SnapshotManagement¶

SnapshotManagement is an extension for DeltaLog to manage Snapshots.

Demo¶

val name = "employees"
val dataPath = s"/tmp/delta/$name"
sql(s"DROP TABLE $name")
sql(s"""
    | CREATE TABLE $name (id bigint, name string, city string)
    | USING delta
    | OPTIONS (path='$dataPath')
    """.stripMargin)

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)

Current Snapshot¶

currentSnapshot: Snapshot

currentSnapshot is a registry with the current Snapshot of a Delta table.

When DeltaLog is created, currentSnapshot is initialized as getSnapshotAtInit and changed every update.

currentSnapshot...FIXME

currentSnapshot is used when:

• DeltaLog is requested to isValid
• SnapshotManagement is requested to...FIXME

Updating Current Snapshot¶

update(
  stalenessAcceptable: Boolean = false): Snapshot

update...FIXME

update is used when:

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

tryUpdate¶

tryUpdate(
  isAsync: Boolean = false): Snapshot

tryUpdate...FIXME

tryUpdate is used when SnapshotManagement is requested to update.

updateInternal¶

updateInternal(
  isAsync: Boolean): Snapshot

updateInternal...FIXME

updateInternal is used when SnapshotManagement is requested to update (and tryUpdate).

Loading Latest Snapshot¶

getSnapshotAtInit: Snapshot

getSnapshotAtInit getLogSegmentFrom for the last checkpoint.

getSnapshotAtInit prints out the following INFO message to the logs:

Loading version [version][startCheckpoint]

getSnapshotAtInit creates a Snapshot for the log segment.

getSnapshotAtInit records the current time in lastUpdateTimestamp registry.

getSnapshotAtInit prints out the following INFO message to the logs:

Returning initial snapshot [snapshot]

getSnapshotAtInit is used when SnapshotManagement is created (and initializes the currentSnapshot registry).

getLogSegmentFrom¶

getLogSegmentFrom(
  startingCheckpoint: Option[CheckpointMetaData]): LogSegment

getLogSegmentFrom getLogSegmentForVersion for the version of the given CheckpointMetaData (if specified) as a start checkpoint version or leaves it undefined.

getLogSegmentFrom is used when SnapshotManagement is requested for getSnapshotAtInit.

getLogSegmentForVersion¶

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

getLogSegmentForVersion...FIXME

getLogSegmentForVersion is used when SnapshotManagement is requested for getLogSegmentFrom, updateInternal and getSnapshotAt.

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, getSnapshotAt and update.

Last Successful Update Timestamp¶

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