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1 .Whirlpools in the Stream Misadventures in Structured Streaming Jayesh Lalwani, Lead Software Engineer, Quantum, Capital One
2 .What is Structured Streaming
3 .Dunderheaded Data stores!
4 .Look Ma! No lookups! Process B Events Process A State • Spark allows you to join Streaming data with Batch Data • BUT…. • Spark doesn’t support pushdown of join predicates. This means that the State is loaded into Spark memory for every microbatch • Caching a Batch dataframe “freezes” it
5 .Solutions • Cache dataframe & Restart the Streaming application • Cache dataframe & Restart the Streaming query • Cache the data outside of Spark • Lookup batch data inside a map/flatmap operation • Stream updates to Process B and do stream to stream join (2.3+ only) – works for limited use cases
6 .Data store? We don’t need no stinkin’ data stores • Outputs supported Batch • Outputs supported Mode Streaming Mode • JSON • JSON • CSV • CSV • Parquet • Parquet • Orc • Text • Text • JDBC WHERE IS JDBC?
7 .My implementation of JDBC sink • Features: • Can write streaming data to JDBC data stores • Uses the same code that is used by Batch JDBC sinks • Modes: Overwrite, Append • Supports Atleast Once out of the box • Supports Exactly Once with some configuration • Fork: https://github.com/GaalDornick/spark • Implementation: https://github.com/GaalDornick/spark/blob/master/sql/core/src/main/scal a/org/apache/spark/sql/execution/streaming/JdbcSink.scala • Unit test: https://github.com/GaalDornick/spark/blob/master/sql/core/src/test/scala /org/apache/spark/sql/execution/streaming/JDBCSinkSuite.scala
8 .Clumsy footed Checkpoints
9 .Missing the point with Checkpoints • Checkpoint stores state • Allows Streaming application to restart on failure • Checkpoint should be stored on a location that is • Resilient to failure • Shared between executors and drivers (read-write many) • Immediately consistent • S3 IS NOT IMMEDIATELY CONSISTENT
10 .Solution • NFS • We use EFS on AWS • On Kubernetes, you need a PV that supports RWX access mode • NFS • Ceph • GlusterFS
11 .Checkpoints had a great fall • https://issues.apache.org/jira/browse/SPARK-21696 • Leads to corrupt checkpoints. • Solved by deleting the checkpoint once a day • This means we lose data • Fixed in 2.2.1
12 .Anthropophagic Aggregations!
13 .Aggregations? What Aggregations? • Structured streaming allows you to aggregate a dataframe • But you cannot aggregate an aggregated data frame • PARTITION BY clause is supported only for time windows Solutions • If possible, aggregate batch data before joining with stream • Implement aggregations without Spark SQL using groupBy..flatMapGroups
14 . Jayesh Lalwani Lead Software Engineer Team Heartbeat Jayesh.Lalwani@capitalone.com