Optimizations in Spark; RDD, DataFrame
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The document presents optimizations in Apache Spark, covering key concepts such as the Spark execution model, shuffle operations, and the differences between SQL and RDDs. It discusses partitioning strategies, optimizing transformations, and various types of joins while highlighting the importance of caching and checkpointing for performance. Additionally, it provides references for further reading on these topics.
![map vs mapPartitions
- Map works the function being utilized at a per element level while mapPartitions
exercises the function at the partition level.
- map: Applies a transformation function on each item of the RDD and returns the result
as a new RDD.
- mapPartition: It is called only once for each partition. The entire content of the
respective partitions is available as a sequential stream of values via the input
argument (Iterarator[T]).
- https://stackoverflow.com/questions/21185092/apache-spark-map-vs-mappartitions](https://image.slidesharecdn.com/webinaroptimizationsinspark-200317080627/85/Optimizations-in-Spark-RDD-DataFrame-24-320.jpg)
Optimizations in Spark; RDD, DataFrame
- 1. Optimizations in Apache Spark Presented By: Sarfaraz Hussain Software Consultant Knoldus Inc.
- 2. About Knoldus Knoldus is a technology consulting firm with focus on modernizing the digital systems at the pace your business demands. DevOps Functional. Reactive. Cloud Native
- 3. 01 Spark Execution Model 02 Optimizing Shuffle Operations 03 Optimizing Functions 04 SQL vs RDD 05 Logical & Physical Plan Agenda 06 Optimizing Joins
- 6. ● Two kinds of operations: 1. Transformation 2. Action ● Dependency are divided into two types: 1. Narrow Dependency 2. Wide Dependency ● Stages Spark Execution Model
- 8. DAG
- 10. Narrow Transformation Wide Transformation map cogroup mapValues groupWith flatMap join filter leftOuterJoin mapPartitions rightOuterJoin groupByKey reduceByKey combineByKey distinct intersection repartition coalesce
- 11. Shuffle Operations What is Shuffle? - Shuffles are data transfers between different executors of a Spark cluster.
- 12. Shuffle Operations 1. In which executors the data needs to be sent? 2. How to send the data? GroupByKey
- 13. Shuffle Operations Where to send data? - Partitioner - The partitioner defines how records will be distributed and thus which records will be completed by each task
- 14. Partitioner Types of partitioner: - Hash Partitioner: Uses Java’s Object.hashCode method to determine the partition as: partition = key.hashCode() % numPartitions. - Range Partitioner: It partition data either based on set of sorted ranges of keys, tuples with the same range will be on the same machine. This method is suitable where there’s a natural ordering in the keys and the keys are non negative. Example: Hash Partitioner - GroupByKey, ReduceByKey Range Partitioner - SortByKey Further reading: https://www.edureka.co/blog/demystifying-partitioning-in-spark
- 15. Partitioner
- 16. Co-partitioned RDD RDDs are co-partitioned if they are partitioned by the same partitioner.
- 18. Co-located RDD Partitions are co-located if they are both loaded into the memory of the same machine (executor).
- 19. Shuffle Operations How to send data? - Serialization - It a mechanism of representing an object as a stream of byte, transferring it through the network, and then reconstructing the same object, and its state on another computer.
- 20. Serializer in Spark - Types of Serializer in Spark - - Java : slow, but robust - Kryo : fast, but has few problem Further Reading: https://spark.apache.org/docs/latest/tuning.html#data-serialization
- 21. Optimizing Functions In Transformation
- 24. map vs mapPartitions - Map works the function being utilized at a per element level while mapPartitions exercises the function at the partition level. - map: Applies a transformation function on each item of the RDD and returns the result as a new RDD. - mapPartition: It is called only once for each partition. The entire content of the respective partitions is available as a sequential stream of values via the input argument (Iterarator[T]). - https://stackoverflow.com/questions/21185092/apache-spark-map-vs-mappartitions
- 26. SQL vs RDD
- 27. SQL RDD SQL is high-level. RDD is low-level API. SQL focus on “WHAT”. RDD focus on “HOW”. Spark takes care of optimizing most SQL queries. Optimizing RDD is developer’s responsibility. SQL are Declarative. RDD are Imperative i.e. we need to specify each step of computation. SQL knows about your data. RDD doesn’t know anything about your data. Does not involves much serialization/deserialization as Catalyst Optimizer takes care to optimize it. RDD involves too many serialization/deserialization
- 28. SQL
- 29. SQL
- 30. RDD
- 31. RDD
- 32. Logical & Physical Plan ● Logical Plan - Unresolved Logical Plan OR Parsed Logical Plan - Resolved Logical Plan OR Logical Plan OR Analyzed Logical Plan - Optimized Logical Plan ● Catalog ● Catalyst Optimizer ● Tungsten ● Physical Plan
- 33. Logical & Physical Plan https://blog.knoldus.com/understanding-sparks-logical-and-physical-plan-in-laymans-term/
- 34. Catalyst Optimizer and Tungsten
- 35. Codegen Once the Best Physical Plan is selected, it’s the time to generate the executable code (DAG of RDDs) for the query that is to be executed in a cluster in a distributed fashion. This process is called Codegen and that’s the job of Spark’s Tungsten Execution Engine.
- 36. Let’s see them in action!
- 40. Physical Plan
- 41. Optimizing Joins Types of Joins - a. Shuffle hash Join b. Sort-merge Join c. Broadcast Join
- 42. Shuffle hash Join - When join keys are not sortable. - It is used when Sort-merge Join is disabled. - spark.sql.join.preferSortMergeJoin is false. - One side is much smaller (at least 3 times) than the other. - Can build hash map.
- 43. Sort-merge Join - spark.sql.join.preferSortMergeJoin is true by default. - Default Join implementation. - Join keys must be sortable. - In our previous example, Sort-merge Join took place. - Use Bucketing : Pre shuffle + sort data based on join key
- 44. Bucketing - Bucketing helps to pre-compute the shuffle and store the data as input table, thus avoiding shuffle at each stage. - SET spark.sql.sources.bucketing.enabled = TRUE
- 45. Broadcast Join - Broadcast smaller Dataframe to all Worker Node. - Perform map-side join. - No shuffle operations take places. - spark.conf.set(“spark.sql.autoBroadcastJoinThreshold”, 10485760)
- 46. Broadcast Join
- 47. Broadcast Join
- 49. Caching/Persisting a. It keeps the lineage intact. b. Data is cached to Executor’s memory and is fetched from the cache. c. Data can be recomputed from scratch if some partitions are lost while cached partitions are not recomputed. (Done by the Resource Manager) d. Subsequent use of a RDD will not lead to computation beyond that point where it is cached. e. The cache is cleared after the SparkContext is destroyed. f. Persisting is unreliable. g. data.persist() OR data.cache()
- 50. Checkpointing a. It breaks the lineage. b. Data is written and fetched from HDFS or local file system. c. Data can not be recomputed from scratch if some partitions are lost as the lineage chain is completely lost. d. Checkpointed data can be used in subsequent job run. e. Checkpointed data is persistent and not removed after SparkContext is destroyed. f. Checkpointing is reliable.
- 51. Checkpointing spark.sparkContext.setCheckpointDir("/hdfs_directory/") myRdd.checkpoint() df.rdd.checkpoint() Why to make a checkpoint? - Busy cluster. - Expensive and long computations.