What is the purpose of watermarking in Spark Structured Streaming (or similar event-time based streaming systems)?

Question

What is the purpose of  watermarking  in Spark Structured Streaming (or similar event-time based streaming systems)?

Answer

To enforce exactly-once processing by marking messages

Answer

To speed up stream processing by skipping over idle periods

Answer

To synchronize the system clocks of all nodes in the cluster

Answer

To define a threshold for event lateness

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