MapReduce是一种编程模型，通过将工作分成独立的任务并在一组机器上并行执行任务，可以处理和生成大量数据。 MapReduce编程风格的灵感来自函数式编程结构map和reduce，它们通常用于处理数据列表。在高层MapReduce程序将输入数据元素列表转换为输出数据元素列表两次，一次在映射阶段，一次在还原阶段。
本章首先介绍MapReduce编程模型，并描述数据如何流经模型的不同阶段。然后示例如何使用Python编写MapReduce作业。
数据流
MapReduce框架由三个主要阶段组成：map，shuffle和sort，以及reduce。
- map
在映射阶段，mapper函数处理一系列键值对。映射器按顺序处理键值对，产生零个或多个输出键值对。
比如将句子转换为单词。输入是包含句子的字符串，映射器将句子拆分为单词并输出单词。
- Shuffle和Sort
映射阶段的中间输出将移动到reducer。将输出从映射器移动到reducer的过程称为洗(shuffling)。
Shuffling由分区函数处理，称为partitioner。partitioner用于控制从映射器到reducer的键值对的流动。reducer知道映射器的输出键和reducer的数量，返回预期的reducer的索引。partitioner程序确保将同一键的所有值发送到同一reducer。默认分区程序是基于哈希的。它计算映射器输出键的哈希值，并根据此结果分配分区。
reducers开始处理数据之前的最后阶段是排序过程。在呈现给reducer之前，每个分区的中间键和值都由Hadoop框架排序。
- Reduce
在reducer阶段，值的迭代器被提供给称为reducer的函数。迭代器把值提供给reducer，这些值是唯一键的一组非唯一值。 reducer聚合每个唯一键的值，并产生零个或多个输出键值对。
数据流|
比如对键的所有值求和。此reducer的输入是键的所有值，reducer对所有值求和。然后，reducer输出键值对中包含输入键和输入键值的总和。
参考资料
- python测试开发项目实战-目录
- python工具书籍下载-持续更新
- python 3.7极速入门教程 - 目录
- 原文地址
- 本文涉及的python测试开发库 谢谢点赞！
- [本文相关海量书籍下载](https://github.com/china-testing/python-api-tesing/blob/master/books.md
Hadoop流
Hadoop流是与Hadoop发行版一起打包的工具，它允许使用任何可执行文件创建MapReduce作业作为映射器和reducer。 Hadoop流实用程序支持Python，shell。
mapper和reducer都是可执行文件，它们从标准输入(stdin),逐行读取输入，并将写入标准输出(stdout)。 Hadoop流公国创建MapReduce作业，将作业提交到集群，并监视其进度直到完成。
mapper初始化时，每个映射任务都会将指定的可执行文件作为单独的进程启动。映射器读取输入文件，并通过stdin将每行显示给可执行文件。在可执行文件处理每行输入后，映射器从stdout收集输出并将每一行转换为键值对。键由第一个制表符前面的行部分组成，值由第一个制表符后面的行部分组成。如果一行不包含制表符，则整行被视为键，值为null。
初始化reducer时，每个reduce任务都会将指定的可执行文件作为单独的进程启动。 reducer将输入键值对转换为通过stdin呈现给可执行文件的行。
reducer从stdout收集可执行文件的结果，并将每一行转换为键值对。与映射器类似，可执行文件通过制表符分隔键和值来指定键值对。
下面我们用python来模拟Hadoop流工具。
mapper.py：在WordCount的map阶段实现逻辑的Python程序。它从stdin读取数据，将行拆分为单词，并将每个单词的中间计数输出到stdout。
#!/usr/bin/env python
# https://github.com/china-testing/python-api-tesing
import sys
# Read each line from STDIN
for line in sys.stdin:
# Get the words in each line
words = line.split()
# Generate the count for each word
for word in words:
# Write the key-value pair to STDOUT to be processed by the reducer.
# The key is anything before the first tab character and the value is
# anything after the first tab character.
print('{0}\t{1}'.format(word, 1))
reducer.py是在WordCount的reduce阶段实现逻辑的Python程序。它从stdin中读取mapper.py的结果，对每个单词的出现次数求和，并将结果写入stdout。
#!/usr/bin/env python
import sys
curr_word = None
curr_count = 0
# Process each key-value pair from the mapper
for line in sys.stdin:
# Get the key and value from the current line
word, count = line.split('\t')
# Convert the count to an int
count = int(count)
# If the current word is the same as the previous word, increment its
# count, otherwise print the words count to STDOUT
if word == curr_word:
curr_count += count
else:
# Write word and its number of occurrences as a key-value pair to STDOUT
if curr_word:
print('{0}\t{1}'.format(curr_word, curr_count))
curr_word = word
curr_count = count
# Output the count for the last word
if curr_word == word:
print('{0}\t{1}'.format(curr_word, curr_count))执行
$ echo 'jack be nimble jack be quick' | ./mapper.py | sort -t 1 | ./reducer.py
be 2
jack 2
nimble 1
quick 1现在我们把'jack be nimble jack be quick'存成/home/hduser_/input2.txt，用hadoop来实现这一过程。
$ hdfs dfs -put /home/hduser_/input2.txt /user/hduser
$ $HADOOP_HOME/bin/hadoop jar $HADOOP_HOME/share/hadoop/tools/lib/hadoop-streaming-2.9.2.jar -files mapper.py,reducer.py -mapper mapper.py -reducer reducer.py -input /user/hduser/input2.txt -output /user/hduser/output
19/01/22 10:44:38 INFO Configuration.deprecation: session.id is deprecated. Instead, use dfs.metrics.session-id
19/01/22 10:44:38 INFO jvm.JvmMetrics: Initializing JVM Metrics with processName=JobTracker, sessionId=
19/01/22 10:44:38 INFO jvm.JvmMetrics: Cannot initialize JVM Metrics with processName=JobTracker, sessionId= - already initialized
19/01/22 10:44:38 ERROR streaming.StreamJob: Error Launching job : Output directory hdfs://localhost:54310/user/hduser/output already exists
Streaming Command Failed!
hduser_@andrew-PC:/home/andrew/code/HadoopWithPython/python/MapReduce/HadoopStreaming$ $HADOOP_HOME/bin/hadoop jar $HADOOP_HOME/share/hadoop/tools/lib/hadoop-streaming-2.9.2.jar -files mapper.py,reducer.py -mapper mapper.py -reducer reducer.py -input /user/hduser/input2.txt -output /user/hduser/output2
19/01/22 10:44:45 INFO Configuration.deprecation: session.id is deprecated. Instead, use dfs.metrics.session-id
19/01/22 10:44:45 INFO jvm.JvmMetrics: Initializing JVM Metrics with processName=JobTracker, sessionId=
19/01/22 10:44:45 INFO jvm.JvmMetrics: Cannot initialize JVM Metrics with processName=JobTracker, sessionId= - already initialized
19/01/22 10:44:46 INFO mapred.FileInputFormat: Total input files to process : 1
19/01/22 10:44:46 INFO mapreduce.JobSubmitter: number of splits:1
19/01/22 10:44:46 INFO mapreduce.JobSubmitter: Submitting tokens for job: job_local208759810_0001
19/01/22 10:44:46 INFO mapred.LocalDistributedCacheManager: Localized file:/home/andrew/code/HadoopWithPython/python/MapReduce/HadoopStreaming/mapper.py as file:/app/hadoop/tmp/mapred/local/1548125086275/mapper.py
19/01/22 10:44:46 INFO mapred.LocalDistributedCacheManager: Localized file:/home/andrew/code/HadoopWithPython/python/MapReduce/HadoopStreaming/reducer.py as file:/app/hadoop/tmp/mapred/local/1548125086276/reducer.py
19/01/22 10:44:46 INFO mapreduce.Job: The url to track the job: http://localhost:8080/
19/01/22 10:44:46 INFO mapred.LocalJobRunner: OutputCommitter set in config null
19/01/22 10:44:46 INFO mapreduce.Job: Running job: job_local208759810_0001
19/01/22 10:44:46 INFO mapred.LocalJobRunner: OutputCommitter is org.apache.hadoop.mapred.FileOutputCommitter
19/01/22 10:44:46 INFO output.FileOutputCommitter: File Output Committer Algorithm version is 1
19/01/22 10:44:46 INFO output.FileOutputCommitter: FileOutputCommitter skip cleanup _temporary folders under output directory:false, ignore cleanup failures: false
19/01/22 10:44:46 INFO mapred.LocalJobRunner: Waiting for map tasks
19/01/22 10:44:46 INFO mapred.LocalJobRunner: Starting task: attempt_local208759810_0001_m_000000_0
19/01/22 10:44:46 INFO output.FileOutputCommitter: File Output Committer Algorithm version is 1
19/01/22 10:44:46 INFO output.FileOutputCommitter: FileOutputCommitter skip cleanup _temporary folders under output directory:false, ignore cleanup failures: false
19/01/22 10:44:46 INFO mapred.Task: Using ResourceCalculatorProcessTree : [ ]
19/01/22 10:44:46 INFO mapred.MapTask: Processing split: hdfs://localhost:54310/user/hduser/input2.txt:0+29
19/01/22 10:44:46 INFO mapred.MapTask: numReduceTasks: 1
19/01/22 10:44:46 INFO mapred.MapTask: (EQUATOR) 0 kvi 26214396(104857584)
19/01/22 10:44:46 INFO mapred.MapTask: mapreduce.task.io.sort.mb: 100
19/01/22 10:44:46 INFO mapred.MapTask: soft limit at 83886080
19/01/22 10:44:46 INFO mapred.MapTask: bufstart = 0; bufvoid = 104857600
19/01/22 10:44:46 INFO mapred.MapTask: kvstart = 26214396; length = 6553600
19/01/22 10:44:46 INFO mapred.MapTask: Map output collector class = org.apache.hadoop.mapred.MapTask$MapOutputBuffer
19/01/22 10:44:46 INFO streaming.PipeMapRed: PipeMapRed exec [/home/andrew/code/HadoopWithPython/python/MapReduce/HadoopStreaming/./mapper.py]
19/01/22 10:44:46 INFO Configuration.deprecation: mapred.work.output.dir is deprecated. Instead, use mapreduce.task.output.dir
19/01/22 10:44:46 INFO Configuration.deprecation: map.input.start is deprecated. Instead, use mapreduce.map.input.start
19/01/22 10:44:46 INFO Configuration.deprecation: mapred.task.is.map is deprecated. Instead, use mapreduce.task.ismap
19/01/22 10:44:46 INFO Configuration.deprecation: mapred.task.id is deprecated. Instead, use mapreduce.task.attempt.id
19/01/22 10:44:46 INFO Configuration.deprecation: mapred.tip.id is deprecated. Instead, use mapreduce.task.id
19/01/22 10:44:46 INFO Configuration.deprecation: mapred.local.dir is deprecated. Instead, use mapreduce.cluster.local.dir
19/01/22 10:44:46 INFO Configuration.deprecation: map.input.file is deprecated. Instead, use mapreduce.map.input.file
19/01/22 10:44:46 INFO Configuration.deprecation: mapred.skip.on is deprecated. Instead, use mapreduce.job.skiprecords
19/01/22 10:44:46 INFO Configuration.deprecation: map.input.length is deprecated. Instead, use mapreduce.map.input.length
19/01/22 10:44:46 INFO Configuration.deprecation: mapred.job.id is deprecated. Instead, use mapreduce.job.id
19/01/22 10:44:46 INFO Configuration.deprecation: user.name is deprecated. Instead, use mapreduce.job.user.name
19/01/22 10:44:46 INFO Configuration.deprecation: mapred.task.partition is deprecated. Instead, use mapreduce.task.partition
19/01/22 10:44:46 INFO streaming.PipeMapRed: R/W/S=1/0/0 in:NA [rec/s] out:NA [rec/s]
19/01/22 10:44:46 INFO streaming.PipeMapRed: Records R/W=1/1
19/01/22 10:44:46 INFO streaming.PipeMapRed: MRErrorThread done
19/01/22 10:44:46 INFO streaming.PipeMapRed: mapRedFinished
19/01/22 10:44:46 INFO mapred.LocalJobRunner:
19/01/22 10:44:46 INFO mapred.MapTask: Starting flush of map output
19/01/22 10:44:46 INFO mapred.MapTask: Spilling map output
19/01/22 10:44:46 INFO mapred.MapTask: bufstart = 0; bufend = 41; bufvoid = 104857600
19/01/22 10:44:46 INFO mapred.MapTask: kvstart = 26214396(104857584); kvend = 26214376(104857504); length = 21/6553600
19/01/22 10:44:46 INFO mapred.MapTask: Finished spill 0
19/01/22 10:44:46 INFO mapred.Task: Task:attempt_local208759810_0001_m_000000_0 is done. And is in the process of committing
19/01/22 10:44:46 INFO mapred.LocalJobRunner: Records R/W=1/1
19/01/22 10:44:46 INFO mapred.Task: Task 'attempt_local208759810_0001_m_000000_0' done.
19/01/22 10:44:46 INFO mapred.LocalJobRunner: Finishing task: attempt_local208759810_0001_m_000000_0
19/01/22 10:44:46 INFO mapred.LocalJobRunner: map task executor complete.
19/01/22 10:44:46 INFO mapred.LocalJobRunner: Waiting for reduce tasks
19/01/22 10:44:46 INFO mapred.LocalJobRunner: Starting task: attempt_local208759810_0001_r_000000_0
19/01/22 10:44:46 INFO output.FileOutputCommitter: File Output Committer Algorithm version is 1
19/01/22 10:44:46 INFO output.FileOutputCommitter: FileOutputCommitter skip cleanup _temporary folders under output directory:false, ignore cleanup failures: false
19/01/22 10:44:46 INFO mapred.Task: Using ResourceCalculatorProcessTree : [ ]
19/01/22 10:44:46 INFO mapred.ReduceTask: Using ShuffleConsumerPlugin: org.apache.hadoop.mapreduce.task.reduce.Shuffle@78674b51
19/01/22 10:44:46 INFO reduce.MergeManagerImpl: MergerManager: memoryLimit=334338464, maxSingleShuffleLimit=83584616, mergeThreshold=220663392, ioSortFactor=10, memToMemMergeOutputsThreshold=10
19/01/22 10:44:46 INFO reduce.EventFetcher: attempt_local208759810_0001_r_000000_0 Thread started: EventFetcher for fetching Map Completion Events
19/01/22 10:44:46 INFO reduce.LocalFetcher: localfetcher#1 about to shuffle output of map attempt_local208759810_0001_m_000000_0 decomp: 55 len: 59 to MEMORY
19/01/22 10:44:46 INFO reduce.InMemoryMapOutput: Read 55 bytes from map-output for attempt_local208759810_0001_m_000000_0
19/01/22 10:44:46 INFO reduce.MergeManagerImpl: closeInMemoryFile -> map-output of size: 55, inMemoryMapOutputs.size() -> 1, commitMemory -> 0, usedMemory ->55
19/01/22 10:44:46 INFO reduce.EventFetcher: EventFetcher is interrupted.. Returning
19/01/22 10:44:46 INFO mapred.LocalJobRunner: 1 / 1 copied.
19/01/22 10:44:46 INFO reduce.MergeManagerImpl: finalMerge called with 1 in-memory map-outputs and 0 on-disk map-outputs
19/01/22 10:44:46 INFO mapred.Merger: Merging 1 sorted segments
19/01/22 10:44:46 INFO mapred.Merger: Down to the last merge-pass, with 1 segments left of total size: 50 bytes
19/01/22 10:44:46 INFO reduce.MergeManagerImpl: Merged 1 segments, 55 bytes to disk to satisfy reduce memory limit
19/01/22 10:44:46 INFO reduce.MergeManagerImpl: Merging 1 files, 59 bytes from disk
19/01/22 10:44:46 INFO reduce.MergeManagerImpl: Merging 0 segments, 0 bytes from memory into reduce
19/01/22 10:44:46 INFO mapred.Merger: Merging 1 sorted segments
19/01/22 10:44:46 INFO mapred.Merger: Down to the last merge-pass, with 1 segments left of total size: 50 bytes
19/01/22 10:44:46 INFO mapred.LocalJobRunner: 1 / 1 copied.
19/01/22 10:44:46 INFO streaming.PipeMapRed: PipeMapRed exec [/home/andrew/code/HadoopWithPython/python/MapReduce/HadoopStreaming/./reducer.py]
19/01/22 10:44:46 INFO Configuration.deprecation: mapred.job.tracker is deprecated. Instead, use mapreduce.jobtracker.address
19/01/22 10:44:46 INFO Configuration.deprecation: mapred.map.tasks is deprecated. Instead, use mapreduce.job.maps
19/01/22 10:44:46 INFO streaming.PipeMapRed: R/W/S=1/0/0 in:NA [rec/s] out:NA [rec/s]
19/01/22 10:44:46 INFO streaming.PipeMapRed: Records R/W=6/1
19/01/22 10:44:46 INFO streaming.PipeMapRed: MRErrorThread done
19/01/22 10:44:46 INFO streaming.PipeMapRed: mapRedFinished
19/01/22 10:44:46 INFO mapred.Task: Task:attempt_local208759810_0001_r_000000_0 is done. And is in the process of committing
19/01/22 10:44:46 INFO mapred.LocalJobRunner: 1 / 1 copied.
19/01/22 10:44:46 INFO mapred.Task: Task attempt_local208759810_0001_r_000000_0 is allowed to commit now
19/01/22 10:44:46 INFO output.FileOutputCommitter: Saved output of task 'attempt_local208759810_0001_r_000000_0' to hdfs://localhost:54310/user/hduser/output2/_temporary/0/task_local208759810_0001_r_000000
19/01/22 10:44:46 INFO mapred.LocalJobRunner: Records R/W=6/1 > reduce
19/01/22 10:44:46 INFO mapred.Task: Task 'attempt_local208759810_0001_r_000000_0' done.
19/01/22 10:44:46 INFO mapred.LocalJobRunner: Finishing task: attempt_local208759810_0001_r_000000_0
19/01/22 10:44:46 INFO mapred.LocalJobRunner: reduce task executor complete.
19/01/22 10:44:47 INFO mapreduce.Job: Job job_local208759810_0001 running in uber mode : false
19/01/22 10:44:47 INFO mapreduce.Job: map 100% reduce 100%
19/01/22 10:44:47 INFO mapreduce.Job: Job job_local208759810_0001 completed successfully
19/01/22 10:44:47 INFO mapreduce.Job: Counters: 35
File System Counters
FILE: Number of bytes read=273356
FILE: Number of bytes written=1217709
FILE: Number of read operations=0
FILE: Number of large read operations=0
FILE: Number of write operations=0
HDFS: Number of bytes read=58
HDFS: Number of bytes written=29
HDFS: Number of read operations=13
HDFS: Number of large read operations=0
HDFS: Number of write operations=4
Map-Reduce Framework
Map input records=1
Map output records=6
Map output bytes=41
Map output materialized bytes=59
Input split bytes=97
Combine input records=0
Combine output records=0
Reduce input groups=4
Reduce shuffle bytes=59
Reduce input records=6
Reduce output records=4
Spilled Records=12
Shuffled Maps =1
Failed Shuffles=0
Merged Map outputs=1
GC time elapsed (ms)=0
Total committed heap usage (bytes)=552599552
Shuffle Errors
BAD_ID=0
CONNECTION=0
IO_ERROR=0
WRONG_LENGTH=0
WRONG_MAP=0
WRONG_REDUCE=0
File Input Format Counters
Bytes Read=29
File Output Format Counters
Bytes Written=29
19/01/22 10:44:47 INFO streaming.StreamJob: Output directory: /user/hduser/output2
$ hdfs dfs -cat /user/hduser/output2/part-00000
be 2
jack 2
nimble 1
quick 1