聊聊flink的ListCheckpointed

序本文主要研究一下flink的ListCheckpointed实例public static class CounterSource extends RichParallelSourceFunction<Long> implements ListCheckpointed<Long> { /** current offset for exactly once semantics / private Long offset; /* flag for job cancellation / private volatile boolean isRunning = true; @Override public void run(SourceContext<Long> ctx) { final Object lock = ctx.getCheckpointLock(); while (isRunning) { // output and state update are atomic synchronized (lock) { ctx.collect(offset); offset += 1; } } } @Override public void cancel() { isRunning = false; } @Override public List<Long> snapshotState(long checkpointId, long checkpointTimestamp) { return Collections.singletonList(offset); } @Override public void restoreState(List<Long> state) { for (Long s : state) offset = s; }}CounterSource是一个有状态的RichParallelSourceFunction，它实现了ListCheckpointed接口，snapshotState方法返回了当前的offset，而restoreState方法则根据传入的state来恢复本地的offset；这里要注意，如果要在failure或者recovery的时候达到exactly-once的语义，这里更新offset的时候要使用SourceContext.getCheckpointLock来进行同步操作ListCheckpointedflink-streaming-java_2.11-1.7.0-sources.jar!/org/apache/flink/streaming/api/checkpoint/ListCheckpointed.java@PublicEvolvingpublic interface ListCheckpointed<T extends Serializable> { /* * Gets the current state of the function. The state must reflect the result of all prior * invocations to this function. * * The returned list should contain one entry for redistributable unit of state. See * the {@link ListCheckpointed class docs} for an illustration how list-style state * redistribution works. * * As special case, the returned list may be null or empty (if the operator has no state) * or it may contain a single element (if the operator state is indivisible). * * @param checkpointId The ID of the checkpoint - a unique and monotonously increasing value. * @param timestamp The wall clock timestamp when the checkpoint was triggered by the master. * * @return The operator state in a list of redistributable, atomic sub-states. * Should not return null, but empty list instead. * * @throws Exception Thrown if the creation of the state object failed. This causes the * checkpoint to fail. The system may decide to fail the operation (and trigger * recovery), or to discard this checkpoint attempt and to continue running * and to try again with the next checkpoint attempt. / List<T> snapshotState(long checkpointId, long timestamp) throws Exception; /* * Restores the state of the function or operator to that of a previous checkpoint. * This method is invoked when the function is executed after a failure recovery. * The state list may be empty if no state is to be recovered by the particular parallel instance * of the function. * * The given state list will contain all the sub states that this parallel * instance of the function needs to handle. Refer to the {@link ListCheckpointed class docs} * for an illustration how list-style state redistribution works. * * Important: When implementing this interface together with {@link RichFunction}, * then the {@code restoreState()} method is called before {@link RichFunction#open(Configuration)}. * * @param state The state to be restored as a list of atomic sub-states. * * @throws Exception Throwing an exception in this method causes the recovery to fail. * The exact consequence depends on the configured failure handling strategy, * but typically the system will re-attempt the recovery, or try recovering * from a different checkpoint. */ void restoreState(List<T> state) throws Exception;}ListCheckpointed定义了两个接口，一个是snapshotState方法，一个是restoreState方法snapshotState方法，方法有个checkpointId参数，是唯一单调递增的数字，而timestamp则是master触发checkpoint的时间戳，该方法要返回当前的state(List结构)restoreState方法会在failure recovery的时候被调用，传递的参数为List类型的state，方法里头可以将state恢复到本地小结stateful function可以通过CheckpointedFunction接口或者ListCheckpointed接口来使用managed operator state；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)ListCheckpointed是CheckpointedFunction的限制版，它只能支持Even-split redistribution模式的list-style stateListCheckpointed定义了两个方法，分别是snapshotState方法及restoreState方法；snapshotState方法在master触发checkpoint的时候被调用，用户需要返回当前的状态，而restoreState方法会在failure recovery的时候被调用，传递的参数为List类型的state，方法里头可以将state恢复到本地docWorking with State listcheckpointed聊聊flink的CheckpointedFunction