Package org.apache.spark.sql

Class ForeachWriter<T>

Object
org.apache.spark.sql.ForeachWriter<T>
All Implemented Interfaces:
Serializable
public abstract class ForeachWriter<T> extends Object implements Serializable
The abstract class for writing custom logic to process data generated by a query. This is often used to write the output of a streaming query to arbitrary storage systems. Any implementation of this base class will be used by Spark in the following way.

  • A single instance of this class is responsible of all the data generated by a single task in a query. In other words, one instance is responsible for processing one partition of the data generated in a distributed manner.

  • Any implementation of this class must be serializable because each task will get a fresh serialized-deserialized copy of the provided object. Hence, it is strongly recommended that any initialization for writing data (e.g. opening a connection or starting a transaction) is done after the open(...) method has been called, which signifies that the task is ready to generate data.

  • The lifecycle of the methods are as follows. 

     For each partition with partitionId: For each batch/epoch of streaming data (if its
 streaming query) with `epochId`: Method `open(partitionId, epochId)` is called. If `open`
 returns true: For each row in the partition and batch/epoch, method `process(row)` is called.
 Method `close(errorOrNull)` is called with error (if any) seen while processing rows.

Important points to note:

  • Spark doesn't guarantee same output for (partitionId, epochId), so deduplication cannot be achieved with (partitionId, epochId). e.g. source provides different number of partitions for some reason, Spark optimization changes number of partitions, etc. Refer SPARK-28650 for more details. If you need deduplication on output, try out foreachBatch instead.

  • The close() method will be called if open() method returns successfully (irrespective of the return value), except if the JVM crashes in the middle.

Scala example: 


   datasetOfString.writeStream.foreach(new ForeachWriter[String] {

     def open(partitionId: Long, version: Long): Boolean = {
       // open connection
     }

     def process(record: String) = {
       // write string to connection
     }

     def close(errorOrNull: Throwable): Unit = {
       // close the connection
     }
   })

Java example: 


  datasetOfString.writeStream().foreach(new ForeachWriter<String>() {

    @Override
    public boolean open(long partitionId, long version) {
      // open connection
    }

    @Override
    public void process(String value) {
      // write string to connection
    }

    @Override
    public void close(Throwable errorOrNull) {
      // close the connection
    }
  });

Since:
2.0.0
See Also:

  • Constructor Summary

    Constructors
    Constructor
    Description
    ForeachWriter()
     

  • Method Summary

    Modifier and Type
    Method
    Description
    abstract void
    close(Throwable errorOrNull)
    Called when stopping to process one partition of new data in the executor side.
    abstract boolean
    open(long partitionId, long epochId)
    Called when starting to process one partition of new data in the executor.
    abstract void
    process(T value)
    Called to process the data in the executor side.

    Methods inherited from class java.lang.Object

    equals, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

  • Constructor Details

    • ForeachWriter

      public ForeachWriter()

  • Method Details

    • close

      public abstract void close(Throwable errorOrNull)
      Called when stopping to process one partition of new data in the executor side. This is guaranteed to be called either open returns true or false. However, close won't be called in the following cases:

      • JVM crashes without throwing a Throwable
      • open throws a Throwable.

      Parameters:
      errorOrNull - the error thrown during processing data or null if there was no error.

    • open

      public abstract boolean open(long partitionId, long epochId)
      Called when starting to process one partition of new data in the executor. See the class docs for more information on how to use the partitionId and epochId.

      Parameters:
      partitionId - the partition id.
      epochId - a unique id for data deduplication.
      Returns:
      true if the corresponding partition and version id should be processed. false indicates the partition should be skipped.

    • process

      public abstract void process(T value)
      Called to process the data in the executor side. This method will be called only if open returns true.
      Parameters:
      value - (undocumented)