SnapshotManagement¶

SnapshotManagement is an extension for DeltaLog to manage Snapshots.

Current Snapshot¶

SnapshotManagement defines currentSnapshot registry with the recently-loaded Snapshot of the delta table.

currentSnapshot is the latest Snapshot initially and can be updated on demand (when installLogSegmentInternal and replaceSnapshot).

currentSnapshot...FIXME

currentSnapshot is used when SnapshotManagement is requested for the following:

unsafeVolatileSnapshot
update (tryUpdate, updateInternal, installLogSegmentInternal, replaceSnapshot)
updateAfterCommit

Loading Latest Snapshot at Initialization¶

getSnapshotAtInit: Snapshot

getSnapshotAtInit finds the LogSegment of the delta table (using the last checkpoint file if available)

In the end, getSnapshotAtInit createSnapshotAtInitInternal.

createSnapshotAtInitInternal¶

createSnapshotAtInitInternal(
  initSegment: Option[LogSegment],
  lastCheckpointOpt: Option[CheckpointMetaData],
  timestamp: Long): CapturedSnapshot

createSnapshotAtInitInternal...FIXME

Fetching Log Files for Version Checkpointed¶

getLogSegmentFrom(
  startingCheckpoint: Option[CheckpointMetaData]): LogSegment

getLogSegmentFrom fetches log files for the version (based on the optional CheckpointMetaData as the starting checkpoint version to start listing log files from).

Fetching Latest Checkpoint and Delta Log Files for Version¶

getLogSegmentForVersion(
  startCheckpoint: Option[Long],
  versionToLoad: Option[Long] = None): LogSegment

getLogSegmentForVersion list all the files (in a transaction log) from the given startCheckpoint (or defaults to 0).

getLogSegmentForVersion filters out unnecessary files and leaves checkpoint and delta files only.

getLogSegmentForVersion filters out checkpoint files of size 0.

getLogSegmentForVersion takes all the files that are older than the requested versionToLoad.

getLogSegmentForVersion splits the files into checkpoint and delta files.

getLogSegmentForVersion finds the latest checkpoint from the list.

In the end, getLogSegmentForVersion creates a LogSegment with the (checkpoint and delta) files.

getLogSegmentForVersion is used when:

SnapshotManagement is requested for getLogSegmentFrom, updateInternal and getSnapshotAt

Listing Files from Version Upwards¶

listFrom(
  startVersion: Long): Iterator[FileStatus]

listFrom...FIXME

Creating Snapshot¶

createSnapshot(
  segment: LogSegment,
  minFileRetentionTimestamp: Long,
  timestamp: Long): Snapshot

createSnapshot readChecksum (for the version of the given LogSegment) and creates a Snapshot.

createSnapshot is used when:

SnapshotManagement is requested for getSnapshotAtInit, updateInternal and getSnapshotAt

Last Successful Update Timestamp¶

SnapshotManagement uses lastUpdateTimestamp internal registry for the timestamp of the last successful update.

Updating Current Snapshot¶

update(
  stalenessAcceptable: Boolean = false): Snapshot

update determines whether to do update asynchronously or not based on the input stalenessAcceptable flag and isSnapshotStale.

With stalenessAcceptable flag turned off (the default value) and the state snapshot is not stale, update updates (with isAsync flag turned off).

update...FIXME

Usage¶

update is used when:

DeltaHistoryManager is requested to getHistory, getActiveCommitAtTime, checkVersionExists
DeltaLog is requested to start a transaction
OptimisticTransactionImpl is requested to doCommit and getNextAttemptVersion
DeltaTableV2 is requested for a Snapshot
TahoeLogFileIndex is requested for a Snapshot
DeltaSource is requested for the getStartingVersion
In Delta commands...

isSnapshotStale¶

isSnapshotStale: Boolean

isSnapshotStale reads spark.databricks.delta.stalenessLimit configuration property.

isSnapshotStale is enabled (true) when any of the following holds:

spark.databricks.delta.stalenessLimit configuration property is 0 (the default)
Internal lastUpdateTimestamp has never been updated (and is below 0) or is at least spark.databricks.delta.stalenessLimit configuration property old

tryUpdate¶

tryUpdate(
  isAsync: Boolean = false): Snapshot

tryUpdate...FIXME

updateInternal¶

updateInternal(
  isAsync: Boolean): Snapshot // (1)

isAsync flag is not used

updateInternal requests the current Snapshot for the LogSegment that is in turn requested for the checkpointVersion to get the LogSegment for.

updateInternal installLogSegmentInternal.

installLogSegmentInternal¶

installLogSegmentInternal(
  previousSnapshot: Snapshot,
  segmentOpt: Option[LogSegment],
  updateTimestamp: Long,
  isAsync: Boolean): Snapshot // (1)!

isAsync is not used

installLogSegmentInternal gives the Snapshot (possibly an InitialSnapshot) of the delta table at the logPath.

installLogSegmentInternal...FIXME

With no LogSegment specified, installLogSegmentInternal prints out the following INFO message to the logs and replaceSnapshot with a new InitialSnapshot (for the logPath).

No delta log found for the Delta table at [logPath]

Replacing Snapshots¶

replaceSnapshot(
  newSnapshot: Snapshot): Unit

replaceSnapshot requests the currentSnapshot to uncache (and drop any cached data) and makes the given newSnapshot the current one.

updateAfterCommit¶

updateAfterCommit(
  committedVersion: Long,
  newChecksumOpt: Option[VersionChecksum],
  preCommitLogSegment: LogSegment): Snapshot

updateAfterCommit...FIXME

updateAfterCommit is used when:

OptimisticTransactionImpl is requested to attempt a commit

Demo¶

import org.apache.spark.sql.delta.DeltaLog
val log = DeltaLog.forTable(spark, dataPath)

import org.apache.spark.sql.delta.SnapshotManagement
assert(log.isInstanceOf[SnapshotManagement], "DeltaLog is a SnapshotManagement")

val snapshot = log.update(stalenessAcceptable = false)

scala> :type snapshot
org.apache.spark.sql.delta.Snapshot

assert(snapshot.version == 0)

Logging¶

As an extension of DeltaLog, use DeltaLog logging to see what happens inside.