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From chandnisingh <...@git.apache.org>
Subject [GitHub] incubator-apex-core pull request: Checkpoint notification to notif...
Date Fri, 18 Dec 2015 00:49:02 GMT
Github user chandnisingh commented on a diff in the pull request:

    --- Diff: api/src/main/java/com/datatorrent/api/Operator.java ---
    @@ -270,4 +271,29 @@ public String toString()
    +  /**
    +   * Operators that need to be notified about checkpoint events should implement this
    +   *
    +   * The notification callbacks in this interface are called outside window boundaries
so the operators should not
    +   * attempt to send any tuples in these callbacks.
    +   *
    +   */
    +  public static interface CheckpointNotificationListener extends CheckpointListener
    +  {
    +    /**
    +     * Notify the operator that a checkpoint is about to be performed.
    +     *
    +     * Operators may need to perform certain tasks just before a checkpoint such as calling
flush on a stream. Having
    +     * this notification helps operators perform such operations optimally by doing them
once before checkpoint as
    +     * opposed to doing it repeatedly at the end of every window.
    +     *
    +     * The method will be called before the checkpoint is performed. It will be called
    +     * {@link Operator#endWindow()} call of the window preceding the checkpoint and before
the checkpoint is
    +     * actually performed.
    +     *
    +     * @param windowId The window id of the window preceding the checkpoint
    +     */
    +    public void checkpoint(long windowId);
    --- End diff --
    IMO it is needed with checkpoint as well. It signifies that state is going to be persisted
with respect to a particular window Id. 
    As mentioned  before this window id is not the same as what the operator caches in beginWindow(...)
when checkpointing is happen between application windows.
    One use case is checkpoint being done incrementally.
    It is consistent with checkpointed(windowId) callback.

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