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The following page has been changed by DougCutting: http://wiki.apache.org/hadoop/HadoopMapReduce The comment on the change is: update for TLP move ------------------------------------------------------------------------------ == Map == As the Map operation is parallelized the input file set is first - split to several pieces called [http://lucene.apache.org/hadoop/docs/api/org/apache/hadoop/mapred/FileSplit.html FileSplits]. If an individual file + split to several pieces called [http://hadoop.apache.org/core/docs/current/api/org/apache/hadoop/mapred/FileSplit.html FileSplits]. If an individual file is so large that it will affect seek time it will be split to several Splits. The splitting does not know anything about the input file's internal logical structure, for example @@ -18, +18 @@ When an individual map task starts it will open a new output writer per configured reduce task. It will then proceed to read - its !FileSplit using the [http://lucene.apache.org/hadoop/docs/api/org/apache/hadoop/mapred/RecordReader.html RecordReader] it gets from the specified + its !FileSplit using the [http://hadoop.apache.org/core/docs/current/api/org/apache/hadoop/mapred/RecordReader.html RecordReader] it gets from the specified - [http://lucene.apache.org/hadoop/docs/api/org/apache/hadoop/mapred/InputFormat.html InputFormat]. !InputFormat parses the input and generates + [http://hadoop.apache.org/core/docs/current/api/org/apache/hadoop/mapred/InputFormat.html InputFormat]. !InputFormat parses the input and generates - key-value pairs. !InputFormat must also handle records that may be split on the !FileSplit boundary. For example [http://svn.apache.org/viewcvs.cgi/lucene/hadoop/trunk/src/java/org/apache/hadoop/mapred/TextInputFormat.java?view=markup TextInputFormat] will read the last line of the !FileSplit past the split boundary and, when reading other than the first !FileSplit, !TextInputFormat ignores the content up to the first newline. + key-value pairs. !InputFormat must also handle records that may be split on the !FileSplit boundary. For example [http://svn.apache.org/viewcvs.cgi/hadoop/core/trunk/src/java/org/apache/hadoop/mapred/TextInputFormat.java?view=markup TextInputFormat] will read the last line of the !FileSplit past the split boundary and, when reading other than the first !FileSplit, !TextInputFormat ignores the content up to the first newline. It is not necessary for the !InputFormat to generate both meaningful keys ''and'' values. For example the @@ -30, +30 @@ offsets. As key-value pairs are read from the !RecordReader they are - passed to the configured [http://lucene.apache.org/hadoop/docs/api/org/apache/hadoop/mapred/Mapper.html Mapper]. The user supplied Mapper does + passed to the configured [http://hadoop.apache.org/core/docs/current/api/org/apache/hadoop/mapred/Mapper.html Mapper]. The user supplied Mapper does - whatever it wants with the input pair and calls [http://lucene.apache.org/hadoop/docs/api/org/apache/hadoop/mapred/OutputCollector.html#collect(org.apache.hadoop.io.WritableComparable,%20org.apache.hadoop.io.Writable) OutputCollector.collect] with key-value pairs of its own choosing. The output it + whatever it wants with the input pair and calls [http://hadoop.apache.org/core/docs/current/api/org/apache/hadoop/mapred/OutputCollector.html#collect(org.apache.hadoop.io.WritableComparable,%20org.apache.hadoop.io.Writable) OutputCollector.collect] with key-value pairs of its own choosing. The output it generates must use one key class and one value class. This is because - the Map output will be written into a [http://wiki.apache.org/lucene-hadoop/SequenceFile SequenceFile] + the Map output will be written into a SequenceFile which has per-file type information and all the records must have the same type (use subclassing if you want to output different data structures). The Map input and output key-value @@ -41, +41 @@ When Mapper output is collected it is partitioned, which means that it will be written to the output specified by the - [http://lucene.apache.org/hadoop/docs/api/org/apache/hadoop/mapred/Partitioner.html Partitioner]. The default [http://lucene.apache.org/hadoop/docs/api/org/apache/hadoop/mapred/lib/HashPartitioner.html HashPartitioner] uses the - hashcode function on the key's class (which means that this hashcode function must be good in order to achieve an even workload across the reduce tasks). See [http://svn.apache.org/viewcvs.cgi/lucene/hadoop/trunk/src/java/org/apache/hadoop/mapred/MapTask.java?view=markup MapTask] for details. + [http://hadoop.apache.org/core/docs/current/api/org/apache/hadoop/mapred/Partitioner.html Partitioner]. The default [http://hadoop.apache.org/core/docs/current/api/org/apache/hadoop/mapred/lib/HashPartitioner.html HashPartitioner] uses the + hashcode function on the key's class (which means that this hashcode function must be good in order to achieve an even workload across the reduce tasks). See [http://svn.apache.org/viewcvs.cgi/hadoop/core/trunk/src/java/org/apache/hadoop/mapred/MapTask.java?view=markup MapTask] for details. N input files will generate M map tasks to be run and each map task will generate as many output files as there are reduce @@ -80, +80 @@ == Reduce == When a reduce task starts, its input is scattered in many files across all the nodes where map tasks ran. If run in distributed mode these need to be first copied to the local - filesystem in a ''copy phase'' (see [http://svn.apache.org/viewcvs.cgi/lucene/hadoop/trunk/src/java/org/apache/hadoop/mapred/ReduceTaskRunner.java?view=markup ReduceTaskRunner]). + filesystem in a ''copy phase'' (see [http://svn.apache.org/viewcvs.cgi/hadoop/core/trunk/src/java/org/apache/hadoop/mapred/ReduceTaskRunner.java?view=markup ReduceTaskRunner]). Once all the data is available locally it is appended to one file in an ''append phase''. The file is then merge sorted so that the key-value pairs for a given key are contiguous (''sort phase''). This makes the actual reduce operation simple: the file is read sequentially and the values are passed to the reduce method with an iterator reading the input file until the next key - value is encountered. See [http://svn.apache.org/viewcvs.cgi/lucene/hadoop/trunk/src/java/org/apache/hadoop/mapred/ReduceTask.java?view=markup ReduceTask] for details. + value is encountered. See [http://svn.apache.org/viewcvs.cgi/hadoop/core/trunk/src/java/org/apache/hadoop/mapred/ReduceTask.java?view=markup ReduceTask] for details. At the end, the output will consist of one output file per executed reduce task. The format of the files can be specified with - [http://lucene.apache.org/hadoop/docs/api/org/apache/hadoop/mapred/JobConf.html#setOutputFormat(java.lang.Class) JobConf.setOutputFormat]. If !SequentialOutputFormat is used then the output key and value + [http://hadoop.apache.org/core/docs/current/api/org/apache/hadoop/mapred/JobConf.html#setOutputFormat(java.lang.Class) JobConf.setOutputFormat]. If !SequentialOutputFormat is used then the output key and value classes must also be specified.
