The task parallelism can be specified at different levels of Flink. Four levels are: operator level, the execution environment (ExecutionEnvironment) level, the client (command line) level, the configuration file (flink-conf.yaml) level
* Each operator, data source or data sink can by calling setParallelism()to the specified method
* The default number of concurrent operating environment by calling setParallelism()to the specified method.env.setParallelism(3);运行环境的并发数可以被每个算子确切的并发数配置所覆盖。
* 对于CLI客户端，并发参数可以通过-p来指定
* 影响所有运行环境的系统级别的默认并发度可以在./conf/flink-conf.yaml的parallelism.defaul项中指定。不建议
当然，你也可以设置最大的并行度
* 你可以通过调用setMaxParallelism()方法来设置最大并发度。
Flink how to determine the number TaskManager? Answer: The maximum degree of parallelism Job divided by the number assigned to each task slot TaskManager
Flink on YARN, the number TaskManager is: max(parallelism) / yarnslots(rounded up). For example, a maximum degree of parallelism of 10, each job has two tasks TaskManager groove 5 will start TaskManager
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