DataStream Transformation

1 map 映射
2 flatMap 而flatMap函数则是两个操作的集合——正是“先映射后扁平化”:
3 reduce reduce()与fold()方法是对同种元素类型数据的RDD进行操作，即必须同构。其返回值返回一个同样类型的新元素
4 fold fold()与reduce()类似，接收与reduce接收的函数签名相同的函数，另外再加上一个初始值作为第一次调用的结果
5 aggregate

  1. Flink 1.6 Release --- 2018/08/09

  2. Flink 1.7 Release --- 2018/11/30

  3. Flink 1.8 Release --- 2019/04/09 (FLIP-6 - Flink Deployment and Process Model - Standalone, Yarn, Mesos, Kubernetes, etc.)

Terminology

  1. To understand the managed state, we have to first start with stateful processing. If the result of processing an event (or a piece of data) is only related to the content of the event itself, it is called stateless processing; otherwise, the result is related to the previously processed event, and it is called stateful processing --- 为了理解托管状态，我们必须首先从状态处理开始。如果处理事件（或数据块）的结果只与事件本身的内容相关，则称为无状态处理；否则，结果与先前处理的事件相关，称为有状态处理。

  2. checkpoint : 检查点

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