聊聊flink的CheckpointedFunction

序
本文主要研究一下flink的CheckpointedFunction
实例
public class BufferingSink
implements SinkFunction<Tuple2<String, Integer>>,
CheckpointedFunction {

private final int threshold;

private transient ListState<Tuple2<String, Integer>> checkpointedState;

private List<Tuple2<String, Integer>> bufferedElements;

public BufferingSink(int threshold) {
this.threshold = threshold;
this.bufferedElements = new ArrayList<>();
}

@Override
public void invoke(Tuple2<String, Integer> value) throws Exception {
bufferedElements.add(value);
if (bufferedElements.size() == threshold) {
for (Tuple2<String, Integer> element: bufferedElements) {
// send it to the sink
}
bufferedElements.clear();
}
}

@Override
public void snapshotState(FunctionSnapshotContext context) throws Exception {
checkpointedState.clear();
for (Tuple2<String, Integer> element : bufferedElements) {
checkpointedState.add(element);
}
}

@Override
public void initializeState(FunctionInitializationContext context) throws Exception {
ListStateDescriptor<Tuple2<String, Integer>> descriptor =
new ListStateDescriptor<>(
“buffered-elements”,
TypeInformation.of(new TypeHint<Tuple2<String, Integer>>() {}));

checkpointedState = context.getOperatorStateStore().getListState(descriptor);

if (context.isRestored()) {
for (Tuple2<String, Integer> element : checkpointedState.get()) {
bufferedElements.add(element);
}
}
}
}

这个BufferingSink实现了CheckpointedFunction接口，它定义了ListState类型的checkpointedState，以及List结构的bufferedElements
在invoke方法里头先将value缓存到bufferedElements，缓存个数触发阈值时，执行sink操作，然后清空bufferedElements
在snapshotState方法里头对bufferedElements进行snapshot操作，在initializeState先创建ListStateDescriptor，然后通过FunctionInitializationContext.getOperatorStateStore().getListState(descriptor)来获取ListState，之后判断state是否有在前一次execution的snapshot中restored，如果有则将ListState中的数据恢复到bufferedElements

CheckpointedFunction
flink-streaming-java_2.11-1.7.0-sources.jar!/org/apache/flink/streaming/api/checkpoint/CheckpointedFunction.java
@PublicEvolving
@SuppressWarnings(“deprecation”)
public interface CheckpointedFunction {

/**
* This method is called when a snapshot for a checkpoint is requested. This acts as a hook to the function to
* ensure that all state is exposed by means previously offered through {@link FunctionInitializationContext} when
* the Function was initialized, or offered now by {@link FunctionSnapshotContext} itself.
*
* @param context the context for drawing a snapshot of the operator
* @throws Exception
*/
void snapshotState(FunctionSnapshotContext context) throws Exception;

/**
* This method is called when the parallel function instance is created during distributed
* execution. Functions typically set up their state storing data structures in this method.
*
* @param context the context for initializing the operator
* @throws Exception
*/
void initializeState(FunctionInitializationContext context) throws Exception;

}

CheckpointedFunction是stateful transformation functions的核心接口，用于跨stream维护state
snapshotState在checkpoint的时候会被调用，用于snapshot state，通常用于flush、commit、synchronize外部系统
initializeState在parallel function被创建时调用，通常用于初始化存储于state的数据

FunctionSnapshotContext
flink-runtime_2.11-1.7.0-sources.jar!/org/apache/flink/runtime/state/FunctionSnapshotContext.java
/**
* This interface provides a context in which user functions that use managed state (i.e. state that is managed by state
* backends) can participate in a snapshot. As snapshots of the backends themselves are taken by the system, this
* interface mainly provides meta information about the checkpoint.
*/
@PublicEvolving
public interface FunctionSnapshotContext extends ManagedSnapshotContext {
}
FunctionSnapshotContext继承了ManagedSnapshotContext接口
ManagedSnapshotContext
flink-runtime_2.11-1.7.0-sources.jar!/org/apache/flink/runtime/state/ManagedSnapshotContext.java
/**
* This interface provides a context in which operators that use managed state (i.e. state that is managed by state
* backends) can perform a snapshot. As snapshots of the backends themselves are taken by the system, this interface
* mainly provides meta information about the checkpoint.
*/
@PublicEvolving
public interface ManagedSnapshotContext {

/**
* Returns the ID of the checkpoint for which the snapshot is taken.
*
* <p>The checkpoint ID is guaranteed to be strictly monotonously increasing across checkpoints.
* For two completed checkpoints <i>A</i> and <i>B</i>, {@code ID_B > ID_A} means that checkpoint
* <i>B</i> subsumes checkpoint <i>A</i>, i.e., checkpoint <i>B</i> contains a later state
* than checkpoint <i>A</i>.
*/
long getCheckpointId();

/**
* Returns timestamp (wall clock time) when the master node triggered the checkpoint for which
* the state snapshot is taken.
*/
long getCheckpointTimestamp();
}
ManagedSnapshotContext定义了getCheckpointId、getCheckpointTimestamp方法
FunctionInitializationContext
flink-runtime_2.11-1.7.0-sources.jar!/org/apache/flink/runtime/state/FunctionInitializationContext.java
/**
* This interface provides a context in which user functions can initialize by registering to managed state (i.e. state
* that is managed by state backends).
*
* <p>
* Operator state is available to all functions, while keyed state is only available for functions after keyBy.
*
* <p>
* For the purpose of initialization, the context signals if the state is empty or was restored from a previous
* execution.
*
*/
@PublicEvolving
public interface FunctionInitializationContext extends ManagedInitializationContext {
}
FunctionInitializationContext继承了ManagedInitializationContext接口
ManagedInitializationContext
flink-runtime_2.11-1.7.0-sources.jar!/org/apache/flink/runtime/state/ManagedInitializationContext.java
/**
* This interface provides a context in which operators can initialize by registering to managed state (i.e. state that
* is managed by state backends).
*
* <p>
* Operator state is available to all operators, while keyed state is only available for operators after keyBy.
*
* <p>
* For the purpose of initialization, the context signals if the state is empty (new operator) or was restored from
* a previous execution of this operator.
*
*/
public interface ManagedInitializationContext {

/**
* Returns true, if state was restored from the snapshot of a previous execution. This returns always false for
* stateless tasks.
*/
boolean isRestored();

/**
* Returns an interface that allows for registering operator state with the backend.
*/
OperatorStateStore getOperatorStateStore();

/**
* Returns an interface that allows for registering keyed state with the backend.
*/
KeyedStateStore getKeyedStateStore();

}
ManagedInitializationContext接口定义了isRestored、getOperatorStateStore、getKeyedStateStore方法
小结

flink有两种基本的state，分别是Keyed State以及Operator State(non-keyed state)；其中Keyed State只能在KeyedStream上的functions及operators上使用；每个operator state会跟parallel operator中的一个实例绑定；Operator State支持parallelism变更时进行redistributing
Keyed State及Operator State都分别有managed及raw两种形式，managed由flink runtime来管理，由runtime负责encode及写入checkpoint；raw形式的state由operators自己管理，flink runtime无法了解该state的数据结构，将其视为raw bytes；所有的datastream function都可以使用managed state，而raw state一般仅限于自己实现operators来使用
stateful function可以通过CheckpointedFunction接口或者ListCheckpointed接口来使用managed operator state；CheckpointedFunction定义了snapshotState、initializeState两个方法；每当checkpoint执行的时候，snapshotState会被调用；而initializeState方法在每次用户定义的function初始化的时候(第一次初始化或者从前一次checkpoint recover的时候)被调用，该方法不仅可以用来初始化state，还可以用于处理state recovery的逻辑
对于manageed operator state，目前仅仅支持list-style的形式，即要求state是serializable objects的List结构，方便在rescale的时候进行redistributed；关于redistribution schemes的模式目前有两种，分别是Even-split redistribution(在restore/redistribution的时候每个operator仅仅得到整个state的sublist)及Union redistribution(在restore/redistribution的时候每个operator得到整个state的完整list)
FunctionSnapshotContext继承了ManagedSnapshotContext接口，它定义了getCheckpointId、getCheckpointTimestamp方法；FunctionInitializationContext继承了ManagedInitializationContext接口，它定义了isRestored、getOperatorStateStore、getKeyedStateStore方法，可以用来判断是否是在前一次execution的snapshot中restored，以及获取OperatorStateStore、KeyedStateStore对象

doc
Using Managed Operator State