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From Piotr Nowojski <pi...@data-artisans.com>
Subject Re: How to broadcast messages to all task manager instances in cluster?
Date Mon, 14 May 2018 09:06:33 GMT

Thanks for the clarification. This might be though. Generally speaking having such static
configuration shared across multiple operators/functions can pose lots of different problems
including synchronisation, fault tolerance etc. 

To be honest you should treat such thing almost like an external system that has an external
state, because from Flink’s perspective that’s exactly what it is - it’s an equivalent
to having an external “configuration service” hosted/stored somewhere outside of Flink.
With it you have to manually take care of fault tolerance (especially it’s state), since
it’s outside of Flink’s control. Especially think about what should happen to your static
configuration if one of your machine fails/restarts, and Flink chooses to restart only part
of the job graph (possible one, many or all of the operators). How will your static configuration
be kept in sync across all of the Task Managers in that case?

It would be easier if you could restructure your job/problem and replace such static configuration
with a configuration stored in the Flink’s state (maybe in one operator? Or on parallel
instances of one task?). Otherwise to make it fully reliable I think you would need to write
quite a lot of code on your own. 

Alternatively you can consider using some third party systems for storing a configuration
like Apache ZooKeeper.


> On 13 May 2018, at 10:38, Di Tang <tangdi.bupt@gmail.com> wrote:
> Thanks Piotr for the response. I have many data streams dependant on the configuration
by getting value from static variables in a class. The way the configuration change works
is to change the static variables' value in the class. Since each task manager only has one
JVM process, as long as the message is broadcast to each task manager, the data streams will
see the change. The logic in data streams is quite simple, just get some parameters from the
static variable. So I think to add connect and flatmap to each of them is too verbose. I am
wondering is there any better way to express.
> Piotr Nowojski <piotr@data-artisans.com <mailto:piotr@data-artisans.com>>
于 2018年5月11日周五 下午7:31写道:
> Hi,
> I don’t quite understand your problem. If you broadcast message as an input to your
operator that depends on this configuration, each instance of your operator will receive this
configuration. It shouldn't matter whether Flink scheduled your operator on one, some or all
of the TaskManagers. It only should matter if operators running your configuration sensitive
code receive the broadcasted message.
> DataStream<> input = xxx;
> DataStream<> controlConfigInput = yyy;
> DataStream<> data = input.
> 	.do()
> 	.something()
> 	.fancy();
> controlConfigInput.broadcast()
> 	.connect(data)
> 	.flatMap(new MyFancyOperatorThatDependsOnConfigStream())
> Or slide 36 from here: https://www.slideshare.net/dataArtisans/apache-flink-datastream-api-basics
> Piotrek
>> On 11 May 2018, at 11:11, Di Tang <tangdi.bupt@gmail.com <mailto:tangdi.bupt@gmail.com>>
>> Hi guys:
>> I have a Flink job which contains multiple pipelines. Each pipeline depends on some
configuration. I want to make the configuration dynamic and effective after change so I created
a data source which periodically poll the database storing the configuration. However, how
can I broadcast the events to all task manager instances?  The datastream.broadcast() only
applies to the parallel instances of operator. And I don't want to connect the configuration
data source to each pipeline because it is too verbose. If Flink cannot explicitly broadcast
messages to task managers, is there any method to guarantee the parallel operator is distributed
on all task managers?
>> Thanks,
>> Di

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