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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
