IndexShuffleBlockResolver
IndexShuffleBlockResolver
is the one and only ShuffleBlockResolver in Spark.
IndexShuffleBlockResolver
manages shuffle block data and uses shuffle index files for faster shuffle data access. IndexShuffleBlockResolver
can write a shuffle block index and data file, find and remove shuffle index and data files per shuffle and
map.
Note
|
Shuffle block data files are more often referred as map outputs files. |
IndexShuffleBlockResolver
is managed exclusively by SortShuffleManager (so BlockManager
can access shuffle block data).
IndexShuffleBlockResolver
is later passed in when SortShuffleManager
creates a ShuffleWriter
for ShuffleHandle
.
Name | Initial Value | Description |
---|---|---|
|
TransportConf for |
Used when |
Creating IndexShuffleBlockResolver Instance
IndexShuffleBlockResolver
takes the following when created:
-
BlockManager (default: unspecified and
SparkEnv
is used to access one)
IndexShuffleBlockResolver
initializes the internal properties.
Note
|
IndexShuffleBlockResolver is created exclusively when SortShuffleManager is created.
|
Writing Shuffle Index and Data Files — writeIndexFileAndCommit
Method
writeIndexFileAndCommit(
shuffleId: Int,
mapId: Int,
lengths: Array[Long],
dataTmp: File): Unit
Internally, writeIndexFileAndCommit
first finds the index file for the input shuffleId
and mapId
.
writeIndexFileAndCommit
creates a temporary file for the index file (in the same directory) and writes offsets (as the moving sum of the input lengths
starting from 0 to the final offset at the end for the end of the output file).
Note
|
The offsets are the sizes in the input lengths exactly.
|
writeIndexFileAndCommit
requests a shuffle block data file for the input shuffleId
and mapId
.
writeIndexFileAndCommit
checks if the given index and data files match each other (aka consistency check).
If the consistency check fails, it means that another attempt for the same task has already written the map outputs successfully and so the input dataTmp
and temporary index files are deleted (as no longer correct).
If the consistency check succeeds, the existing index and data files are deleted (if they exist) and the temporary index and data files become "official", i.e. renamed to their final names.
In case of any IO-related exception, writeIndexFileAndCommit
throws a IOException
with the messages:
fail to rename file [indexTmp] to [indexFile]
or
fail to rename file [dataTmp] to [dataFile]
Note
|
writeIndexFileAndCommit is used when ShuffleWriter is requested to write records to shuffle system, i.e. SortShuffleWriter, BypassMergeSortShuffleWriter, and UnsafeShuffleWriter.
|
Creating ManagedBuffer to Read Shuffle Block Data File — getBlockData
Method
getBlockData(blockId: ShuffleBlockId): ManagedBuffer
Note
|
getBlockData is a part of ShuffleBlockResolver contract.
|
Internally, getBlockData
finds the index file for the input shuffle blockId
.
Note
|
ShuffleBlockId knows shuffleId and mapId .
|
getBlockData
discards blockId.reduceId
bytes of data from the index file.
Note
|
getBlockData uses Guava’s com.google.common.io.ByteStreams to skip the bytes.
|
getBlockData
reads the start and end offsets from the index file and then creates a FileSegmentManagedBuffer
to read the data file for the offsets (using transportConf internal property).
Note
|
The start and end offsets are the offset and the length of the file segment for the block data. |
In the end, getBlockData
closes the index file.
Checking Consistency of Shuffle Index and Data Files and Returning Block Lengths — checkIndexAndDataFile
Internal Method
checkIndexAndDataFile(index: File, data: File, blocks: Int): Array[Long]
checkIndexAndDataFile
first checks if the size of the input index
file is exactly the input blocks
multiplied by 8
.
checkIndexAndDataFile
returns null
when the numbers, and hence the shuffle index and data files, don’t match.
checkIndexAndDataFile
reads the shuffle index
file and converts the offsets into lengths of each block.
checkIndexAndDataFile
makes sure that the size of the input shuffle data
file is exactly the sum of the block lengths.
checkIndexAndDataFile
returns the block lengths if the numbers match, and null
otherwise.
Note
|
checkIndexAndDataFile is used exclusively when IndexShuffleBlockResolver writes shuffle index and data files.
|
Requesting Shuffle Block Index File (from DiskBlockManager) — getIndexFile
Internal Method
getIndexFile(shuffleId: Int, mapId: Int): File
getIndexFile
requests BlockManager
for the current DiskBlockManager
.
Note
|
getIndexFile uses SparkEnv to access the current BlockManager unless specified when IndexShuffleBlockResolver is created.
|
getIndexFile
then requests DiskBlockManager
for the shuffle index file given the input shuffleId
and mapId
(as ShuffleIndexBlockId
)
Note
|
getIndexFile is used when IndexShuffleBlockResolver writes shuffle index and data files, creates a ManagedBuffer to read a shuffle block data file, and ultimately removes the shuffle index and data files.
|
Requesting Shuffle Block Data File — getDataFile
Method
getDataFile(shuffleId: Int, mapId: Int): File
Note
|
getDataFile uses SparkEnv to access the current BlockManager unless specified when IndexShuffleBlockResolver is created.
|
getDataFile
then requests DiskBlockManager
for the shuffle block data file given the input shuffleId
, mapId
, and the special reduce id 0
(as ShuffleDataBlockId
).
Note
|
|
Removing Shuffle Index and Data Files (For Single Map) — removeDataByMap
Method
removeDataByMap(shuffleId: Int, mapId: Int): Unit
removeDataByMap
finds and deletes the shuffle data for the input shuffleId
and mapId
first followed by finding and deleting the shuffle data index file.
When removeDataByMap
fails deleting the files, you should see a WARN message in the logs.
WARN Error deleting data [path]
or
WARN Error deleting index [path]
Note
|
removeDataByMap is used exclusively when SortShuffleManager unregisters a shuffle, i.e. removes a shuffle from a shuffle system.
|
Stopping IndexShuffleBlockResolver — stop
Method
stop(): Unit
Note
|
stop is a part of ShuffleBlockResolver contract.
|
stop
is a noop operation, i.e. does nothing when called.