Flink: State accumulation during restoring from a savepoint

This ticket captures my findings when restoring a BEAM job from a savepoint on a Flink runner.

 

*The problem*

When job is restored from a savepoint taken a few hours ago than we see that checkpoint size starts growing rediciously high which leads to the job running out of heap space error. We use filesystem state backend, which keeps state on the heap.  

 

*Job structure*

Job has two paths the data lake path and the aggregate data path. 

*Data lake path*

Data lake path is a dumb sink of all records received by the job. The records are flushed to S3.

Datalake trigger:
```

input.apply( WithTimestamps.of(extractTimestamp).withAllowedTimestampSkew(standardDays(7)))
.apply(Window.into(FixedWindows.of(standardMinutes(1)))

.triggering(
AfterWatermark.pastEndOfWindow()

.withLateFirings(AfterProcessingTime.pastFirstElementInPane().plusDelayOf(lateFiringsPeriod))

)
.withAllowedLateness(standardMinutes(30), FIRE_ALWAYS)

.discardingFiredPanes()); <-- IMPORTANT
```

 

*Aggregate path*

Aggregate path has some group by key, count etc transformations 

Aggregate trigger:
```

input.apply( WithTimestamps.of(extractTimestamp).withAllowedTimestampSkew(standardDays(7)))

.apply(Window.into(FixedWindows.of(WINDOW_SIZE))
.triggering(

AfterWatermark.pastEndOfWindow()
.withLateFirings(AfterProcessingTime.pastFirstElementInPane().plusDelayOf(lateFiringsPeriod))

)
.withAllowedLateness(standardMinutes(30), FIRE_ALWAYS)

.accumulatingFiredPanes());
```

 

*My investigation*

Our team has written a tool to collect input watermarks from the Flink API. It turned out that it's a common situation that for pretty much every operator some sub operators (running on particular Flink slot) are running slower thus watermark is falling behind other slots. Look at the graph below: 

!Screen Shot 2019-01-18 at 12.07.03 copy.png!

Y axis represents an event time, X wall clock time. Graph shows input watermarks for some operator in the job. Each line represents an input watermark for a specific slot (job runs with parallelism 8) for which operator is running on.

At 17:55 the difference between slowest slot and quickest one is already 20 mins. This means that 20 mins of data will be buffered in memory until slowest slot's watermark will cross the end of the 1 minute window of the buffered data. 

Unfortunately it's very hard to tell why some slots are doing better then the other( I believe data is properly balanced when it comes to hashing etc)  

 

Below I present checkpoint size growing in size. Normally when not restoring from a job checkpoint size is around 3 GB

!Screen Shot 2019-01-18 at 11.38.02.png!

 

*My findings*

If the state is being accumulated because of watermark slowing down on some operator slots (Flink specific) than introducing *early firings* *with discarding mode* should help ... and indeed helped.

 

*My worry is that introducing an early firings as a way to avoid OOM error during catchup seems hacky to me. The other downside is that early firings introduce speculative results which might not be acceptable in some cases.*

 

*Setup:*
* Job reads records from 32 Kinesis shards.
* Job parallelism 8
* Running on Beam 2.7 Flink 1.5
*Hardware:*
** Master:1 x m5.xlarge
** Core instances: 5 x r4.2xlarge
* *YARN session configuration:*
**
```

/usr/bin/flink run --class streaming.Main -m yarn-cluster --yarnstreaming --yarnjobManagerMemory 6272
--yarntaskManagerMemory 26000 -yD classloader.resolve-order=parent-first -yD parallelism.default=8 -yD
containerized.heap-cutoff-ratio=0.15 -yD state.backend=filesystem -yD yarn.maximum-failed-containers=-1
-yD jobmanager.web.checkpoints.history=1000 -yD akka.ask.timeout=60s -XX:GCLogFileSize=20M -XX:NumberOfGCLogFiles=2
-XX:+PrintGCDetails -XX:+PrintGCTimeStamps -XX:+PrintTenuringDistribution -XX:+PrintGCCause -XX:+PrintGCDateStamps
-XX:+UseG1GC /home/hadoop/job.jar --runner=FlinkRunner --awsRegion=eu-west-1 --appName=XXX --input=kinesis://XXX
--outputFileSystemType=S3 --outputFileSystemRoot=XXX --outputDirectory=structured-streaming --externalizedCheckpointsEnabled=true
--checkpointingInterval=300000 --checkpointTimeoutMillis=360000 --failOnCheckpointingErrors=false --minPauseBetweenCheckpoints=60000
--parallelism=8
```

 
 

 

Imported from Jira [BEAM-6465](https://issues.apache.org/jira/browse/BEAM-6465). Original Jira may contain additional context.
Reported by: pawelbartoszek.