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本文主要研究一下flink的CheckpointedFunction
實例
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);
}
}
}
}
- 這個BufferingSink實現(xiàn)了CheckpointedFunction接口心铃，它定義了ListState類型的checkpointedState葡盗，以及List結(jié)構(gòu)的bufferedElements
- 在invoke方法里頭先將value緩存到bufferedElements，緩存?zhèn)€數(shù)觸發(fā)閾值時巨双，執(zhí)行sink操作副女，然后清空bufferedElements
- 在snapshotState方法里頭對bufferedElements進(jìn)行snapshot操作锌仅，在initializeState先創(chuàng)建ListStateDescriptor守谓，然后通過FunctionInitializationContext.getOperatorStateStore().getListState(descriptor)來獲取ListState，之后判斷state是否有在前一次execution的snapshot中restored钓株，如果有則將ListState中的數(shù)據(jù)恢復(fù)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的核心接口实牡，用于跨stream維護(hù)state
- snapshotState在checkpoint的時候會被調(diào)用，用于snapshot state轴合，通常用于flush创坞、commit、synchronize外部系統(tǒng)
- initializeState在parallel function初始化的時候(第一次初始化或者從前一次checkpoint recover的時候)被調(diào)用受葛，通常用來初始化state题涨，以及處理state recovery的邏輯
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繼承了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定義了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繼承了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接口定義了isRestored总滩、getOperatorStateStore纲堵、getKeyedStateStore方法
小結(jié)
- flink有兩種基本的state，分別是Keyed State以及Operator State(
non-keyed state)闰渔；其中Keyed State只能在KeyedStream上的functions及operators上使用席函；每個operator state會跟parallel operator中的一個實例綁定；Operator State支持parallelism變更時進(jìn)行redistributing - Keyed State及Operator State都分別有managed及raw兩種形式冈涧，managed由flink runtime來管理茂附，由runtime負(fù)責(zé)encode及寫入checkpoint；raw形式的state由operators自己管理督弓，flink runtime無法了解該state的數(shù)據(jù)結(jié)構(gòu)营曼，將其視為raw bytes；所有的datastream function都可以使用managed state咽筋，而raw state一般僅限于自己實現(xiàn)operators來使用
- stateful function可以通過CheckpointedFunction接口或者ListCheckpointed接口來使用managed operator state溶推；CheckpointedFunction定義了snapshotState徊件、initializeState兩個方法奸攻；每當(dāng)checkpoint執(zhí)行的時候蒜危，snapshotState會被調(diào)用；而initializeState方法在每次用戶定義的function初始化的時候(
第一次初始化或者從前一次checkpoint recover的時候)被調(diào)用睹耐，該方法不僅可以用來初始化state辐赞，還可以用于處理state recovery的邏輯 - 對于manageed operator state，目前僅僅支持list-style的形式硝训，即要求state是serializable objects的List結(jié)構(gòu)响委，方便在rescale的時候進(jìn)行redistributed；關(guān)于redistribution schemes的模式目前有兩種窖梁，分別是Even-split redistribution(
在restore/redistribution的時候每個operator僅僅得到整個state的sublist)及Union redistribution(在restore/redistribution的時候每個operator得到整個state的完整list) - FunctionSnapshotContext繼承了ManagedSnapshotContext接口赘风，它定義了getCheckpointId、getCheckpointTimestamp方法纵刘；FunctionInitializationContext繼承了ManagedInitializationContext接口邀窃，它定義了isRestored、getOperatorStateStore假哎、getKeyedStateStore方法瞬捕，可以用來判斷是否是在前一次execution的snapshot中restored，以及獲取OperatorStateStore舵抹、KeyedStateStore對象
