mxm commented on a change in pull request #6980: [FLINK-5697] [kinesis] Add
periodic per-shard watermark support
URL: https://github.com/apache/flink/pull/6980#discussion_r234580470
##########
File path:
flink-connectors/flink-connector-kinesis/src/main/java/org/apache/flink/streaming/connectors/kinesis/FlinkKinesisConsumer.java
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@@ -78,6 +79,22 @@
* A custom assigner implementation can be set via {@link
#setShardAssigner(KinesisShardAssigner)} to optimize the
* hash function or use static overrides to limit skew.
*
+ * <p>In order for the consumer to emit watermarks, a timestamp assigner needs
to be set via {@link
+ * #setPeriodicWatermarkAssigner(AssignerWithPeriodicWatermarks)} and the auto
watermark emit
+ * interval configured via {@link
+ * org.apache.flink.api.common.ExecutionConfig#setAutoWatermarkInterval(long)}.
+ *
+ * <p>Watermarks can only advance when all shards of a subtask continuously
deliver records. To
+ * avoid an inactive or closed shard to block the watermark progress, the idle
timeout should be
+ * configured via configuration property {@link
+ * ConsumerConfigConstants#SHARD_IDLE_INTERVAL_MILLIS}. By default, shards
won't be considered
+ * idle and watermark calculation will wait for newer records to arrive from
all shards.
+ *
+ * <p>Note that re-sharding of the Kinesis stream while an application (that
relies on
+ * the Kinesis records for watermarking) is running can lead to incorrect late
events.
+ * This depends on how shards are assigned to subtasks and applies regardless
of whether watermarks
+ * are generated in the source or a downstream operator.
Review comment:
Good to mention this here. The re-sharding logic can corrupt the
watermarking logic, but that is not unique to this change.
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