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From Radu Tudoran <radu.tudo...@huawei.com>
Subject FW: What's the advantage of using BroadcastState?
Date Tue, 28 Aug 2018 09:32:43 GMT
Thanks. For the explanation – I was suspected it might be like this and I wanted to double
check before building inconsistent programs ☺)

Would it be interesting for the community to have also something that would also be able to
share/broadcast items from one task to the other tasks. Spark for example has this as the
broadcast behavior, and it can be useful for different algorithms and applications. For example
if one build a clustering algorithm running in parallel, then you can have a task updating
the cluster centroids, that would advertise to the others afterwards (just one of the examples).
I am thinking we can achieve this using a combination of the broadcast stream and the iterative
operators. Basically if a task makes an update to the state, this value should be push back
(upstream) such that afterwards it can be broadcasted to all the task using it.

Let me know what you think..

Dr. Radu Tudoran
Staff Research Engineer - Big Data Expert
IT R&D Division

[cid:image007.jpg@01CD52EB.AD060EE0]
HUAWEI TECHNOLOGIES Duesseldorf GmbH
German Research Center
Munich Office
Riesstrasse 25, 80992 München

E-mail: radu.tudoran@huawei.com<mailto:radu.tudoran@huawei.com>
Mobile: +49 15209084330
Telephone: +49 891588344173

HUAWEI TECHNOLOGIES Duesseldorf GmbH
Hansaallee 205, 40549 Düsseldorf, Germany, www.huawei.com<http://www.huawei.com/>
Registered Office: Düsseldorf, Register Court Düsseldorf, HRB 56063,
Managing Director: Bo PENG, Qiuen Peng, Shengli Wang
Sitz der Gesellschaft: Düsseldorf, Amtsgericht Düsseldorf, HRB 56063,
Geschäftsführer: Bo PENG, Qiuen Peng, Shengli Wang
This e-mail and its attachments contain confidential information from HUAWEI, which is intended
only for the person or entity whose address is listed above. Any use of the information contained
herein in any way (including, but not limited to, total or partial disclosure, reproduction,
or dissemination) by persons other than the intended recipient(s) is prohibited. If you receive
this e-mail in error, please notify the sender by phone or email immediately and delete it!

From: Fabian Hueske [mailto:fhueske@gmail.com]
Sent: Tuesday, August 28, 2018 9:53 AM
To: Xingcan Cui <xingcanc@gmail.com<mailto:xingcanc@gmail.com>>
Cc: Radu Tudoran <radu.tudoran@huawei.com<mailto:radu.tudoran@huawei.com>>; user
<user@flink.apache.org<mailto:user@flink.apache.org>>
Subject: Re: What's the advantage of using BroadcastState?

Hi,

Xingcan is right. There is no hidden state synchronization happening.
You have to ensure that the broadcast state is the same at every parallel instance. Hence,
it should only be modified by the processBroadcastElement() method that receives the same
broadcasted elements on all task instance.
The API tries to help users to not violate the contract, however it is not bullet proof. Side-passing
information in a local variable (as suggested by you) cannot be prevented and would lead to
inconsistencies.

Best, Fabian



Am Mo., 27. Aug. 2018 um 16:51 Uhr schrieb Xingcan Cui <xingcanc@gmail.com<mailto:xingcanc@gmail.com>>:
Hi Radu,

I cannot make a full understanding of your question but I guess the answer is NO.

The broadcast state pattern just provides you with an automatic data broadcasting and a bunch
of map states to cache the "low-throughput” patterns. Also, to keep consistency, it forbid
the `processElement()` to modify the states. But this API does not really broadcast the states.
You should keep the logic for `processBraodcastElement()` deterministic. Maybe the equation
below could make the pattern clear.

<identical input> + <deterministic logic> = <identical states> = <broadcast
state>

Best,
Xingcan

On Aug 27, 2018, at 10:23 PM, Radu Tudoran <radu.tudoran@huawei.com<mailto:radu.tudoran@huawei.com>>
wrote:

Hi Fabian,

Thanks for the blog post about broadcast state. I have a question with respect to the update
capabilities of the broadcast state:

Assume you do whatever processing logic in the main processElement function .. and at a given
context marker you 1) would change a local field marker, to 2) signal that next time the broadcast
function is triggered a special pattern should be created and broadcasted.

My question is: is such a behavior allowed? Would the new special Pattern that originates
in an operator be shared across the other instances of the KeyedProcessFunction?


public static class PatternEvaluator
 extends KeyedBroadcastProcessFunction<Long, Action, Pattern, Tuple2<Long, Pattern>>
{
public bolean test = false;

  @Override
  public void processElement(
     Action action,
     ReadOnlyContext ctx,
     Collector<Tuple2<Long, Pattern>> out) throws Exception {

   //…logic

   if (..whatever context) {
      Test = true;
   }

   }
 @Override
 public void processBroadcastElement(
     Pattern pattern,
     Context ctx,
     Collector<Tuple2<Long, Pattern>> out) throws Exception {
   // store the new pattern by updating the broadcast state

 BroadcastState<Void, Pattern> bcState =
     ctx.getBroadcastState(new MapStateDescriptor<>("patterns", Types.VOID, Types.POJO(Pattern.class)));
   // storing in MapState with null as VOID default value
   bcState.put(null, pattern);
   If (test) {
       bcState.put(null, new Pattern(test) );
   }

 }
}


Dr. Radu Tudoran
Staff Research Engineer - Big Data Expert
IT R&D Division

<image001.png>
HUAWEI TECHNOLOGIES Duesseldorf GmbH
German Research Center
Munich Office
Riesstrasse 25, 80992 München

E-mail: radu.tudoran@huawei.com<mailto:radu.tudoran@huawei.com>
Mobile: +49 15209084330
Telephone: +49 891588344173

HUAWEI TECHNOLOGIES Duesseldorf GmbH
Hansaallee 205, 40549 Düsseldorf, Germany, www.huawei.com<http://www.huawei.com/>
Registered Office: Düsseldorf, Register Court Düsseldorf, HRB 56063,
Managing Director: Bo PENG, Qiuen Peng, Shengli Wang
Sitz der Gesellschaft: Düsseldorf, Amtsgericht Düsseldorf, HRB 56063,
Geschäftsführer: Bo PENG, Qiuen Peng, Shengli Wang
This e-mail and its attachments contain confidential information from HUAWEI, which is intended
only for the person or entity whose address is listed above. Any use of the information contained
herein in any way (including, but not limited to, total or partial disclosure, reproduction,
or dissemination) by persons other than the intended recipient(s) is prohibited. If you receive
this e-mail in error, please notify the sender by phone or email immediately and delete it!

From: Fabian Hueske [mailto:fhueske@gmail.com]
Sent: Monday, August 20, 2018 9:40 AM
To: Paul Lam <paullin3280@gmail.com<mailto:paullin3280@gmail.com>>
Cc: Rong Rong <walterddr@gmail.com<mailto:walterddr@gmail.com>>; Hequn Cheng <chenghequn@gmail.com<mailto:chenghequn@gmail.com>>;
user <user@flink.apache.org<mailto:user@flink.apache.org>>
Subject: Re: What's the advantage of using BroadcastState?

Hi,

I've recently published a blog post about Broadcast State [1].

Cheers,
Fabian

[1] https://data-artisans.com/blog/a-practical-guide-to-broadcast-state-in-apache-flink

2018-08-20 3:58 GMT+02:00 Paul Lam <paullin3280@gmail.com<mailto:paullin3280@gmail.com>>:
Hi Rong, Hequn

Your answers are very helpful! Thank you!

Best Regards,
Paul Lam

在 2018年8月19日,23:30,Rong Rong <walterddr@gmail.com<mailto:walterddr@gmail.com>>
写道:

Hi Paul,

To add to Hequn's answer. Broadcast state can typically be used as "a low-throughput stream
containing a set of rules which we want to evaluate against all elements coming from another
stream" [1]
So to add to the difference list is: whether it is "broadcast" across all keys if processing
a keyed stream. This is typically when it is not possible to derive same key field using KeySelector
in CoStream.
Another additional difference is performance: BroadcastStream is "stored locally and is used
to process all incoming elements on the other stream" thus requires to carefully manage the
size of the BroadcastStream.

[1]: https://ci.apache.org/projects/flink/flink-docs-release-1.6/dev/stream/state/broadcast_state.html

On Sun, Aug 19, 2018 at 1:40 AM Hequn Cheng <chenghequn@gmail.com<mailto:chenghequn@gmail.com>>
wrote:
Hi Paul,

There are some differences:
1. The BroadcastStream can broadcast data for you, i.e, data will be broadcasted to all downstream
tasks automatically.
2. To guarantee that the contents in the Broadcast State are the same across all parallel
instances of our operator, read-write access is only given to the broadcast side
3. For BroadcastState, flink guarantees that upon restoring/rescaling there will be no duplicates
and no missing data. In case of recovery with the same or smaller parallelism, each task reads
its checkpointed state. Upon scaling up, each task reads its own state, and the remaining
tasks (p_new-p_old) read checkpoints of previous tasks in a round-robin manner. While MapState
doesn't have such abilities.

Best, Hequn

On Sun, Aug 19, 2018 at 11:18 AM, Paul Lam <paullin3280@gmail.com<mailto:paullin3280@gmail.com>>
wrote:
Hi,

AFAIK, the difference between a BroadcastStream and a normal DataStream is that the BroadcastStream
is with a BroadcastState, but it seems that the functionality of BroadcastState can also be
achieved by MapState in a CoMapFunction or something since the control stream is still broadcasted
without being turned into BroadcastStream. So, I’m wondering what’s the advantage of using
BroadcastState? Thanks a lot!

Best Regards,
Paul Lam

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