apache-flink - Flink 在没有窗口的情况下减少键控流

Question

我阅读了源代码，reduce 会将每个结果转发给下游。

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我想在没有窗口的情况下按键减少流，

    stream.keyBy(key)
          .reduce((a, b) -> {
                //reduce
                return a+b;
          });

如果reduce on window，flink会在watermark到达的时候forword元素到下游，那么flink如何判断reduce finish without window。

Answer 1

根据官方文档https://ci.apache.org/projects/flink/flink-docs-stable/dev/stream/operators/

Reduce KeyedStream → DataStream

A "rolling" reduce on a keyed data stream. Combines the current element with the last reduced value and emits the new value.

Window Reduce WindowedStream → DataStream

Applies a functional reduce function to the window and returns the reduced value.

主要区别在于:

• 当 reduce 在窗口中完成时，该函数将当前值与窗口值合并。
• 当 reduce 在 KeyedStream 中完成时，该函数将当前值与最新值组合。