Stateful vs Stateless Transforms

Concepts covered: paStatefulVsStateless, paWatermarks

Transforms divide into two categories that matter much more in streaming than in batch. A stateless transform processes one event at a time and produces output that depends only on that event. A stateful transform produces output that depends on more than one event: a count, a sum, a window, a join with another stream. The category changes the cost, the complexity, and the failure-recovery story. In batch, both categories look about the same because the engine has all the data in memory at once. In streaming, the difference is structural and shapes nearly every design decision. Stateless Transforms Stateless transforms are cheap and easy in both batch and streaming. They scale linearly with event count, recover trivially from failure, and require no state store. A streaming pipeline of pur

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