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From "Tzu-Li (Gordon) Tai" <tzuli...@apache.org>
Subject Re: Flink Checkpoint runs slow for low load stream
Date Tue, 04 Oct 2016 08:25:14 GMT

Helping out here: this is the PR for async Kafka offset committing - https://github.com/apache/flink/pull/2574.
It has already been merged into the master and release-1.1 branches, so you can try out the
changes now if you’d like.
The change should also be included in the 1.1.3 release, which the Flink community is discussing
to release soon.

Will definitely be helpful if you can provide feedback afterwards!

Best Regards,

On October 3, 2016 at 9:40:14 PM, Chakravarthy varaga (chakravarthyvp@gmail.com) wrote:

Hi Stephan,

    Is the Async kafka offset commit released in 1.3.1?


On Wed, Sep 28, 2016 at 9:49 AM, Chakravarthy varaga <chakravarthyvp@gmail.com> wrote:
Hi Stephan,

     That should be great. Let me know once the fix is done and the snapshot version to
use, I'll check and revert then.
     Can you also share the JIRA that tracks the issue?
     With regards to offset commit issue, I'm not sure as to how to proceed here. Probably
I'll use your fix first and see if the problem reoccurs.

Thanks much

On Tue, Sep 27, 2016 at 7:46 PM, Stephan Ewen <sewen@apache.org> wrote:

Flink stores in checkpoints in your case only the Kafka offsets (few bytes) and the custom
state (e).

Here is an illustration of the checkpoint and what is stored (from the Flink docs).

I am quite puzzled why the offset committing problem occurs only for one input, and not for
the other.
I am preparing a fix for 1.2, possibly going into 1.1.3 as well.
Could you try out a snapshot version to see if that fixes your problem?


On Tue, Sep 27, 2016 at 2:24 PM, Chakravarthy varaga <chakravarthyvp@gmail.com> wrote:
Hi Stefan,

     Thanks a million for your detailed explanation. I appreciate it.

     -  The zookeeper bundled with kafka was used to start zookeeper. There is
only 1 instance (standalone) of zookeeper running on my localhost (ubuntu 14.04)
     -  There is only 1 Kafka broker (version: )

     With regards to Flink cluster there's only 1 JM & 2 TMs started with no HA. I
presume this does not use zookeeper anyways as it runs as standalone cluster.

     BTW., The kafka connector version that I use is as suggested in the flink connectors
     Do you see any issues with versions?
     1) Do you have benchmarks wrt., to checkpointing in flink?

     2) There isn't detailed explanation on what states are stored as part of the checkpointing
process. For ex.,  If I have pipeline like source -> map -> keyBy -> map -> sink,
my assumption on what's stored is:
         a) The source stream's custom watermarked records
         b) Intermediate states of each of the transformations in the pipeline
         c) Delta of Records stored from the previous sink
         d) Custom States (SayValueState as in my case) - Essentially this is what
I bother about storing.
         e) All of my operators

      Is my understanding right?

     3) Is there a way in Flink to checkpoint only d) as stated above

     4) Can you apply checkpointing to only streams and certain operators (say I wish
to store aggregated values part of the transformation)

Best Regards

On Mon, Sep 26, 2016 at 6:18 PM, Stephan Ewen <sewen@apache.org> wrote:
Thanks, the logs were very helpful!

TL:DR - The offset committing to ZooKeeper is very slow and prevents proper starting of checkpoints.

Here is what is happening in detail:

  - Between the point when the TaskManager receives the "trigger checkpoint" message and
when the point when the KafkaSource actually starts the checkpoint is a long time (many seconds)
- for one of the Kafka Inputs (the other is fine).
  - The only way this delayed can be introduced is if another checkpoint related operation
(such as trigger() or notifyComplete() ) is still in progress when the checkpoint is started.
Flink does not perform concurrent checkpoint operations on a single operator, to ease the
concurrency model for users.
  - The operation that is still in progress must be the committing of the offsets (to ZooKeeper
or Kafka). That also explains why this only happens once one side receives the first record.
Before that, there is nothing to commit.

What Flink should fix:
  - The KafkaConsumer should run the commit operations asynchronously, to not block the "notifyCheckpointComplete()"

What you can fix:
  - Have a look at your Kafka/ZooKeeper setup. One Kafka Input works well, the other does
not. Do they go against different sets of brokers, or different ZooKeepers? Is the metadata
for one input bad?
  - In the next Flink version, you may opt-out of committing offsets to Kafka/ZooKeeper all
together. It is not important for Flink's checkpoints anyways.


On Mon, Sep 26, 2016 at 5:13 PM, Chakravarthy varaga <chakravarthyvp@gmail.com> wrote:
Hi Stefan,

    Please find my responses below.

    - What source are you using for the slow input?
     [CVP] - Both stream as pointed out in my first mail, are Kafka Streams
  - How large is the state that you are checkpointing?
     [CVP] - I have enabled checkpointing on the StreamEnvironment as below.
         final StreamExecutionEnvironment streamEnv = StreamExecutionEnvironment.getExecutionEnvironment();
         streamEnv.setStateBackend(new FsStateBackend("file:///tmp/flink/checkpoints"));

      In terms of the state stored, the KS1 stream has payload of 100K events/second,
while KS2 have about 1 event / 10 minutes... basically the operators perform flatmaps on 8
fields of tuple (all fields are primitives). If you look at the states' sizes in dashboard
they are in Kb...

  - Can you try to see in the log if actually the state snapshot takes that long, or if it
simply takes long for the checkpoint barriers to travel through the stream due to a lot of
    [CVP] -There are no back pressure atleast from the sample computation in the flink
dashboard. 100K/second is low load for flink's benchmarks. I could not quite get the barriers
vs snapshot state. I have attached the Task Manager log (DEBUG) info if that will interest
     I have attached the checkpoints times' as .png from the dashboard. Basically if you
look at checkpoint IDs 28 & 29 &30- you'd see that the checkpoints take more than
a minute in each case. Before these checkpoints, the KS2 stream did not have any events. As
soon as an event(should be in bytes) was generated, the checkpoints went slow and subsequently
a minute more for every checkpoint thereafter.

   This log was collected from the standalone flink cluster with 1 job manager & 2 TMs.
1 TM was running this application with checkpointing (parallelism=1)
    Please let me know if you need further info.,

On Fri, Sep 23, 2016 at 6:26 PM, Stephan Ewen <sewen@apache.org> wrote:

Let's try to figure that one out. Can you give us a bit more information?

  - What source are you using for the slow input?
  - How large is the state that you are checkpointing?
  - Can you try to see in the log if actually the state snapshot takes that long, or if it
simply takes long for the checkpoint barriers to travel through the stream due to a lot of


On Fri, Sep 23, 2016 at 3:35 PM, Fabian Hueske <fhueske@gmail.com> wrote:

I'm not so much familiar with the internals of the checkpointing system, but maybe Stephan
(in CC) has an idea what's going on here.

Best, Fabian

2016-09-23 11:33 GMT+02:00 Chakravarthy varaga <chakravarthyvp@gmail.com>:
Hi Aljoscha & Fabian,

    I have a stream application that has 2 stream source as below.

     KeyedStream<String, String> ks1 = ds1.keyBy("*") ;
     KeyedStream<Tuple2<String, V>, String> ks2 = ds2.flatMap(split T into
k-v pairs).keyBy(0);

     //X is a CoFlatMapFunction that inserts and removes elements from ks2 into a key-value
state member. Elements from ks1 are matched against that state. the CoFlatMapFunction operator
maintains ValueState<Tuple2<Long, Long>>;

     //ks1 is streaming about 100K events/sec from kafka topic
     //ks2 is streaming about 1 event every 10 minutes... Precisely when the 1st event
is consumed from this stream, checkpoint takes 2 minutes straight away.

    The version of flink is 1.1.2.

I tried to use checkpoint every 10 Secs using a FsStateBackend... What I notice is that the
checkpoint duration is almost 2 minutes for many cases, while for the other cases it varies
from 100 ms to 1.5 minutes frequently. I'm attaching the snapshot of the dashboard for your

     Is this an issue with flink checkpointing?

 Best Regards

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