org.apache.spark.sql

Class ForeachWriter<T>

Object

org.apache.spark.sql.ForeachWriter<T>

All Implemented Interfaces:

java.io.Serializable

public abstract class ForeachWriter<T>
extends Object
implements scala.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:

• The partitionId and epochId can be used to deduplicate generated data when failures cause reprocessing of some input data. This depends on the execution mode of the query. If the streaming query is being executed in the micro-batch mode, then every partition represented by a unique tuple (partitionId, epochId) is guaranteed to have the same data. Hence, (partitionId, epochId) can be used to deduplicate and/or transactionally commit data and achieve exactly-once guarantees. However, if the streaming query is being executed in the continuous mode, then this guarantee does not hold and therefore should not be used for deduplication.

• 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:

Serialized Form

Constructor Summary

Constructors

Constructor and Description
ForeachWriter()

Method Summary

Modifier and Type	Method and 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 Object

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

Constructor Detail

ForeachWriter

public ForeachWriter()

Method Detail

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)