MapReduce
MapReduce is a programming model for data processing. The model is simple, yet not too simple to express useful programs in. Hadoop can run MapReduce programs written
in various languages; in this chapter, we shall look at the same program expressed in Java, Ruby, Python, and C++. Most important, MapReduce programs are inherently parallel, thus putting very large-scale data analysis into the hands of anyone with enough machines
at their disposal. MapReduce comes into its own for large datasets, so let’s start by looking at one.
2.1 Analyzing the Data with Hadoop 使用Hadoop分析数据
To take advantage of the parallel processing that Hadoop provides, we need to express our query as a MapReduce job. After some local, small-scale testing, we will be able to
run it on a cluster of machines.
利用Hadoop提供的并发处理的优势，我们须要使用MapReduce job来表达一个查询，通过一个本地化、小范围的測试，我们就能够在集群机器上执行了。
2.2 Map and Reduce
MapReduce works by breaking the processing into two phases: the map phase and the reduce phase.
Each phase has key-value pairs as input and output, the types of which may be chosen by the programmer. The programmer also specifies two functions: the map function and the reduce function.
map函数和 reduce函数 输入输出键值对
2.3 Scaling Out 横向扩展
Data Flow 数据流
A MapReduce job is a unit of work that the client wants to be performed: it consists of the input data, the MapReduce program, and configuration information. Hadoop runs the job by dividing it into tasks, of which there are two types: map tasks and reduce tasks.
job 是client运行的一个工作单元。由输入数据、程序和配置信息组成。