Re: Broadcast state before events stream consumption

Posted by Konstantin Knauf-2 on
URL: http://deprecated-apache-flink-user-mailing-list-archive.369.s1.nabble.com/Broadcast-state-before-events-stream-consumption-tp25978p26024.html

Hi Chirag, Hi Vadim, 

from the top of my head, I see two options here: 

* Buffer the "fast" stream inside the KeyedBroadcastProcessFunction until relevant (whatever this means in your use case) broadcast events have arrived. Advantage: operationally easy, events are emitted as early as possible. Disadvantage: state size might become very large, depending on the nature of the broadcast stream it might be hard to know, when the "relevant broadcast events have arrived".

* Start your job and only consume the broadcast stream (by configuration). Once the stream is "fully processed", i.e. has caught up, take a savepoint. Finally, start the job from this savepoint with the correct "fast" stream. There is a small race condition between taking the savepoint and restarting the job, which might matter (or not) depending on your use case. 

This topic is related to event-time alignment in sources, which has been actively discussed in the community in the past and we might be able to solve this in a similar way in the future. 

Cheers, 

Konstantin

On Fri, Feb 8, 2019 at 5:48 PM Chirag Dewan <[hidden email]> wrote:

Hi Vadim,

I would be interested in this too. 

Presently, I have to read my lookup source in the open method and keep it in a cache. By doing that I cannot make use of the broadcast state until ofcourse the first emit comes on the Broadcast stream.

The problem with waiting the event stream is the lack of knowledge that I have read all the data from the lookup source. There is no possibility of having a special marker in the data as well for my use case.

So pre loading the data seems to be the only option right now.

Thanks,

Chirag

On Friday, 8 February, 2019, 7:45:37 pm IST, Vadim Vararu <[hidden email]> wrote:

Hi all,

I need to use the broadcast state mechanism (https://ci.apache.org/projects/flink/flink-docs-stable/dev/stream/state/broadcast_state.html) for the next scenario.

I have a reference data stream (slow) and an events stream (fast running) and I want to do a kind of lookup in the reference stream for each

event. The broadcast state mechanism seems to fit perfect the scenario. 

From documentation:

As an example where broadcast state can emerge as a natural fit, one can imagine a low-throughput stream containing a set of rules which we want to evaluate against all elements coming from another stream.

However, I am not sure what is the correct way to delay the consumption of the fast running stream until the slow one is fully read (in case of a file) or until a marker is emitted (in case of some other source). Is there any way to accomplish that? It doesn't seem to be a rare use case.

Thanks, Vadim.

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