Class EventTimeMapper<T>

Type Parameters:
T - the event type

public class EventTimeMapper<T> extends Object
A utility that helps a source emit events according to a given EventTimePolicy. Generally this class should be used if a source needs to emit watermarks. The mapper deals with the following concerns:

1. Reading partition by partition

Upon restart it can happen that partition P1 has one very recent event and P2 has an old one. If we poll P1 first and emit its recent event, it will advance the watermark. When we poll P2 later on, its event will be behind the watermark and can be dropped as late. This utility tracks the event timestamps for each source partition individually and allows the processor to emit the watermark that is correct with respect to all the partitions.

2. Some partition having no data

It can happen that some partition does not have any events at all while others do, or the processor doesn't get any external partitions assigned to it. If we simply wait for the timestamps in all partitions to advance to some point, we won't be emitting any watermarks. This utility supports the idle timeout: if there's no new data from a partition after the timeout elapses, it will be marked as idle, allowing the processor's watermark to advance as if that partition didn't exist. If all partitions are idle or there are no partitions, the processor will emit a special idle message and the downstream will exclude this processor from watermark coalescing.

3. Wrapping of events

Events may need to be wrapped with the extracted timestamp if EventTimePolicy.wrapFn() is set.

4. Throttling of Watermarks

Watermarks are only consumed by windowing operations and emitting watermarks more frequently than the given EventTimePolicy.watermarkThrottlingFrameSize() is wasteful since they are broadcast to all processors. The mapper ensures that watermarks are emitted according to the throttling frame size.


The API is designed to be used as a flat-mapping step in the Traverser that holds the output data. Your source can follow this pattern:

 public boolean complete() {
     if (traverser == null) {
         List<Record> records = poll();
         if (records.isEmpty()) {
             traverser = eventTimeMapper.flatMapIdle();
         } else {
             traverser = traverseIterable(records)
                 .flatMap(event -> eventTimeMapper.flatMapEvent(
                      event, event.getPartition()));
         traverser = traverser.onFirstNull(() -> traverser = null);
     emitFromTraverser(traverser, event -> {
         if (!(event instanceof Watermark)) {
             // store your offset after event was emitted
             offsetsMap.put(event.getPartition(), event.getOffset());
     return false;
Other methods:
  • Call addPartitions(int) and removePartition(int) to change your partition count initially or whenever the count changes.
  • If you support state snapshots, save the value returned by getWatermark(int) for all partitions to the snapshot. When restoring the state, call restoreWatermark(int, long).
    You should save the value under your external partition key so that the watermark value can be restored to correct processor instance. The key should also be wrapped using broadcastKey(), because the external partitions don't match Hazelcast partitions. This way, all processor instances will see all keys and they can restore the partitions they handle and ignore others.
Jet 3.0
  • Field Details

  • Constructor Details

  • Method Details

    • flatMapEvent

      @Nonnull public Traverser<Object> flatMapEvent(@Nullable T event, int partitionIndex, long nativeEventTime)
      Flat-maps the given event by (possibly) prepending it with a watermark. Designed to use when emitting from traverser:
           Traverser t = traverserIterable(...)
               .flatMap(event -> eventTimeMapper.flatMapEvent(
                       event, event.getPartition(), nativeEventTime));
      event - the event to flat-map. If null, it's equivalent to the behavior of flatMapIdle()
      partitionIndex - the source partition index the event came from
      nativeEventTime - native event time in case no timestampFn was supplied or NO_NATIVE_TIME if the event has no native timestamp
      a traverser over the given event and the watermark (if it was due)
    • flatMapIdle

      @Nonnull public Traverser<Object> flatMapIdle()
      Call this method when there is no event to emit. It returns a traverser over the watermark, if it was due.
    • flatMapEvent

      public Traverser<Object> flatMapEvent(long now, @Nullable T event, int partitionIndex, long nativeEventTime)
      A lower-level variant of flatMapEvent(T, int, long) that accepts an explicit result of a System.nanoTime() call. Use this variant if you're calling it in a hot loop, in order to avoid repeating the expensive System.nanoTime() call.
    • addPartitions

      public void addPartitions(int addedCount)
      Adds addedCount partitions. Added partitions will be initially considered active and having watermark value equal to the last emitted watermark. Their indices will follow current highest index.

      You can call this method whenever new partitions are detected.

      addedCount - number of added partitions, must be >= 0
    • removePartition

      public Traverser<Object> removePartition(int partitionIndex)
      Removes a partition that will no longer have events. If we were waiting for a watermark from it, the returned traverser might contain a watermark to emit.

      Note that the indexes of partitions with index larger than the given partitionIndex will be decremented by 1.

      partitionIndex - the index of the removed partition
    • partitionCount

      public int partitionCount()
      Returns the current partition count.
    • getWatermark

      public long getWatermark(int partitionIndex)
      Watermark value to be saved to state snapshot for the given source partition index. The returned value should be restored to a processor handling the same partition after restart.

      Method is meant to be used from Processor.saveToSnapshot().

      partitionIndex - 0-based source partition index.
      A value to save to state snapshot
    • restoreWatermark

      public void restoreWatermark(int partitionIndex, long wm)
      Restore watermark value from state snapshot.

      Method is meant to be used from Processor.restoreFromSnapshot(Inbox).

      See getWatermark(int).

      partitionIndex - 0-based source partition index.
      wm - watermark value to restore