zoukankan      html  css  js  c++  java
  • MapReduce(3): Partitioner, Combiner and Shuffling

    Partitioner:

    Partitioning and Combining take place between Map and Reduce phases. It is to club the data which should go to the same reducer based on keys. The number of partitioners is equal to the number of reducers. That means a partitioner will divide the data according to the number of reducers. Therefore, the data passed from a single partitioner is processed by a single Reducer. HashPartitioner is the default Partitioner in hadoop.

    A partitioner partitions the key-value pairs of intermediate Map-outputs. It partitions the data using a user-defined condition, which works like a hash function. The total number of partitions is same as the number of Reducer tasks for the job. Records having the same key value go into the same partition (within each mapper).

    Partition doing jobs on local machine.

    Combiner:

    Combiner is a 'mini-reducer' (semi-reducer), used to process reducer's work before transfering data onto reducers. It can reduce network congestion. An example is shown below:

    Shuffle:

    shuffle notify master to copy files onto reducer machines. In the final output of map task there can be multiple partitions and these partitions should go to different reduce task. Shuffling is basically transferring map output partitions to the corresponding reduce tasks. Map task notified application master about completion of map task and application master notifies corresponding reducer to copy the map output into reduce machine. As shuffling can start even before the map phase has finished so this saves some time and completes the tasks in lesser time.

    References:

    https://www.cnblogs.com/hadoop-dev/p/5910459.html

    https://blog.csdn.net/bitcarmanlee/article/details/60137837

    http://geekdirt.com/blog/map-reduce-in-detail/

    Using hash function to map immediate K,V pairs

    https://en.wikipedia.org/wiki/Hash_function

    https://www.tutorialspoint.com/map_reduce/map_reduce_partitioner.htm

    https://data-flair.training/blogs/hadoop-partitioner-tutorial/

  • 相关阅读:
    简单的倒计时 时间显示
    git submodule
    使用选择器语法来查找元素
    yo bootstrap mui 使用对比
    flexbox 兼容安卓4.3
    mac 下 php 安装 中的坑
    微信网页开发
    能发送http请求(get,post)的工具
    h5宣传页制作过程中遇到的问题
    功能模块图、业务流程图、处理流程图、ER图,数据库表图(概念模型和物理模型)画法
  • 原文地址:https://www.cnblogs.com/rhyswang/p/10946833.html
Copyright © 2011-2022 走看看