Package org.apache.beam.sdk.transforms.splittabledofn

Class RestrictionTracker<RestrictionT,​PositionT>

  • Direct Known Subclasses:
    ByteKeyRangeTracker, OffsetRangeTracker, PeriodicSequence.OutputRangeTracker
    public abstract class RestrictionTracker<RestrictionT,​PositionT>
extends java.lang.Object
    Manages access to the restriction and keeps track of its claimed part for a splittable DoFn.

    The restriction may be modified by different threads, however the system will ensure sufficient locking such that no methods on the restriction tracker will be called concurrently.

    RestrictionTrackers should implement RestrictionTracker.HasProgress otherwise poor auto-scaling of workers and/or splitting may result if the progress is an inaccurate representation of the known amount of completed and remaining work.

    • Constructor Detail

      • RestrictionTracker

        public RestrictionTracker()

    • Method Detail

      • tryClaim

        public abstract boolean tryClaim​(PositionT position)
        Attempts to claim the block of work in the current restriction identified by the given position. Each claimed position MUST be a valid split point.

        If this succeeds, the DoFn MUST execute the entire block of work. If this fails:

        This method is required to be implemented.

      • currentRestriction

        public abstract RestrictionT currentRestriction()
        Returns a restriction accurately describing the full range of work the current DoFn.ProcessElement call will do, including already completed work.

        The current restriction returned by method may be updated dynamically due to due to concurrent invocation of other methods of the RestrictionTracker, For example, trySplit(double).

        This method is required to be implemented.

      • trySplit

        public abstract @Nullable SplitResult<RestrictionT> trySplit​(double fractionOfRemainder)
        Splits current restriction based on fractionOfRemainder.

        If splitting the current restriction is possible, the current restriction is split into a primary and residual restriction pair. This invocation updates the currentRestriction() to be the primary restriction effectively having the current DoFn.ProcessElement execution responsible for performing the work that the primary restriction represents. The residual restriction will be executed in a separate DoFn.ProcessElement invocation (likely in a different process). The work performed by executing the primary and residual restrictions as separate DoFn.ProcessElement invocations MUST be equivalent to the work performed as if this split never occurred.

        The fractionOfRemainder should be used in a best effort manner to choose a primary and residual restriction based upon the fraction of the remaining work that the current DoFn.ProcessElement invocation is responsible for. For example, if a DoFn.ProcessElement was reading a file with a restriction representing the offset range [100, 200) and has processed up to offset 130 with a fractionOfRemainder of 0.7, the primary and residual restrictions returned would be [100, 179), [179, 200) (note: currentOffset + fractionOfRemainder * remainingWork = 130 + 0.7 * 70 = 179).

        fractionOfRemainder = 0 means a checkpoint is required.

        The API is recommended to be implemented for a batch pipeline to improve parallel processing performance.

        The API is recommended to be implemented for batch pipeline given that it is very important for pipeline scaling and end to end pipeline execution.

        The API is required to be implemented for a streaming pipeline.

        Parameters:
        fractionOfRemainder - A hint as to the fraction of work the primary restriction should represent based upon the current known remaining amount of work.
        Returns:
        a SplitResult if a split was possible, otherwise returns null. If the fractionOfRemainder == 0, a null result MUST imply that the restriction tracker is done and there is no more work left to do.

      • checkDone

        public abstract void checkDone()
                        throws java.lang.IllegalStateException
        Checks whether the restriction has been fully processed.

        Called by the SDK harness after DoFn.ProcessElement returns.

        Must throw an exception with an informative error message, if there is still any unclaimed work remaining in the restriction.

        This method is required to be implemented in order to prevent data loss during SDK processing.

        Throws:
        java.lang.IllegalStateException