http://flink-forward.org/kb_sessions/no-shard-left-behind-dynamic-work-rebalancing-in-apache-beam/ The Apache Beam (incubating) programming model is designed to support several advanced data processing features such as autoscaling and dynamic work rebalancing. In this talk, we will first explain how dynamic work rebalancing not only provides a general and robust solution to the problem of stragglers in traditional data processing pipelines, but also how it allows autoscaling to be truly effective. We will then present how dynamic work rebalancing works as implemented in Google Cloud Dataflow and which path other Apache Beam runners link Apache Flink can follow to benefit from it.

1. Confidential + Proprietary
No shard left behind
Dynamic Work Rebalancing
and other adaptive features in
Apache Beam (incubating)
Malo Denielou malo@google.com

2. Apache Beam is a unified
programming model
designed to provide
efficient and portable
data processing pipelines.

3. Apache Beam (incubating)
1. The Beam Programming Model
2. SDKs for writing Beam pipelines -- starting with Java
3. Runners for existing distributed processing
backends
○ Apache Flink (thanks to data Artisans)
○ Apache Spark (thanks to Cloudera and PayPal)
○ Google Cloud Dataflow (fully managed service)
○ Local runner for testing Beam Model: Fn Runners
Apache
Flink
Apache
Spark
Beam Model: Pipeline Construction
Other
LanguagesBeam Java
Beam
Python
Execution Execution
Cloud
Dataflow
Execution

4. Apache Beam abstractions
PCollection – a parallel collection of timestamped elements that are in windows.
Sources & Readers – produce PCollections of timestamped elements and a watermark.
ParDo – flatmap over elements of a PCollection.
(Co)GroupByKey – shuffle & group {{K: V}} → {K: [V]}.
Side inputs – global view of a PCollection used for broadcast / joins.
Window – reassign elements to zero or more windows; may be data-dependent.
Triggers – user flow control based on window, watermark, element count, lateness.

5. 1.Classic Batch 2. Batch with Fixed
Windows
3. Streaming
5. Streaming With
Retractions
4. Streaming with
Speculative + Late Data
6. Streaming With
Sessions

6. Data processing for realistic workloads
Workload
Time
Streaming pipelines have variable input Batch pipelines have stages of different sizes

7. The curse of configuration
Workload
Time
Workload
Time
Over-provisioning resources? Under-provisioning on purpose?
A considerable effort is spent to finely tune all the parameters of the jobs.

8. Ideal case
Workload
Time
A system that adapts.

9. The straggler problem in batchWorkers
Time
Tasks do not finish evenly on the workers.
● Data is not evenly distributed among tasks
● Processing time is uneven between tasks
● Runtime constraints
Effects are cumulative per stage!

10. Common straggler mitigation techniques
● Split files into equal sizes?
● Pre-emptively over split?
● Detect slow workers and reexecute?
● Sample the data and split based on partial execution
All have major costs, but do not solve completely the problem.
Workers
Time

11. No amount of upfront heuristic tuning (be it
manual or automatic) is enough to guarantee
good performance: the system will always hit
unpredictable situations at run-time.
A system that's able to dynamically adapt and
get out of a bad situation is much more
powerful than one that heuristically hopes to
avoid getting into it.

12. Beam abstractions empower runners
A bundle is group of elements of a PCollection processed and committed together.
APIs (ParDo/DoFn):
• startBundle()
• processElement() n times
• finishBundle()
Streaming runner:
• small bundles, low-latency pipelining across stages, overhead of frequent commits.
Classic batch runner:
• large bundles, fewer large commits, more efficient, long synchronous stages.
Other runner strategies may strike a different balance.

13. Beam abstractions empower runners
Efficiency at runner’s discretion
“Read from this source, splitting it 1000 ways”
➔ user decides
“Read from this source”
➔ runner decides
APIs:
• long getEstimatedSize()
• List<Source> splitIntoBundles(size)

14. Beam abstractions empower runners
Efficiency at runner’s discretion
“Read from this source, splitting it 1000 ways”
➔ user decides
“Read from this source”
➔ runner decides
APIs:
• long getEstimatedSize()
• List<Source> splitIntoBundles(size)
Runner
Source
gs://logs/*
Size?

15. Beam abstractions empower runners
Efficiency at runner’s discretion
“Read from this source, splitting it 1000 ways”
➔ user decides
“Read from this source”
➔ runner decides
APIs:
• long getEstimatedSize()
• List<Source> splitIntoBundles(size)
Runner
Source
gs://logs/*
50TB

16. Beam abstractions empower runners
Efficiency at runner’s discretion
“Read from this source, splitting it 1000 ways”
➔ user decides
“Read from this source”
➔ runner decides
APIs:
• long getEstimatedSize()
• List<Source> splitIntoBundles(size)
Runner
(cluster utilization, quota,
bandwidth, throughput,
concurrent stages, …)
Source
gs://logs/*
Split in
chunks of
500GB

17. Beam abstractions empower runners
Efficiency at runner’s discretion
“Read from this source, splitting it 1000 ways”
➔ user decides
“Read from this source”
➔ runner decides
APIs:
• long getEstimatedSize()
• List<Source> splitIntoBundles(size)
Runner
Source
gs://logs/*
List<Source>

18. Solving the straggler problem: Dynamic Work Rebalancing

19. Solving the straggler problem: Dynamic Work Rebalancing
Workers
Time
Done work Active work Predicted completion
Now Average

20. Solving the straggler problem: Dynamic Work Rebalancing
Workers
Time
Done work Active work Predicted completion
Now Average

21. Solving the straggler problem: Dynamic Work Rebalancing
Workers
Time
Done work Active work Predicted completion
Now Average
Workers
Time
Now Average

22. Solving the straggler problem: Dynamic Work Rebalancing
Workers
Time
Done work Active work Predicted completion
Now Average
Workers
Time
Now Average

23. Dynamic Work Rebalancing in the wild
A classic MapReduce job (read from Google Cloud Storage,
GroupByKey, write to Google Cloud Storage), 400 workers.
Dynamic Work Rebalancing disabled to demonstrate
stragglers.
X axis: time (total ~20min.); Y axis: workers

24. Dynamic Work Rebalancing in the wild
A classic MapReduce job (read from Google Cloud Storage,
GroupByKey, write to Google Cloud Storage), 400 workers.
Dynamic Work Rebalancing disabled to demonstrate
stragglers.
X axis: time (total ~20min.); Y axis: workers
Same job,
Dynamic Work Rebalancing enabled.
X axis: time (total ~15min.); Y axis: workers

25. Dynamic Work Rebalancing in the wild
A classic MapReduce job (read from Google Cloud Storage,
GroupByKey, write to Google Cloud Storage), 400 workers.
Dynamic Work Rebalancing disabled to demonstrate
stragglers.
X axis: time (total ~20min.); Y axis: workers
Same job,
Dynamic Work Rebalancing enabled.
X axis: time (total ~15min.); Y axis: workers
Savings

26. Dynamic Work Rebalancing with Autoscaling
Initial allocation of 80
workers, based on size
Multiple rounds of
upsizing, enabled by
dynamic work
rebalancing
Upscales to 1000
workers.
● tasks are balanced
● no oversplitting or
manual tuning

27. Apache Beam enable dynamic adaptation
Beam Readers provide simple progress signals, which enable runners to take action based on
execution-time characteristics.
All Beam runners can implement autoscaling and Dynamic Work Rebalancing.
APIs for how much work is pending.
• bounded: double getFractionConsumed()
• unbounded: long getBacklogBytes()
APIs for splitting:
• bounded:
• Source splitAtFraction(double)
• int getParallelismRemaining()

28. Apache Beam is a unified
programming model
designed to provide
efficient and portable
data processing pipelines.

29. To learn more
Read our blog posts!
• No shard left behind: Dynamic work rebalancing in Google Cloud Dataflow
https://cloud.google.com/blog/big-data/2016/05/no-shard-left-behind-dyna
mic-work-rebalancing-in-google-cloud-dataflow
• Comparing Cloud Dataflow autoscaling to Spark and Hadoop
https://cloud.google.com/blog/big-data/2016/03/comparing-cloud-dataflow-
autoscaling-to-spark-and-hadoop
Join the Apache Beam community!
https://beam.incubator.apache.org/