) >> >> >> On 25.04.19 17:12, Robert Bradshaw wrote: >>> > On Thu, Apr 25, 2019 at 4:58 PM Maximilian Michels <m...@apache.org> >>> wrote: >>> >> >>> >> I forgot to give an example, just to clarify for others: >>> >> >>> >>> What was the specific example that was less natural? >>> >> >>> >> Basically every time we use ListState to express ValueState, e.g. >>> >> >>> >> next_index, = list(state.read()) or [0] >>> >> >>> >> Taken from: >>> >> >>> https://github.com/apache/beam/pull/8363/files#diff-ba1a2aed98079ccce869cd660ca9d97dR301 >>> > >>> > Yes, ListState is much less natural here. I think generally >>> > CombiningValue is often a better replacement. E.g. the Java example >>> > reads >>> > >>> > >>> > public void processElement( >>> > ProcessContext context, @StateId("index") ValueState<Integer> >>> index) { >>> > int current = firstNonNull(index.read(), 0); >>> > context.output(KV.of(current, context.element())); >>> > index.write(current+1); >>> > } >>> > >>> > >>> > which is replaced with bag state >>> > >>> > >>> > def process(self, element, state=DoFn.StateParam(INDEX_STATE)): >>> > next_index, = list(state.read()) or [0] >>> > yield (element, next_index) >>> > state.clear() >>> > state.add(next_index + 1) >>> > >>> > >>> > whereas CombiningState would be more natural (than ListState, and >>> > arguably than even ValueState), giving >>> > >>> > >>> > def process(self, element, index=DoFn.StateParam(INDEX_STATE)): >>> > yield element, index.read() >>> > index.add(1) >>> > >>> > >>> > >>> > >>> >> >>> >> -Max >>> >> >>> >> On 25.04.19 16:40, Robert Bradshaw wrote: >>> >>> https://github.com/apache/beam/pull/8402 >>> >>> >>> >>> On Thu, Apr 25, 2019 at 4:26 PM Robert Bradshaw <rober...@google.com> >>> wrote: >>> >>>> >>> >>>> Oh, this is for the indexing example. >>> >>>> >>> >>>> I actually think using CombiningState is more cleaner than >>> ValueState. >>> >>>> >>> >>>> >>> https://github.com/apache/beam/blob/release-2.12.0/sdks/python/apache_beam/runners/portability/fn_api_runner_test.py#L262 >>> >>>> >>> >>>> (The fact that one must specify the accumulator coder is, however, >>> >>>> unfortunate. We should probably infer that if we can.) >>> >>>> >>> >>>> On Thu, Apr 25, 2019 at 4:19 PM Robert Bradshaw < >>> rober...@google.com> wrote: >>> >>>>> >>> >>>>> The desire was to avoid the implicit disallowed combination wart in >>> >>>>> Python (until we could make sense of it), and also ValueState >>> could be >>> >>>>> surprising with respect to older values overwriting newer ones. >>> What >>> >>>>> was the specific example that was less natural? >>> >>>>> >>> >>>>> On Thu, Apr 25, 2019 at 3:01 PM Maximilian Michels <m...@apache.org> >>> wrote: >>> >>>>>> >>> >>>>>> @Pablo: Thanks for following up with the PR! :) >>> >>>>>> >>> >>>>>> @Brian: I was wondering about this as well. It makes the Python >>> state >>> >>>>>> code a bit unnatural. I'd suggest to add a ValueState wrapper >>> around >>> >>>>>> ListState/CombiningState. >>> >>>>>> >>> >>>>>> @Robert: Like Reuven pointed out, we can disallow ValueState for >>> merging >>> >>>>>> windows with state. >>> >>>>>> >>> >>>>>> @Reza: Great. Let's make sure it has Python examples out of the >>> box. >>> >>>>>> Either Pablo or me could help there. >>> >>>>>> >>> >>>>>> Thanks, >>> >>>>>> Max >>> >>>>>> >>> >>>>>> On 25.04.19 04:14, Reza Ardeshir Rokni wrote: >>> >>>>>>> Pablo, Kenneth and I have a new blog ready for publication which >>> covers >>> >>>>>>> how to create a "looping timer" it allows for default values to >>> be >>> >>>>>>> created in a window when no incoming elements exists. We just >>> need to >>> >>>>>>> clear a few bits before publication, but would be great to have >>> that >>> >>>>>>> also include a python example, I wrote it in java... >>> >>>>>>> >>> >>>>>>> Cheers >>> >>>>>>> >>> >>>>>>> Reza >>> >>>>>>> >>> >>>>>>> On Thu, 25 Apr 2019 at 04:34, Reuven Lax <re...@google.com >>> >>>>>>> <mailto:re...@google.com>> wrote: >>> >>>>>>> >>> >>>>>>> Well state is still not implemented for merging windows >>> even for >>> >>>>>>> Java (though I believe the idea was to disallow ValueState >>> there). >>> >>>>>>> >>> >>>>>>> On Wed, Apr 24, 2019 at 1:11 PM Robert Bradshaw < >>> rober...@google.com >>> >>>>>>> <mailto:rober...@google.com>> wrote: >>> >>>>>>> >>> >>>>>>> It was unclear what the semantics were for ValueState >>> for merging >>> >>>>>>> windows. (It's also a bit weird as it's inherently a >>> race condition >>> >>>>>>> wrt element ordering, unlike Bag and CombineState, >>> though you can >>> >>>>>>> always implement it as a CombineState that always >>> returns the latest >>> >>>>>>> value which is a bit more explicit about the dangers >>> here.) >>> >>>>>>> >>> >>>>>>> On Wed, Apr 24, 2019 at 10:08 PM Brian Hulette >>> >>>>>>> <bhule...@google.com <mailto:bhule...@google.com>> >>> wrote: >>> >>>>>>> > >>> >>>>>>> > That's a great idea! I thought about this too after >>> those >>> >>>>>>> posts came up on the list recently. I started to look >>> into it, >>> >>>>>>> but I noticed that there's actually no implementation >>> of >>> >>>>>>> ValueState in userstate. Is there a reason for that? I >>> started >>> >>>>>>> to work on a patch to add it but I was just curious if >>> there was >>> >>>>>>> some reason it was omitted that I should be aware of. >>> >>>>>>> > >>> >>>>>>> > We could certainly replicate the example without >>> ValueState >>> >>>>>>> by using BagState and clearing it before each write, >>> but it >>> >>>>>>> would be nice if we could draw a direct parallel. >>> >>>>>>> > >>> >>>>>>> > Brian >>> >>>>>>> > >>> >>>>>>> > On Fri, Apr 12, 2019 at 7:05 AM Maximilian Michels >>> >>>>>>> <m...@apache.org <mailto:m...@apache.org>> wrote: >>> >>>>>>> >> >>> >>>>>>> >> > It would probably be pretty easy to add the >>> corresponding >>> >>>>>>> code snippets to the docs as well. >>> >>>>>>> >> >>> >>>>>>> >> It's probably a bit more work because there is no >>> section >>> >>>>>>> dedicated to >>> >>>>>>> >> state/timer yet in the documentation. Tracked here: >>> >>>>>>> >> https://jira.apache.org/jira/browse/BEAM-2472 >>> >>>>>>> >> >>> >>>>>>> >> > I've been going over this topic a bit. I'll add >>> the >>> >>>>>>> snippets next week, if that's fine by y'all. >>> >>>>>>> >> >>> >>>>>>> >> That would be great. The blog posts are a great >>> way to get >>> >>>>>>> started with >>> >>>>>>> >> state/timers. >>> >>>>>>> >> >>> >>>>>>> >> Thanks, >>> >>>>>>> >> Max >>> >>>>>>> >> >>> >>>>>>> >> On 11.04.19 20:21, Pablo Estrada wrote: >>> >>>>>>> >> > I've been going over this topic a bit. I'll add >>> the >>> >>>>>>> snippets next week, >>> >>>>>>> >> > if that's fine by y'all. >>> >>>>>>> >> > Best >>> >>>>>>> >> > -P. >>> >>>>>>> >> > >>> >>>>>>> >> > On Thu, Apr 11, 2019 at 5:27 AM Robert Bradshaw >>> >>>>>>> <rober...@google.com <mailto:rober...@google.com> >>> >>>>>>> >> > <mailto:rober...@google.com <mailto: >>> rober...@google.com>>> >>> >>>>>>> wrote: >>> >>>>>>> >> > >>> >>>>>>> >> > That's a great idea! It would probably be >>> pretty easy >>> >>>>>>> to add the >>> >>>>>>> >> > corresponding code snippets to the docs as >>> well. >>> >>>>>>> >> > >>> >>>>>>> >> > On Thu, Apr 11, 2019 at 2:00 PM Maximilian >>> Michels >>> >>>>>>> <m...@apache.org <mailto:m...@apache.org> >>> >>>>>>> >> > <mailto:m...@apache.org <mailto: >>> m...@apache.org>>> wrote: >>> >>>>>>> >> > > >>> >>>>>>> >> > > Hi everyone, >>> >>>>>>> >> > > >>> >>>>>>> >> > > The Python SDK still lacks documentation >>> on state >>> >>>>>>> and timers. >>> >>>>>>> >> > > >>> >>>>>>> >> > > As a first step, what do you think about >>> updating >>> >>>>>>> these two blog >>> >>>>>>> >> > posts >>> >>>>>>> >> > > with the corresponding Python code? >>> >>>>>>> >> > > >>> >>>>>>> >> > > >>> >>>>>>> >>> https://beam.apache.org/blog/2017/02/13/stateful-processing.html >>> >>>>>>> >> > > >>> >>>>>>> >>> https://beam.apache.org/blog/2017/08/28/timely-processing.html >>> >>>>>>> >> > > >>> >>>>>>> >> > > Thanks, >>> >>>>>>> >> > > Max >>> >>>>>>> >> > >>> >>>>>>> >>> >> -- This email may be confidential and privileged. 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