MapReduce applications must have the characteristic of "Map" and "Reduce," meaning that the task or job can be divided into smaller pieces to be processed in parallel.
MapReduce应用程序必须具备“映射”和“缩减”的性质，也就是说任务或作业可以分割为小片段以进行并行处理。
The JobTracker, as the single control point for launching the MapReduce applications, decides how many TaskTracker and subordinate tasks to be created and then assigns each sub-task to TaskTracker.
JobTracker作为启动MapReduce应用程序的单一控制点，它决定应该创建多少个TaskTracker和子任务，然后把每个子任务分配给TaskTracker。
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