seniorApache Spark
Stream-Stream Joins and Watermarking.
Updated May 5, 2026
Short answer
Watermarking allows Spark to join two streams by limiting how much state it must keep.
Deep explanation
In stream-stream joins, Spark must buffer data indefinitely to wait for matches. Watermarking defines a threshold (e.g., '10 minutes late') after which Spark drops old data from memory.
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