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

ListCheckpointed

flink-streaming-java_2.11-1.7.0-sources.jar!/org/apache/flink/streaming/api/checkpoint/ListCheckpointed.java

@PublicEvolving
public 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.
     *
     * <p>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.
     *
     * <p>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.
     *
     * <p>The given state list will contain all the <i>sub states</i> 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.
     *
     * <p><b>Important:</b> 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 state
• ListCheckpointed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

doc

• Working with State listcheckpointed
• 聊聊flink的CheckpointedFunction