On Mon, Dec 4, 2017 at 3:19 PM, Eugene Kirpichov <[email protected]> wrote: > Hi, > > After a recent investigation of a data loss bug caused by unintuitive > behavior of some kinds of triggers, we had a discussion about how we can > protect against future issues like this, and I summarized it in > https://issues.apache.org/jira/browse/BEAM-3288 . Copying here: > > Current Beam trigger semantics are rather confusing and in some cases > extremely unsafe, especially if the pipeline includes multiple chained GBKs. > One example of that is https://issues.apache.org/jira/browse/BEAM-3169 . > > There's multiple issues: > > The API allows users to specify terminating top-level triggers (e.g. > "trigger a pane after receiving 10000 elements in the window, and that's > it"), but experience from user support shows that this is nearly always a > mistake and the user did not intend to drop all further data. > > In general, triggers are the only place in Beam where data is being dropped > without making a lot of very loud noise about it - a practice for which the > PTransform style guide uses the language: "never, ever, ever do this". > > Continuation triggers are still worse. For context: continuation trigger is > the trigger that's set on the output of a GBK and controls further > aggregation of the results of this aggregation by downstream GBKs. The > output shouldn't just use the same trigger as the input, because e.g. if the > input trigger said "wait for an hour before emitting a pane", that doesn't > mean that we should wait for another hour before emitting a result of > aggregating the result of the input trigger. Continuation triggers try to > simulate the behavior "as if a pane of the input propagated through the > entire pipeline", but the implementation of individual continuation triggers > doesn't do that. E.g. the continuation of "first N elements in pane" trigger > is "first 1 element in pane", and if the results of a first GBK are further > grouped by a second GBK onto more coarse key (e.g. if everything is grouped > onto the same key), that effectively means that, of the keys of the first > GBK, only one survives and all others are dropped (what happened in the data > loss bug). > > The ultimate fix to all of these things is > https://s.apache.org/beam-sink-triggers . However, it is a huge model > change, and meanwhile we have to do something. The options are, in order of > increasing backward incompatibility (but incompatibility in a "rejecting > something that previously was accepted but extremely dangerous" kind of > way): > > Make the continuation trigger of most triggers be the "always-fire" trigger. > Seems that this should be the case for all triggers except the watermark > trigger. This will definitely increase safety, but lead to more eager firing > of downstream aggregations. It also will violate a user's expectation that a > fire-once trigger fires everything downstream only once, but that > expectation appears impossible to satisfy safely.
Note that firing more often for multiply stacked triggers, especially in the case of accumulating mode, easily leads to data corruption bugs where elements are duplicated and/or overcounted. I suppose that bad data is slightly easier to detect than missing data, but automatically turning once triggers into more-than-once triggers (automatically, or manually by prohibiting the former) isn't a straightforward fix. > Make the continuation trigger of some triggers be the "invalid" trigger, > i.e. require the user to set it explicitly: there's in general no good and > safe way to infer what a trigger on a second GBK "truly" should be, based on > the trigger of the PCollection input into a first GBK. This is especially > true for terminating triggers. I think this should be on the table until we come up with good semantics, which may not be until sink triggers. It's backwards incompatible, but in a bug-fixing and verbose way (as opposed silently changing semantics/behavior/outputs with the first option). Note that sometimes for composite operations one does not have access to the second grouping operation, so we must at least provide an opt-out flag. > Prohibit top-level terminating triggers entirely. This will ensure that the > only data that ever gets dropped is "droppably late" data. The AfterWatermark.withEarlyFirings(...) is a top-level terminating trigger that we should keep allowing, and has perfectly reasonable continuation semantics. I suppose we could only allow it if allowed lateness is 0, but that makes things even more context sensitive. > Do people think that these options are sensible? > +Kenn Knowles +Thomas Groh +Ben Chambers is this a fair summary of our > discussion? I think this is a fair summary of the discussions we've had. - Robert
