小编给大家分享一下spark中delta写操作ACID事务的示例分析，相信大部分人都还不怎么了解，因此分享这篇文章给大家参考一下，希望大家阅读完这篇文章后大有收获，下面让我们一起去了解一下吧！
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##分析
直接到WriteIntoDelta.run
override def run(sparkSession: SparkSession): Seq[Row] = {
deltaLog.withNewTransaction { txn =>
val actions = write(txn, sparkSession)
val operation = DeltaOperations.Write(mode, Option(partitionColumns),
options.replaceWhere, options.userMetadata)
txn.commit(actions, operation)
}
Seq.empty
}我们来看一下deltaLog.withNewTrancation方法 ：
def withNewTransaction[T](thunk: OptimisticTransaction => T): T = {
try {
update()
val txn = new OptimisticTransaction(this)
OptimisticTransaction.setActive(txn)
thunk(txn)
} finally {
OptimisticTransaction.clearActive()
}
}首先update方法直接同步调用updateInternal用来更新当前deltalog的snapshot,具体的updateInternal如下：
val segment = getLogSegmentForVersion(currentSnapshot.logSegment.checkpointVersion)
if (segment.version == currentSnapshot.version) {
// Exit early if there is no new file
lastUpdateTimestamp = clock.getTimeMillis()
return currentSnapshot
}
logInfo(s"Loading version ${segment.version}" +
segment.checkpointVersion.map(v => s"starting from checkpoint version $v."))
val newSnapshot = createSnapshot(
segment,
minFileRetentionTimestamp,
segment.lastCommitTimestamp)
...
currentSnapshot.uncache()
currentSnapshot = newSnapshot首先先通过getLogSegmentForVersion方法获取当前最新的snapshot，之后更新到内存
设置OptimisticTransaction，并在当前事务中执行当前语句
val actions = write(txn, sparkSession)
val operation = DeltaOperations.Write(mode, Option(partitionColumns),
options.replaceWhere, options.userMetadata)
txn.commit(actions, operation)
val atcions = write(txn, sparksession)我们已经在spark delta写操作ACID事务前传--写文件基础类FileFormat/FileCommitProtocol分析分析了，即会返回Seq[AddAction],而实际的数据文件已经存储到了文件目录下
val operation = DeltaOperations.Write(mode, Option(partitionColumns)记录了这是一个delta write Operation
txn.commit(actions, operation) 是该提交delta log的关键:
def commit(actions: Seq[Action], op: DeltaOperations.Operation): Long = recordDeltaOperation(
deltaLog,
"delta.commit") {
commitStartNano = System.nanoTime()
val version = try {
// Try to commit at the next version.
var finalActions = prepareCommit(actions, op)
// Find the isolation level to use for this commit
val noDataChanged = actions.collect { case f: FileAction => f.dataChange }.forall(_ == false)
val isolationLevelToUse = if (noDataChanged) {
// If no data has changed (i.e. its is only being rearranged), then SnapshotIsolation
// provides Serializable guarantee. Hence, allow reduced conflict detection by using
// SnapshotIsolation of what the table isolation level is.
SnapshotIsolation
} else {
Serializable
}
val isBlindAppend = {
val dependsOnFiles = readPredicates.nonEmpty || readFiles.nonEmpty
val onlyAddFiles =
finalActions.collect { case f: FileAction => f }.forall(_.isInstanceOf[AddFile])
onlyAddFiles && !dependsOnFiles
}
if (spark.sessionState.conf.getConf(DeltaSQLConf.DELTA_COMMIT_INFO_ENABLED)) {
commitInfo = CommitInfo(
clock.getTimeMillis(),
op.name,
op.jsonEncodedValues,
Map.empty,
Some(readVersion).filter(_ >= 0),
None,
Some(isBlindAppend),
getOperationMetrics(op),
getUserMetadata(op))
finalActions = commitInfo +: finalActions
}
// Register post-commit hooks if any
lazy val hasFileActions = finalActions.collect { case f: FileAction => f }.nonEmpty
if (DeltaConfigs.SYMLINK_FORMAT_MANIFEST_ENABLED.fromMetaData(metadata) && hasFileActions) {
registerPostCommitHook(GenerateSymlinkManifest)
}
val commitVersion = doCommit(snapshot.version + 1, finalActions, 0, isolationLevelToUse)
logInfo(s"Committed delta #$commitVersion to ${deltaLog.logPath}")
postCommit(commitVersion, finalActions)
commitVersion
} catch {
case e: DeltaConcurrentModificationException =>
recordDeltaEvent(deltaLog, "delta.commit.conflict." + e.conflictType)
throw e
case NonFatal(e) =>
recordDeltaEvent(
deltaLog, "delta.commit.failure", data = Map("exception" -> Utils.exceptionString(e)))
throw e
}
runPostCommitHooks(version, actions)
version
}prepareCommit用来做一些提交前的检查，以及增加一些actions,
如果是第一次提交还得增加Protocol，如{"protocol":{"minReaderVersion":1,"minWriterVersion":2}} 如metadata变化了，还得增加newMetadata，如 {"metaData":{"id":"2b2457e3-ce74-4897-abbd-04a94692304a","format":{"provider":"parquet","options":{}},"schemaString":"{\"type\":\"struct\",\"fields\":[{\"name\":\"value\",\"type\":\"string\",\"nullable\":true,\"metadata\":{}}]}","partitionColumns":[],"configuration":{},"createdTime":1609398723678}}
如果配置了spark.databricks.delta.commitInfo.enabled（默认是true）则还会增加commitInfo信息等，如{"commitInfo":{"timestamp":1609400013646,"operation":"WRITE","operationParameters":{"mode":"Append","partitionBy":"[]"},"readVersion":0,"isBlindAppend":true,"operationMetrics":{"numFiles":"1","numOutputBytes":"306","numOutputRows":"0"}}}， 如果做了Presto / Athena兼容，还会注册GenerateSymlinkManifest postCommitHook,在commit成功后还会进行调用
doCommit(snapshot.version + 1, finalActions, 0, isolationLevelToUse) 同步提交最终的action到deltalog:
protected def doCommit(
attemptVersion: Long,
actions: Seq[Action],
attemptNumber: Int,
isolationLevel: IsolationLevel): Long = deltaLog.lockInterruptibly {
try {
...
deltaLog.store.write(
deltaFile(deltaLog.logPath, attemptVersion),
actions.map(_.json).toIterator)
val commitTime = System.nanoTime()
val postCommitSnapshot = deltaLog.update()
if (postCommitSnapshot.version < attemptVersion) {
recordDeltaEvent(deltaLog, "delta.commit.inconsistentList", data = Map(
"committedVersion" -> attemptVersion,
"currentVersion" -> postCommitSnapshot.version
))
throw new IllegalStateException(
s"The committed version is $attemptVersion " +
s"but the current version is ${postCommitSnapshot.version}.")
}
// Post stats
var numAbsolutePaths = 0
var pathHolder: Path = null
val distinctPartitions = new mutable.HashSet[Map[String, String]]
val adds = actions.collect {
case a: AddFile =>
pathHolder = new Path(new URI(a.path))
if (pathHolder.isAbsolute) numAbsolutePaths += 1
distinctPartitions += a.partitionValues
a
}
...
attemptVersion
} catch {
case e: java.nio.file.FileAlreadyExistsException =>
checkAndRetry(attemptVersion, actions, attemptNumber, isolationLevel)
}
}deltaLog.store.write( deltaFile(deltaLog.logPath, attemptVersion), actions.map(_.json).toIterator)` 方法直接调用HDFSLogStore的write方法,而最终调用writeInternal方法，这里attemptVersion是当前的version+1 我们看一下writeInternal方法：
private def writeInternal(path: Path, actions: Iterator[String], overwrite: Boolean): Unit = {
val fc: FileContext = try {
getFileContext(path)
} catch {
case e: IOException if e.getMessage.contains(noAbstractFileSystemExceptionMessage) =>
val newException = DeltaErrors.incorrectLogStoreImplementationException(sparkConf, e)
logError(newException.getMessage, newException.getCause)
throw newException
}
if (!overwrite && fc.util.exists(path)) {
// This is needed for the tests to throw error with local file system
throw new FileAlreadyExistsException(path.toString)
}
val tempPath = createTempPath(path)
var streamClosed = false // This flag is to avoid double close
var renameDone = false // This flag is to save the delete operation in most of cases.
val stream = fc.create(
tempPath, EnumSet.of(CREATE), CreateOpts.checksumParam(ChecksumOpt.createDisabled()))
try {
actions.map(_ + "\n").map(_.getBytes(UTF_8)).foreach(stream.write)
stream.close()
streamClosed = true
try {
val renameOpt = if (overwrite) Options.Rename.OVERWRITE else Options.Rename.NONE
fc.rename(tempPath, path, renameOpt)
renameDone = true
// TODO: this is a workaround of HADOOP-16255 - remove this when HADOOP-16255 is resolved
tryRemoveCrcFile(fc, tempPath)
} catch {
case e: org.apache.hadoop.fs.FileAlreadyExistsException =>
throw new FileAlreadyExistsException(path.toString)
}
} finally {
if (!streamClosed) {
stream.close()
}
if (!renameDone) {
fc.delete(tempPath, false)
}
}
}如果文件存在，则抛出异常。否则写入该log文件，这里注意：如果是本地文件的话，需要加同步进行rename操作，因为本地文件的rename操作，即使目标文件存在了也不会报异常，其他的文件类型，则不需要加同步。
如果deltaLog.store.write没有发生异常，则获取最新的snaphost，进行记录，返回传入的attemptVersion，
如果发生了异常，则执行checkAndRetry(attemptVersion, actions, attemptNumber, isolationLevel)，进行重试,重试的时候，我们得查找以下从事务提交到失败这段时间的由其他程序提交的Actions，如果和当前actions没有冲突则继续提交，否则抛出异常
postCommit 进行checkpoint操作，每隔 10 个提交，Delta Lake 会在 _delta_log 子目录下自动生成一个 Parquet 格式的 checkpoint 文件，便于快速replays，且清除过期的deltalog,默认保存30天(这里会把addfile和removefile合并了，比如先ad d A文件，之后remove A 合并完了A就没记录了):
def checkpoint(): Unit = recordDeltaOperation(this, "delta.checkpoint") {
val checkpointMetaData = checkpoint(snapshot)
val json = JsonUtils.toJson(checkpointMetaData)
store.write(LAST_CHECKPOINT, Iterator(json), overwrite = true)
doLogCleanup()
}runPostCommitHooks 如果注册了postCommitHooks，就执行
至此整个delta写deltalog的流程就结束了， 整个流程如下：
update 获取最新的snapshot
|
v
write()写入delta data
|
v
commit 事务提交 -> prepareCommit 用来做一些提交前的检查，以及增加一些actions
|
v
doCommit 真正写入deltalog，会一直重试直到冲突
|
v
postCommit 进行checkpoint操作,合并Addfile和RemoveFile,便于快速replays，且清除过期的delta log
|
v
runPostCommitHooks 如果存在hook则执行对应的hook
|
v
结束
以上是“spark中delta写操作ACID事务的示例分析”这篇文章的所有内容，感谢各位的阅读！相信大家都有了一定的了解，希望分享的内容对大家有所帮助，如果还想学习更多知识，欢迎关注创新互联行业资讯频道！
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