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From Hequn Cheng <chenghe...@gmail.com>
Subject Re: Flink Checkpoint Barrier in case of Join
Date Wed, 03 Oct 2018 03:01:33 GMT
Hi Anil,

The join operator behaviors same as other operators.
When a non-source task receives a barrier from one of its inputs, it blocks
that input until it receives a barrier from all inputs. When barriers have
been received from all the inputs, the task takes a snapshot of its current
state and broadcasts the barrier to its outputs. Then, the task unblocks
its input channels to continue its computation[1].

Best, Hequn

On Wed, Oct 3, 2018 at 5:31 AM Anil <anilsingh.jsr@gmail.com> wrote:

> I'm trying to understand when will Flink's Stream Barrier (for checkpoint)
> be
> emitted by the join operator.
> Consider a query like -
> select * from  stream_1 a1 INNER JOIN  stream_2 a2 on a2.orderId =
> a1.orderId group by HOP(a1.proctime, INTERVAL '1' HOUR, INTERVAL '1' DAY),
> a1.restaurantId
> Since I'm using a Hopping window on 1 day here, Flink will have to cache my
> entire 1 day events.
> The join operator will receive stream barrier from the previous operator.
> Join operator will emit one stream barrier but I'm not sure on what basis
> and when will it be emitted.
> Any help will be appreciated. Thanks!
> From Flink's documentation -
> ```A core element in Flink’s distributed snapshotting are the stream
> barriers. These barriers are injected into the data stream and flow with
> the
> records as part of the data stream.  The point where the barriers for
> snapshot n are injected (let’s call it Sn) is the position in the source
> stream up to which the snapshot covers the data. The barriers then flow
> downstream. When an intermediate operator has received a barrier for
> snapshot n from all of its input streams, it emits a barrier for snapshot n
> into all of its outgoing streams. Once a sink operator (the end of a
> streaming DAG) has received the barrier n from all of its input streams, it
> acknowledges that snapshot n to the checkpoint coordinator. After all sinks
> have acknowledged a snapshot, it is considered completed.```
> --
> Sent from:
> http://apache-flink-user-mailing-list-archive.2336050.n4.nabble.com/

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