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DiskBlockObjectWriter is a disk writer of BlockManager.

DiskBlockObjectWriter is an OutputStream (Java) that BlockManager offers for writing data blocks to disk.

DiskBlockObjectWriter is used when:

Creating Instance

DiskBlockObjectWriter takes the following to be created:

DiskBlockObjectWriter is created when:

Buffer Size

DiskBlockObjectWriter is given a buffer size when created.

The buffer size is specified by BlockManager and is based on spark.shuffle.file.buffer configuration property (in most cases) or is hardcoded to 32k (in some cases but is in fact the default value).

The buffer size is exactly the buffer size of the BufferedOutputStream.


DiskBlockObjectWriter manages a SerializationStream for writing a key-value record:


DiskBlockObjectWriter can be in one of the following states (that match the state of the underlying output streams):

  • Initialized
  • Open
  • Closed

Writing Out Record

  key: Any,
  value: Any): Unit

write opens the underlying stream unless open already.

write requests the SerializationStream to write the key and then the value.

In the end, write updates the write metrics.

write is used when:


commitAndGet(): FileSegment

With streamOpen enabled, commitAndGet...FIXME

Otherwise, commitAndGet returns a new FileSegment (with the File, committedPosition and 0 length).

commitAndGet is used when:

Committing Writes and Closing Resources

close(): Unit

Only if initialized, close commitAndGet followed by closeResources. Otherwise, close does nothing.

close is used when:



revertPartialWritesAndClose(): File


revertPartialWritesAndClose is used when...FIXME

Writing Bytes (From Byte Array Starting From Offset)

  kvBytes: Array[Byte],
  offs: Int,
  len: Int): Unit


write is used when...FIXME

Opening DiskBlockObjectWriter

open(): DiskBlockObjectWriter

open opens the DiskBlockObjectWriter, i.e. initializes and re-sets bs and objOut internal output streams.

Internally, open makes sure that DiskBlockObjectWriter is not closed (hasBeenClosed flag is disabled). If it was, open throws a IllegalStateException:

Writer already closed. Cannot be reopened.

Unless DiskBlockObjectWriter has already been initialized (initialized flag is enabled), open initializes it (and turns initialized flag on).

Regardless of whether DiskBlockObjectWriter was already initialized or not, open requests SerializerManager to wrap mcs output stream for encryption and compression (for blockId) and sets it as bs.

open requests the SerializerInstance to serialize bs output stream and sets it as objOut.


open uses the SerializerInstance that was used to create the DiskBlockObjectWriter.

In the end, open turns streamOpen flag on.

open is used when DiskBlockObjectWriter writes out a record or bytes from a specified byte array and the stream is not open yet.


initialize(): Unit

initialize creates a FileOutputStream to write to the file (with theappend enabled) and takes the FileChannel associated with this file output stream.

initialize creates a TimeTrackingOutputStream (with the ShuffleWriteMetricsReporter and the FileOutputStream).

With checksumEnabled, initialize...FIXME

In the end, initialize creates a BufferedOutputStream.

checksumEnabled Flag

DiskBlockObjectWriter defines checksumEnabled flag to...FIXME

checksumEnabled is false by default and can be enabled using setChecksum.


  checksum: Checksum): Unit


setChecksum is used when:

Recording Bytes Written

recordWritten(): Unit

recordWritten increases the numRecordsWritten counter.

recordWritten requests the ShuffleWriteMetricsReporter to incRecordsWritten.

recordWritten updates the bytes written metric every 16384 bytes written (based on the numRecordsWritten counter).

recordWritten is used when:

  • ShuffleExternalSorter is requested to writeSortedFile
  • DiskBlockObjectWriter is requested to write
  • UnsafeSorterSpillWriter is requested to write

Updating Bytes Written Metric

updateBytesWritten(): Unit

updateBytesWritten requests the FileChannel for the file position (i.e., the number of bytes from the beginning of the file to the current position) that is used to incBytesWritten (using the ShuffleWriteMetricsReporter and the reportedPosition counter).

In the end, updateBytesWritten updates the reportedPosition counter to the current file position (so it can report incBytesWritten properly).


mcs: ManualCloseOutputStream

DiskBlockObjectWriter creates a custom BufferedOutputStream (Java) when requested to initialize.

The BufferedOutputStream is closed (and dereferenced) in closeResources.

The BufferedOutputStream is used to create the OutputStream when requested to open.


bs: OutputStream

DiskBlockObjectWriter creates an OutputStream when requested to open. The OutputStream can be encrypted and compressed if enabled.

The OutputStream is closed (and dereferenced) in closeResources.

The OutputStream is used to create the SerializationStream when requested to open.

The OutputStream is requested for the following: