Flink源码之Checkpoint执行流程

Checkpoint完整流程如上图所示：

JobMaster的CheckpointCoordinator向所有SourceTask发送RPC触发一次CheckPoint
SourceTask向下游广播CheckpointBarrier
SouceTask完成状态快照后向JobMaster发送快照结果
非SouceTask在Barrier对齐后完成状态快照向JobMaster发送快照结果
JobMaster保存SubTask快照结果
JobMaster收到所有SubTask快照结果后保存快照信息，想SubTask通知Checkpoint完成

以下对整个流程具体说明。

CheckpointCoordinator

JobMaster将JobGraph转换为ExecutionGraph时，如果开启Checkpoint，会为ExecutionGraph生成一个CheckpointCoordinator

DefaultExecutionGraphBuilder.buildGraph//在此会将JobGraph转换为ExecutionGraph
    DefaultExecutionGraph::new
    DefaultExecutionGraph::attachJobGraph //创建ExecutionJobVertex
    	DefaultExecutionTopology.fromExecutionGraph //创建ExecutionTopology
    DefaultExecutionGraph::enableCheckpointing //创建CheckpointCoordinator
    	DefaultExecutionGraph::createCheckpointPlanCalculator//创建DefaultCheckpointPlanCalculator
    	CheckpointCoordinator::new

CheckpointCoordinator封装了StateBackend和CheckpointStorage

StateBackend负责管理状态：

HashMapStateBackend //内存
EmbeddedRocksDBStateBackend //内存+磁盘

CheckpointStorage则是负责存储StateBackend管理的状态：

JobManagerCheckpointStorage //checkpoint state放入JobManager内存
FileSystemCheckpointStorage //配置state.checkpoints.dir时

在为StreamTask构造SubtaskCheckpointCoordinatorImpl时会调用：

CheckpointStorage::createCheckpointStorage

创建CheckpointStorageAccess用于执行Checkpoint时解析状态存储位置

MemoryBackendCheckpointStorageAccess //对应JobManagerCheckpointStorage
FsCheckpointStorageAccess //对应FileSystemCheckpointStorage

CheckpointCoordinator在执行状态快照时会调用

CheckpointStorageAccess::resolveCheckpointStorageLocation

生成CheckpointStreamFactory用于生成读写状态数据流

MemCheckpointStreamFactory //对应JobManagerCheckpointStorage
FsCheckpointStreamFactory //对应FileSystemCheckpointStorage

Checkpoint触发流程

JobMaster状态转换为running后，通过CheckpointCoordinator向SourceTask发送TriggerCheckpoint

JobMaster端触发流程

JobMaster::start  //RPCServer启动
JobMaster::onStart
JobMaster::startJobExecution
JobMaster::startJobMasterServices //获取RM地址后与RM建立连接
JobMaster::startScheduling
SchedulerBase::startScheduling
DefaultScheduler::startSchedulingInternal
SchedulerBase::transitionToRunning
	DefaultExecutionGraph::transitionToRunning //调用ExecutionGraph监听器通知状态变化
		CheckpointCoordinatorDeActivator::jobStatusChanges//触发checkpoint
			CheckpointCoordinator::startCheckpointScheduler
				CheckpointCoordinator::scheduleTriggerWithDelay //定时不断触发Checkpoint
				CheckpointCoordinator::triggerCheckpoint
				CheckpointCoordinator::startTriggeringCheckpoint
				DefaultCheckpointPlanCalculator::calculateCheckpointPlan//Plan中会隔离出SourceTask作为作为Trigger Checkpoint的入口
				CheckpointCoordinator::createPendingCheckpoint
				CheckpointCoordinator::triggerCheckpointRequest
				CheckpointCoordinator::triggerTasks 
					Execution::triggerCheckpoint //向每个SourceTask发送TriggerCheckpoint请求
                    Execution::triggerCheckpointHelper
                    TaskManagerGateway::triggerCheckpoint//向TaskExecutor发RPC

StreamTask端执行流程

SourceTask

SourceTask由JobMaster RPC直接触发，执行时先广播CheckpointBarrier，然后对状态执行异步快照

TaskExecutor::triggerCheckpoint
Task::triggerCheckpointBarrier
AbstractInvokable::triggerCheckpointAsync
SourceStreamTask::triggerCheckpointAsync
StreamTask::triggerCheckpointAsync
StreamTask::triggerCheckpointAsyncInMailbox
StreamTask::performCheckpoint
SubtaskCheckpointCoordinatorImpl::checkpointState
	OperatorChain.broadcastEvent //广播CheckpointBarrier
CheckpointStorage::createCheckpointStorage//为JobId创建CheckpointStorageAccess
SubtaskCheckpointCoordinatorImpl::takeSnapshotSync
CheckpointStorageWorkerView::resolveCheckpointStorageLocation//CheckpointStorageAccess创建 CheckpointStreamFactory
	OperatorChain::snapshotState //对每个Operator
		RegularOperatorChain::buildOperatorSnapshotFutures
		RegularOperatorChain::checkpointStreamOperator
			AbstractStreamOperator::snapshotState
			StreamOperatorStateHandler::snapshotState//调用Operator/Keyed Backend的snapshot
				StateSnapshotContextSynchronousImpl::new
				AbstractUdfStreamOperator::snapshotState //调用UDF中snapshotState方法，一般用于更新OperatorState
				DefaultOperatorStateBackend::snapshot
					SnapshotStrategyRunner::snapshot
					  DefaultOperatorStateBackendSnapshotStrategy::syncPrepareResources//深copy operator state,便于后续进行异步快照
					  DefaultOperatorStateBackendSnapshotStrategy::asyncSnapshot//异步快照					  	  CheckpointStateOutputStream::closeAndGetHandle
						OperatorStreamStateHandle::new //包装元信息及数据StreamStateHandle
					
				HeapKeyedStateBackend::snapshot
					SnapshotStrategyRunner::snapshot
					    HeapSnapshotStrategy::syncPrepareResources
						HeapSnapshotStrategy::asyncSnapshot //采用COWSateTable异步快照
							CheckpointStateOutputStream::closeAndGetHandle
							KeyGroupsStateHandle::new //包装KeyGroup及数据StreamStateHandle
SubtaskCheckpointCoordinatorImpl::finishAndReportAsync //向JobMaster发送checkpoint的结果
	AsyncCheckpointRunnable::new 
	AsyncCheckpointRunnable::run
		AsyncCheckpointRunnable::finalizeNonFinishedSnapshots
			OperatorSnapshotFinalizer::new //等待TaskSnapshot状态信息序列化完成
		AsyncCheckpointRunnable::reportCompletedSnapshotStates
			TaskStateManagerImpl::reportTaskStateSnapshots
				RpcCheckpointResponder::acknowledgeCheckpoint//向JobMaster发送Ack，带上State信息

非SourceTask

在StreamTask启动后调用StreamTask::processInput不断读取数据进行处理，非SourceTask在收到上游的CheckpointBarrier对齐后触发Checkpoint,

StreamTask::processInput
StreamOneInputProcessor::processInput
StreamTaskNetworkInput::emitNext(StreamTaskNetworkOutput)
AbstractStreamTaskNetworkInput::emitNext //循环不断从buffer中读取StreamElement
处理
	CheckpointedInputGate::pollNext
	CheckpointedInputGate::handleEvent
		SingleCheckpointBarrierHandler::processBarrier
		SingleCheckpointBarrierHandler::markCheckpointAlignedAndTransformState
			WaitingForFirstBarrier::barrierReceived
			AbstractAlignedBarrierHandlerState::barrierReceived
			 SingleCheckpointBarrierHandler.ControllerImpl::allBarriersReceived//判断对齐
			 AbstractAlignedBarrierHandlerState::triggerGlobalCheckpoint
			  SingleCheckpointBarrierHandler.ControllerImpl::triggerGlobalCheckpoint
			  SingleCheckpointBarrierHandler::triggerCheckpoint
			  	CheckpointBarrierHandler::notifyCheckpoint //触发StreamTask Checkpoint
			  		StreamTask::triggerCheckpointOnBarrier
			  			StreamTask::performCheckpoint //后续调用过程与SourceTask一样
			  			SubtaskCheckpointCoordinatorImpl::checkpointState

根据调用栈看出，非SourceStreamTask执行Checkpoint只是触发时机不同，SourceTask由JobMaster RPC定时不断触发，非SourceTask则是在上游的CheckpointBarrier对齐后触发Checkpoint，最终执行逻辑都是将当前算子的信息写入CheckpointStorage后向JobMaster发送确认信息。

StreamTask向JobMaster ACK信息中包含状态元信息及StreamStateHandle，根据状态存储位置分为：

ByteStreamStateHandle //对应JobManagerCheckpointStorage,将状态序列化为byte[]发送给JobMaster
FileStateHandle //对应FileSystemCheckpointStorage,将状态写入文件系统后将文件路径发送给JobMaster

JobMaster端完成流程

JobMaster收到StreamTask的acknowledgeCheckpoint后：

JobMaster::acknowledgeCheckpoint
SchedulerBase::acknowledgeCheckpoint
ExecutionGraphHandler::acknowledgeCheckpoint
CheckpointCoordinator::receiveAcknowledgeMessage
	PendingCheckpoint::acknowledgeTask //某一个Task的确认
		PendingCheckpoint::updateOperatorState//更新SubTask状态信息
	CheckpointCoordinator::completePendingCheckpoint//所有Task Ack后
		PendingCheckpoint::finalizeCheckpoint
			Checkpoints.storeCheckpointMetadata//保存CheckpointMetadata
				CompletedCheckpoint::new
		CheckpointCoordinator::sendAcknowledgeMessages//向Task通知Checkpoint完成消息
			ExecutionVertex::notifyCheckpointComplete
				TaskManagerGateway.notifyCheckpointComplete

JobMaster收到所有StreamTask的Checkpoint状态信息后，标志一次Checkpoint完成，这时会通知StreamTask CheckPoint完成消息，便于SubTask监听Checkpoint完成后做后续动作。