Flink uses watermarks to indicate when a stream has become complete up through some point in time. Various operations on streams wait for watermarks in order to know when they can safely stop waiting for further input, and so go ahead and produce their results. These operations include event-time windowing, interval and temporal joins, pattern matching, and sorting (by timestamp).
Events that are late have timestamps less than equal to the current watermark. They have missed their chance to influence the results of those operations that rely on watermarks for triggering. But otherwise, Flink doesn't care if events are late or not. It's not that late events are automatically dropped in all circumstances -- it's just that these temporal operations won't wait long enough to accommodate their extreme out-of-order-ness (lateness). So yes, your ALL_EVENTS view will contain all of the events, including late ones. When your job starts running, it takes some time for an initial watermark to be produced. During that period of time, the current watermark is NULL, and no events will be considered late. Hope this helps clarify things. Regards, David On Sat, Feb 12, 2022 at 12:01 AM M Singh <mans2si...@yahoo.com> wrote: > I thought a little more about your references Martijn and wanted to > confirm one thing - the table is specifying the watermark and the > downstream view needs to check if it wants all events or only the non-late > events. Please let my understanding is correct. > > Thanks again for your references. > > Mans > > On Friday, February 11, 2022, 05:02:49 PM EST, M Singh < > mans2si...@yahoo.com> wrote: > > > > Hi Martijn: > > Thanks for the reference. > > My understanding was that if we use watermark then any event with event > time (in the above example) < event_time - 30 seconds will be dropped > automatically. > > My question [1] is will the downstream (ALL_EVENTS) view which is > selecting the events from the table receive events which are late ? If > late events are dropped at the table level then do we still need the second > predicate check (ts > CURRENT_WATERMARK(ts)) to filter out late events at > the view level. > > If the table does not drop late events, then will all downstream views/etc > need to add this check (ts > CURRENT_WATERMARK(ts)) ? > > I am still not clear on this concept of whether downstream view need to > check for late events with this predicate or will they never receive late > events. > > Thanks again for your time. > > > On Friday, February 11, 2022, 01:55:09 PM EST, Martijn Visser < > mart...@ververica.com> wrote: > > > Hi, > > There's a Flink SQL Cookbook recipe on CURRENT_WATERMARK, I think this > would cover your questions [1]. > > Best regards, > > Martijn > > [1] > https://github.com/ververica/flink-sql-cookbook/blob/main/other-builtin-functions/03_current_watermark/03_current_watermark.md > > On Fri, 11 Feb 2022 at 16:45, M Singh <mans2si...@yahoo.com> wrote: > > Hi: > > The flink docs ( > https://nightlies.apache.org/flink/flink-docs-master/docs/dev/table/functions/systemfunctions/) > indicates that the CURRENT_WATERMARK(rowtime) can return null: > > Note that this function can return NULL, and you may have to consider > this case. For example, if you want to filter out late data you can use: > > WHERE > CURRENT_WATERMARK(ts) IS NULL > OR ts > CURRENT_WATERMARK(ts) > > > I have the following questions that if the table is defined with a > watermark eg: > > CREATE TABLE `MYEVENTS` ( > `name` STRING, > `event_time` TIMESTAMP_LTZ(3), > ... > WATERMARK FOR event_time AS event_time - INTERVAL '30' SECONDS) > WITH (...) > > > 1. If we define the water mark as above, will the late events still be > propagated to a view or table which is selecting from MYEVENTS table: > > CREATE TEMPORARY VIEW `ALL_EVENTS` AS > SELECT * FROM MYEVENTS; > > 2. Can CURRENT_WATERMARK(event_time) still return null ? If so, what are > the conditions for returning null ? > > > > Thanks > > > >
