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From Aljoscha Krettek <aljos...@apache.org>
Subject Re: Event-time and first watermark
Date Fri, 04 Aug 2017 10:21:44 GMT

How are you defining the watermark, i.e. what kind of watermark extractor are you using?


> On 3. Aug 2017, at 17:45, Gwenhael Pasquiers <gwenhael.pasquiers@ericsson.com>
> We're not using a Window but a more basic ProcessFunction to handle sessions. We made
this choice because we have to handle (millions of) sessions that can last from 10 seconds
to 24 hours so we wanted to handle things manually using the State class.
> We're using the watermark as an event-time "clock" to:
> * compute "lateness" of a message relatively to the watermark (most recent message from
the stream)
> * fire timer events 
> We're using event-time instead of processing time because our stream will be late and
data arrive by hourly bursts.
> Maybe we're misusing the watermark ?
> B.R.
> -----Original Message-----
> From: Nico Kruber [mailto:nico@data-artisans.com] 
> Sent: jeudi 3 août 2017 16:30
> To: user@flink.apache.org
> Cc: Gwenhael Pasquiers <gwenhael.pasquiers@ericsson.com>
> Subject: Re: Event-time and first watermark
> Hi Gwenhael,
> "A Watermark(t) declares that event time has reached time t in that stream, meaning that
there should be no more elements from the stream with a timestamp t’ <= t (i.e. events
with timestamps older or equal to the watermark)." [1]
> Therefore, they should be behind the actual event with timestamp t.
> What is it that you want to achieve in the end? What do you want to use the watermark
for? They are basically a means to defining when an event time window ends.
> Nico
> [1] https://ci.apache.org/projects/flink/flink-docs-release-1.2/dev/
> event_time.html#event-time-and-watermarks
> On Thursday, 3 August 2017 10:24:35 CEST Gwenhael Pasquiers wrote:
>> Hi,
>> From my tests it seems that the initial watermark value is 
>> Long.MIN_VALUE even though my first data passed through the timestamp 
>> extractor before arriving into my ProcessFunction. It looks like the 
>> watermark "lags" behind the data by one message.
>> Is there a way to have a watermark more "up to date" ? Or is the only 
>> way to compute it myself into my ProcessFunction ?
>> Thanks.

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