MAPREDUCE-5637. Convert Hadoop Streaming document to APT (Akira AJISAKA via jeagles)
git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/trunk@1592789 13f79535-47bb-0310-9956-ffa450edef68
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Jonathan Turner Eagles
parent
f4b687b873
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bd54137afa
@ -182,6 +182,9 @@ Release 2.5.0 - UNRELEASED
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MAX_CHUNKS_IDEAL, MIN_RECORDS_PER_CHUNK and SPLIT_RATIO to be configurable.
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(Tsuyoshi OZAWA via szetszwo)
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MAPREDUCE-5637. Convert Hadoop Streaming document to APT (Akira AJISAKA via
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jeagles)
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OPTIMIZATIONS
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BUG FIXES
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@ -0,0 +1,792 @@
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~~ Licensed under the Apache License, Version 2.0 (the "License");
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~~ you may not use this file except in compliance with the License.
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~~ You may obtain a copy of the License at
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~~
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~~ http://www.apache.org/licenses/LICENSE-2.0
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~~
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~~ Unless required by applicable law or agreed to in writing, software
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~~ distributed under the License is distributed on an "AS IS" BASIS,
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~~ WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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~~ See the License for the specific language governing permissions and
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~~ limitations under the License. See accompanying LICENSE file.
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---
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Hadoop Streaming
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---
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---
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${maven.build.timestamp}
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Hadoop Streaming
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%{toc|section=1|fromDepth=0|toDepth=4}
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* Hadoop Streaming
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Hadoop streaming is a utility that comes with the Hadoop distribution. The
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utility allows you to create and run Map/Reduce jobs with any executable or
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script as the mapper and/or the reducer. For example:
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+---+
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hadoop jar hadoop-streaming-${project.version}.jar \
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-input myInputDirs \
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-output myOutputDir \
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-mapper /bin/cat \
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-reducer /usr/bin/wc
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+---+
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* How Streaming Works
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In the above example, both the mapper and the reducer are executables that
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read the input from stdin (line by line) and emit the output to stdout. The
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utility will create a Map/Reduce job, submit the job to an appropriate
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cluster, and monitor the progress of the job until it completes.
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When an executable is specified for mappers, each mapper task will launch the
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executable as a separate process when the mapper is initialized. As the
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mapper task runs, it converts its inputs into lines and feed the lines to the
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stdin of the process. In the meantime, the mapper collects the line oriented
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outputs from the stdout of the process and converts each line into a
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key/value pair, which is collected as the output of the mapper. By default,
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the <prefix of a line up to the first tab character> is the <<<key>>> and the
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rest of the line (excluding the tab character) will be the <<<value>>>. If
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there is no tab character in the line, then entire line is considered as key
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and the value is null. However, this can be customized by setting
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<<<-inputformat>>> command option, as discussed later.
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When an executable is specified for reducers, each reducer task will launch
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the executable as a separate process then the reducer is initialized. As the
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reducer task runs, it converts its input key/values pairs into lines and
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feeds the lines to the stdin of the process. In the meantime, the reducer
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collects the line oriented outputs from the stdout of the process, converts
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each line into a key/value pair, which is collected as the output of the
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reducer. By default, the prefix of a line up to the first tab character is
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the key and the rest of the line (excluding the tab character) is the value.
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However, this can be customized by setting <<<-outputformat>>> command
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option, as discussed later.
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This is the basis for the communication protocol between the Map/Reduce
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framework and the streaming mapper/reducer.
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User can specify <<<stream.non.zero.exit.is.failure>>> as <<<true>>> or
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<<<false>>> to make a streaming task that exits with a non-zero status to be
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<<<Failure>>> or <<<Success>>> respectively. By default, streaming tasks
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exiting with non-zero status are considered to be failed tasks.
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* Streaming Command Options
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Streaming supports streaming command options as well as
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{{{Generic_Command_Options}generic command options}}. The general command
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line syntax is shown below.
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<<Note:>> Be sure to place the generic options before the streaming options,
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otherwise the command will fail. For an example, see
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{{{Making_Archives_Available_to_Tasks}Making Archives Available to Tasks}}.
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+---+
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hadoop command [genericOptions] [streamingOptions]
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+---+
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The Hadoop streaming command options are listed here:
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*-------------*--------------------*------------------------------------------*
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|| Parameter || Optional/Required || Description |
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*-------------+--------------------+------------------------------------------+
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| -input directoryname or filename | Required | Input location for mapper
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*-------------+--------------------+------------------------------------------+
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| -output directoryname | Required | Output location for reducer
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*-------------+--------------------+------------------------------------------+
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| -mapper executable or JavaClassName | Required | Mapper executable
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*-------------+--------------------+------------------------------------------+
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| -reducer executable or JavaClassName | Required | Reducer executable
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*-------------+--------------------+------------------------------------------+
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| -file filename | Optional | Make the mapper, reducer, or combiner executable
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| | | available locally on the compute nodes
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*-------------+--------------------+------------------------------------------+
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| -inputformat JavaClassName | Optional | Class you supply should return
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| | | key/value pairs of Text class. If not
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| | | specified, TextInputFormat is used as
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| | | the default
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*-------------+--------------------+------------------------------------------+
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| -outputformat JavaClassName | Optional | Class you supply should take
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| | | key/value pairs of Text class. If
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| | | not specified, TextOutputformat is
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| | | used as the default
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*-------------+--------------------+------------------------------------------+
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| -partitioner JavaClassName | Optional | Class that determines which reduce a
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| | | key is sent to
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*-------------+--------------------+------------------------------------------+
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| -combiner streamingCommand | Optional | Combiner executable for map output
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| or JavaClassName | |
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*-------------+--------------------+------------------------------------------+
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| -cmdenv name=value | Optional | Pass environment variable to streaming
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| | | commands
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*-------------+--------------------+------------------------------------------+
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| -inputreader | Optional | For backwards-compatibility: specifies a record
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| | | reader class (instead of an input format class)
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*-------------+--------------------+------------------------------------------+
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| -verbose | Optional | Verbose output
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*-------------+--------------------+------------------------------------------+
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| -lazyOutput | Optional | Create output lazily. For example, if the output
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| | | format is based on FileOutputFormat, the output file
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| | | is created only on the first call to Context.write
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*-------------+--------------------+------------------------------------------+
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| -numReduceTasks | Optional | Specify the number of reducers
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*-------------+--------------------+------------------------------------------+
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| -mapdebug | Optional | Script to call when map task fails
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*-------------+--------------------+------------------------------------------+
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| -reducedebug | Optional | Script to call when reduce task fails
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*-------------+--------------------+------------------------------------------+
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** Specifying a Java Class as the Mapper/Reducer
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You can supply a Java class as the mapper and/or the reducer.
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+---+
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hadoop jar hadoop-streaming-${project.version}.jar \
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-input myInputDirs \
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-output myOutputDir \
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-inputformat org.apache.hadoop.mapred.KeyValueTextInputFormat \
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-mapper org.apache.hadoop.mapred.lib.IdentityMapper \
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-reducer /usr/bin/wc
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+---+
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You can specify <<<stream.non.zero.exit.is.failure>>> as <<<true>>> or
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<<<false>>> to make a streaming task that exits with a non-zero status to be
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<<<Failure>>> or <<<Success>>> respectively. By default, streaming tasks
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exiting with non-zero status are considered to be failed tasks.
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** Packaging Files With Job Submissions
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You can specify any executable as the mapper and/or the reducer. The
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executables do not need to pre-exist on the machines in the cluster; however,
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if they don't, you will need to use "-file" option to tell the framework to
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pack your executable files as a part of job submission. For example:
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+---+
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hadoop jar hadoop-streaming-${project.version}.jar \
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-input myInputDirs \
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-output myOutputDir \
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-mapper myPythonScript.py \
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-reducer /usr/bin/wc \
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-file myPythonScript.py
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+---+
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The above example specifies a user defined Python executable as the mapper.
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The option "-file myPythonScript.py" causes the python executable shipped
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to the cluster machines as a part of job submission.
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In addition to executable files, you can also package other auxiliary files
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(such as dictionaries, configuration files, etc) that may be used by the
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mapper and/or the reducer. For example:
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+---+
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hadoop jar hadoop-streaming-${project.version}.jar \
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-input myInputDirs \
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-output myOutputDir \
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-mapper myPythonScript.py \
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-reducer /usr/bin/wc \
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-file myPythonScript.py \
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-file myDictionary.txt
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+---+
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** Specifying Other Plugins for Jobs
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Just as with a normal Map/Reduce job, you can specify other plugins for a
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streaming job:
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+---+
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-inputformat JavaClassName
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-outputformat JavaClassName
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-partitioner JavaClassName
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-combiner streamingCommand or JavaClassName
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+---+
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The class you supply for the input format should return key/value pairs of
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Text class. If you do not specify an input format class, the TextInputFormat
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is used as the default. Since the TextInputFormat returns keys of
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LongWritable class, which are actually not part of the input data, the keys
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will be discarded; only the values will be piped to the streaming mapper.
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The class you supply for the output format is expected to take key/value
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pairs of Text class. If you do not specify an output format class, the
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TextOutputFormat is used as the default.
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** Setting Environment Variables
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To set an environment variable in a streaming command use:
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+---+
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-cmdenv EXAMPLE_DIR=/home/example/dictionaries/
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+---+
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* Generic Command Options
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Streaming supports {{{Streaming_Command_Options}streaming command options}}
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as well as generic command options. The general command line syntax is shown
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below.
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<<Note:>> Be sure to place the generic options before the streaming options,
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otherwise the command will fail. For an example, see
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{{{Making_Archives_Available_to_Tasks}Making Archives Available to Tasks}}.
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+---+
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hadoop command [genericOptions] [streamingOptions]
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+---+
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The Hadoop generic command options you can use with streaming are listed
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here:
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*-------------*--------------------*------------------------------------------*
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|| Parameter || Optional/Required || Description |
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*-------------+--------------------+------------------------------------------+
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| -conf configuration_file | Optional | Specify an application configuration
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| | | file
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*-------------+--------------------+------------------------------------------+
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| -D property=value | Optional | Use value for given property
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*-------------+--------------------+------------------------------------------+
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| -fs host:port or local | Optional | Specify a namenode
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*-------------+--------------------+------------------------------------------+
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| -files | Optional | Specify comma-separated files to be copied to the
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| | | Map/Reduce cluster
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*-------------+--------------------+------------------------------------------+
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| -libjars | Optional | Specify comma-separated jar files to include in the
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| | | classpath
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*-------------+--------------------+------------------------------------------+
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| -archives | Optional | Specify comma-separated archives to be unarchived on
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| | | the compute machines
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*-------------+--------------------+------------------------------------------+
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** Specifying Configuration Variables with the -D Option
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You can specify additional configuration variables by using
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"-D \<property\>=\<value\>".
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*** Specifying Directories
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To change the local temp directory use:
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+---+
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-D dfs.data.dir=/tmp
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+---+
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To specify additional local temp directories use:
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+---+
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-D mapred.local.dir=/tmp/local
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-D mapred.system.dir=/tmp/system
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-D mapred.temp.dir=/tmp/temp
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+---+
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<<Note:>> For more details on job configuration parameters see:
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{{{./mapred-default.xml}mapred-default.xml}}
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*** Specifying Map-Only Jobs
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Often, you may want to process input data using a map function only. To do
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this, simply set <<<mapreduce.job.reduces>>> to zero. The Map/Reduce
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framework will not create any reducer tasks. Rather, the outputs of the
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mapper tasks will be the final output of the job.
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+---+
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-D mapreduce.job.reduces=0
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+---+
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To be backward compatible, Hadoop Streaming also supports the "-reducer NONE"
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option, which is equivalent to "-D mapreduce.job.reduces=0".
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*** Specifying the Number of Reducers
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To specify the number of reducers, for example two, use:
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+---+
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hadoop jar hadoop-streaming-${project.version}.jar \
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-D mapreduce.job.reduces=2 \
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-input myInputDirs \
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-output myOutputDir \
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-mapper /bin/cat \
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-reducer /usr/bin/wc
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+---+
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*** Customizing How Lines are Split into Key/Value Pairs
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As noted earlier, when the Map/Reduce framework reads a line from the stdout
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of the mapper, it splits the line into a key/value pair. By default, the
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prefix of the line up to the first tab character is the key and the rest of
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the line (excluding the tab character) is the value.
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However, you can customize this default. You can specify a field separator
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other than the tab character (the default), and you can specify the nth
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(n >= 1) character rather than the first character in a line (the default) as
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the separator between the key and value. For example:
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+---+
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hadoop jar hadoop-streaming-${project.version}.jar \
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-D stream.map.output.field.separator=. \
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-D stream.num.map.output.key.fields=4 \
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-input myInputDirs \
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-output myOutputDir \
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-mapper /bin/cat \
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-reducer /bin/cat
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+---+
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In the above example, "-D stream.map.output.field.separator=." specifies "."
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as the field separator for the map outputs, and the prefix up to the fourth
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"." in a line will be the key and the rest of the line (excluding the fourth
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".") will be the value. If a line has less than four "."s, then the whole
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line will be the key and the value will be an empty Text object (like the one
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created by new Text("")).
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Similarly, you can use "-D stream.reduce.output.field.separator=SEP" and
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"-D stream.num.reduce.output.fields=NUM" to specify the nth field separator
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in a line of the reduce outputs as the separator between the key and the
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value.
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Similarly, you can specify "stream.map.input.field.separator" and
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"stream.reduce.input.field.separator" as the input separator for Map/Reduce
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inputs. By default the separator is the tab character.
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** Working with Large Files and Archives
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The -files and -archives options allow you to make files and archives
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available to the tasks. The argument is a URI to the file or archive that you
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have already uploaded to HDFS. These files and archives are cached across
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jobs. You can retrieve the host and fs_port values from the fs.default.name
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config variable.
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<<Note:>> The -files and -archives options are generic options. Be sure to
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place the generic options before the command options, otherwise the command
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will fail.
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*** Making Files Available to Tasks
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The -files option creates a symlink in the current working directory of the
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tasks that points to the local copy of the file.
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In this example, Hadoop automatically creates a symlink named testfile.txt in
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the current working directory of the tasks. This symlink points to the local
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copy of testfile.txt.
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+---+
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-files hdfs://host:fs_port/user/testfile.txt
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+---+
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User can specify a different symlink name for -files using #.
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+---+
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-files hdfs://host:fs_port/user/testfile.txt#testfile
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+---+
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Multiple entries can be specified like this:
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+---+
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-files hdfs://host:fs_port/user/testfile1.txt,hdfs://host:fs_port/user/testfile2.txt
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+---+
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*** Making Archives Available to Tasks
|
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The -archives option allows you to copy jars locally to the current working
|
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directory of tasks and automatically unjar the files.
|
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In this example, Hadoop automatically creates a symlink named testfile.jar in
|
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the current working directory of tasks. This symlink points to the directory
|
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that stores the unjarred contents of the uploaded jar file.
|
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|
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+---+
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-archives hdfs://host:fs_port/user/testfile.jar
|
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+---+
|
||||
|
||||
User can specify a different symlink name for -archives using #.
|
||||
|
||||
+---+
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-archives hdfs://host:fs_port/user/testfile.tgz#tgzdir
|
||||
+---+
|
||||
|
||||
In this example, the input.txt file has two lines specifying the names of the
|
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two files: cachedir.jar/cache.txt and cachedir.jar/cache2.txt. "cachedir.jar"
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||||
is a symlink to the archived directory, which has the files "cache.txt" and
|
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"cache2.txt".
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||||
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||||
+---+
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hadoop jar hadoop-streaming-${project.version}.jar \
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-archives 'hdfs://hadoop-nn1.example.com/user/me/samples/cachefile/cachedir.jar' \
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-D mapreduce.job.maps=1 \
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-D mapreduce.job.reduces=1 \
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-D mapreduce.job.name="Experiment" \
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-input "/user/me/samples/cachefile/input.txt" \
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-output "/user/me/samples/cachefile/out" \
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-mapper "xargs cat" \
|
||||
-reducer "cat"
|
||||
|
||||
$ ls test_jar/
|
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cache.txt cache2.txt
|
||||
|
||||
$ jar cvf cachedir.jar -C test_jar/ .
|
||||
added manifest
|
||||
adding: cache.txt(in = 30) (out= 29)(deflated 3%)
|
||||
adding: cache2.txt(in = 37) (out= 35)(deflated 5%)
|
||||
|
||||
$ hdfs dfs -put cachedir.jar samples/cachefile
|
||||
|
||||
$ hdfs dfs -cat /user/me/samples/cachefile/input.txt
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||||
cachedir.jar/cache.txt
|
||||
cachedir.jar/cache2.txt
|
||||
|
||||
$ cat test_jar/cache.txt
|
||||
This is just the cache string
|
||||
|
||||
$ cat test_jar/cache2.txt
|
||||
This is just the second cache string
|
||||
|
||||
$ hdfs dfs -ls /user/me/samples/cachefile/out
|
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Found 2 items
|
||||
-rw-r--r-- 1 me supergroup 0 2013-11-14 17:00 /user/me/samples/cachefile/out/_SUCCESS
|
||||
-rw-r--r-- 1 me supergroup 69 2013-11-14 17:00 /user/me/samples/cachefile/out/part-00000
|
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|
||||
$ hdfs dfs -cat /user/me/samples/cachefile/out/part-00000
|
||||
This is just the cache string
|
||||
This is just the second cache string
|
||||
+---+
|
||||
|
||||
* More Usage Examples
|
||||
|
||||
** Hadoop Partitioner Class
|
||||
|
||||
Hadoop has a library class,
|
||||
{{{../../api/org/apache/hadoop/mapred/lib/KeyFieldBasedPartitioner.html}
|
||||
KeyFieldBasedPartitioner}}, that is useful for many applications. This class
|
||||
allows the Map/Reduce framework to partition the map outputs based on certain
|
||||
key fields, not the whole keys. For example:
|
||||
|
||||
+---+
|
||||
hadoop jar hadoop-streaming-${project.version}.jar \
|
||||
-D stream.map.output.field.separator=. \
|
||||
-D stream.num.map.output.key.fields=4 \
|
||||
-D map.output.key.field.separator=. \
|
||||
-D mapreduce.partition.keypartitioner.options=-k1,2 \
|
||||
-D mapreduce.job.reduces=12 \
|
||||
-input myInputDirs \
|
||||
-output myOutputDir \
|
||||
-mapper /bin/cat \
|
||||
-reducer /bin/cat \
|
||||
-partitioner org.apache.hadoop.mapred.lib.KeyFieldBasedPartitioner
|
||||
+---+
|
||||
|
||||
Here, <-D stream.map.output.field.separator=.> and
|
||||
<-D stream.num.map.output.key.fields=4> are as explained in previous example.
|
||||
The two variables are used by streaming to identify the key/value pair of
|
||||
mapper.
|
||||
|
||||
The map output keys of the above Map/Reduce job normally have four fields
|
||||
separated by ".". However, the Map/Reduce framework will partition the map
|
||||
outputs by the first two fields of the keys using the
|
||||
<-D mapred.text.key.partitioner.options=-k1,2> option. Here,
|
||||
<-D map.output.key.field.separator=.> specifies the separator for the
|
||||
partition. This guarantees that all the key/value pairs with the same first
|
||||
two fields in the keys will be partitioned into the same reducer.
|
||||
|
||||
<This is effectively equivalent to specifying the first two fields as the
|
||||
primary key and the next two fields as the secondary. The primary key is used
|
||||
for partitioning, and the combination of the primary and secondary keys is
|
||||
used for sorting.> A simple illustration is shown here:
|
||||
|
||||
Output of map (the keys)
|
||||
|
||||
+---+
|
||||
11.12.1.2
|
||||
11.14.2.3
|
||||
11.11.4.1
|
||||
11.12.1.1
|
||||
11.14.2.2
|
||||
+---+
|
||||
|
||||
Partition into 3 reducers (the first 2 fields are used as keys for partition)
|
||||
|
||||
+---+
|
||||
11.11.4.1
|
||||
-----------
|
||||
11.12.1.2
|
||||
11.12.1.1
|
||||
-----------
|
||||
11.14.2.3
|
||||
11.14.2.2
|
||||
+---+
|
||||
|
||||
Sorting within each partition for the reducer(all 4 fields used for sorting)
|
||||
|
||||
+---+
|
||||
11.11.4.1
|
||||
-----------
|
||||
11.12.1.1
|
||||
11.12.1.2
|
||||
-----------
|
||||
11.14.2.2
|
||||
11.14.2.3
|
||||
+---+
|
||||
|
||||
** Hadoop Comparator Class
|
||||
|
||||
Hadoop has a library class,
|
||||
{{{../../api/org/apache/hadoop/mapreduce/lib/partition/KeyFieldBasedComparator.html}
|
||||
KeyFieldBasedComparator}}, that is useful for many applications. This class
|
||||
provides a subset of features provided by the Unix/GNU Sort. For example:
|
||||
|
||||
+---+
|
||||
hadoop jar hadoop-streaming-${project.version}.jar \
|
||||
-D mapreduce.job.output.key.comparator.class=org.apache.hadoop.mapreduce.lib.partition.KeyFieldBasedComparator \
|
||||
-D stream.map.output.field.separator=. \
|
||||
-D stream.num.map.output.key.fields=4 \
|
||||
-D mapreduce.map.output.key.field.separator=. \
|
||||
-D mapreduce.partition.keycomparator.options=-k2,2nr \
|
||||
-D mapreduce.job.reduces=1 \
|
||||
-input myInputDirs \
|
||||
-output myOutputDir \
|
||||
-mapper /bin/cat \
|
||||
-reducer /bin/cat
|
||||
+---+
|
||||
|
||||
The map output keys of the above Map/Reduce job normally have four fields
|
||||
separated by ".". However, the Map/Reduce framework will sort the outputs by
|
||||
the second field of the keys using the
|
||||
<-D mapreduce.partition.keycomparator.options=-k2,2nr> option. Here, <-n>
|
||||
specifies that the sorting is numerical sorting and <-r> specifies that the
|
||||
result should be reversed. A simple illustration is shown below:
|
||||
|
||||
Output of map (the keys)
|
||||
|
||||
+---+
|
||||
11.12.1.2
|
||||
11.14.2.3
|
||||
11.11.4.1
|
||||
11.12.1.1
|
||||
11.14.2.2
|
||||
+---+
|
||||
|
||||
Sorting output for the reducer (where second field used for sorting)
|
||||
|
||||
+---+
|
||||
11.14.2.3
|
||||
11.14.2.2
|
||||
11.12.1.2
|
||||
11.12.1.1
|
||||
11.11.4.1
|
||||
+---+
|
||||
|
||||
** Hadoop Aggregate Package
|
||||
|
||||
Hadoop has a library package called
|
||||
{{{../../org/apache/hadoop/mapred/lib/aggregate/package-summary.html}
|
||||
Aggregate}}. Aggregate provides a special reducer class and a special
|
||||
combiner class, and a list of simple aggregators that perform aggregations
|
||||
such as "sum", "max", "min" and so on over a sequence of values. Aggregate
|
||||
allows you to define a mapper plugin class that is expected to generate
|
||||
"aggregatable items" for each input key/value pair of the mappers. The
|
||||
combiner/reducer will aggregate those aggregatable items by invoking the
|
||||
appropriate aggregators.
|
||||
|
||||
To use Aggregate, simply specify "-reducer aggregate":
|
||||
|
||||
+---+
|
||||
hadoop jar hadoop-streaming-${project.version}.jar \
|
||||
-input myInputDirs \
|
||||
-output myOutputDir \
|
||||
-mapper myAggregatorForKeyCount.py \
|
||||
-reducer aggregate \
|
||||
-file myAggregatorForKeyCount.py \
|
||||
+---+
|
||||
|
||||
The python program myAggregatorForKeyCount.py looks like:
|
||||
|
||||
+---+
|
||||
#!/usr/bin/python
|
||||
|
||||
import sys;
|
||||
|
||||
def generateLongCountToken(id):
|
||||
return "LongValueSum:" + id + "\t" + "1"
|
||||
|
||||
def main(argv):
|
||||
line = sys.stdin.readline();
|
||||
try:
|
||||
while line:
|
||||
line = line[:-1];
|
||||
fields = line.split("\t");
|
||||
print generateLongCountToken(fields[0]);
|
||||
line = sys.stdin.readline();
|
||||
except "end of file":
|
||||
return None
|
||||
if __name__ == "__main__":
|
||||
main(sys.argv)
|
||||
+---+
|
||||
|
||||
** Hadoop Field Selection Class
|
||||
|
||||
Hadoop has a library class,
|
||||
{{{../../api/org/apache/hadoop/mapred/lib/FieldSelectionMapReduce.html}
|
||||
FieldSelectionMapReduce}}, that effectively allows you to process text data
|
||||
like the unix "cut" utility. The map function defined in the class treats
|
||||
each input key/value pair as a list of fields. You can specify the field
|
||||
separator (the default is the tab character). You can select an arbitrary
|
||||
list of fields as the map output key, and an arbitrary list of fields as the
|
||||
map output value. Similarly, the reduce function defined in the class treats
|
||||
each input key/value pair as a list of fields. You can select an arbitrary
|
||||
list of fields as the reduce output key, and an arbitrary list of fields as
|
||||
the reduce output value. For example:
|
||||
|
||||
+---+
|
||||
hadoop jar hadoop-streaming-${project.version}.jar \
|
||||
-D mapreduce.map.output.key.field.separator=. \
|
||||
-D mapreduce.partition.keypartitioner.options=-k1,2 \
|
||||
-D mapreduce.fieldsel.data.field.separator=. \
|
||||
-D mapreduce.fieldsel.map.output.key.value.fields.spec=6,5,1-3:0- \
|
||||
-D mapreduce.fieldsel.reduce.output.key.value.fields.spec=0-2:5- \
|
||||
-D mapreduce.map.output.key.class=org.apache.hadoop.io.Text \
|
||||
-D mapreduce.job.reduces=12 \
|
||||
-input myInputDirs \
|
||||
-output myOutputDir \
|
||||
-mapper org.apache.hadoop.mapred.lib.FieldSelectionMapReduce \
|
||||
-reducer org.apache.hadoop.mapred.lib.FieldSelectionMapReduce \
|
||||
-partitioner org.apache.hadoop.mapred.lib.KeyFieldBasedPartitioner
|
||||
+---+
|
||||
|
||||
The option "-D
|
||||
mapreduce.fieldsel.map.output.key.value.fields.spec=6,5,1-3:0-" specifies
|
||||
key/value selection for the map outputs. Key selection spec and value
|
||||
selection spec are separated by ":". In this case, the map output key will
|
||||
consist of fields 6, 5, 1, 2, and 3. The map output value will consist of all
|
||||
fields (0- means field 0 and all the subsequent fields).
|
||||
|
||||
The option "-D mapreduce.fieldsel.reduce.output.key.value.fields.spec=0-2:5-"
|
||||
specifies key/value selection for the reduce outputs. In this case, the
|
||||
reduce output key will consist of fields 0, 1, 2 (corresponding to the
|
||||
original fields 6, 5, 1). The reduce output value will consist of all fields
|
||||
starting from field 5 (corresponding to all the original fields).
|
||||
|
||||
* Frequently Asked Questions
|
||||
|
||||
** How do I use Hadoop Streaming to run an arbitrary set of (semi) independent
|
||||
tasks?
|
||||
|
||||
Often you do not need the full power of Map Reduce, but only need to run
|
||||
multiple instances of the same program - either on different parts of the
|
||||
data, or on the same data, but with different parameters. You can use Hadoop
|
||||
Streaming to do this.
|
||||
|
||||
** How do I process files, one per map?
|
||||
|
||||
As an example, consider the problem of zipping (compressing) a set of files
|
||||
across the hadoop cluster. You can achieve this by using Hadoop Streaming
|
||||
and custom mapper script:
|
||||
|
||||
* Generate a file containing the full HDFS path of the input files. Each map
|
||||
task would get one file name as input.
|
||||
|
||||
* Create a mapper script which, given a filename, will get the file to local
|
||||
disk, gzip the file and put it back in the desired output directory.
|
||||
|
||||
** How many reducers should I use?
|
||||
|
||||
See MapReduce Tutorial for details: {{{./MapReduceTutorial.html#Reducer}
|
||||
Reducer}}
|
||||
|
||||
** If I set up an alias in my shell script, will that work after -mapper?
|
||||
|
||||
For example, say I do: alias c1='cut -f1'. Will -mapper "c1" work?
|
||||
|
||||
Using an alias will not work, but variable substitution is allowed as shown
|
||||
in this example:
|
||||
|
||||
+---+
|
||||
$ hdfs dfs -cat /user/me/samples/student_marks
|
||||
alice 50
|
||||
bruce 70
|
||||
charlie 80
|
||||
dan 75
|
||||
|
||||
$ c2='cut -f2'; hadoop jar hadoop-streaming-${project.version}.jar \
|
||||
-D mapreduce.job.name='Experiment' \
|
||||
-input /user/me/samples/student_marks \
|
||||
-output /user/me/samples/student_out \
|
||||
-mapper "$c2" -reducer 'cat'
|
||||
|
||||
$ hdfs dfs -cat /user/me/samples/student_out/part-00000
|
||||
50
|
||||
70
|
||||
75
|
||||
80
|
||||
+---+
|
||||
|
||||
** Can I use UNIX pipes?
|
||||
|
||||
For example, will -mapper "cut -f1 | sed s/foo/bar/g" work?
|
||||
|
||||
Currently this does not work and gives an "java.io.IOException: Broken pipe"
|
||||
error. This is probably a bug that needs to be investigated.
|
||||
|
||||
** What do I do if I get the "No space left on device" error?
|
||||
|
||||
For example, when I run a streaming job by distributing large executables
|
||||
(for example, 3.6G) through the -file option, I get a "No space left on
|
||||
device" error.
|
||||
|
||||
The jar packaging happens in a directory pointed to by the configuration
|
||||
variable stream.tmpdir. The default value of stream.tmpdir is /tmp. Set the
|
||||
value to a directory with more space:
|
||||
|
||||
+---+
|
||||
-D stream.tmpdir=/export/bigspace/...
|
||||
+---+
|
||||
|
||||
** How do I specify multiple input directories?
|
||||
|
||||
You can specify multiple input directories with multiple '-input' options:
|
||||
|
||||
+---+
|
||||
hadoop jar hadoop-streaming-${project.version}.jar \
|
||||
-input '/user/foo/dir1' -input '/user/foo/dir2' \
|
||||
(rest of the command)
|
||||
+---+
|
||||
|
||||
** How do I generate output files with gzip format?
|
||||
|
||||
Instead of plain text files, you can generate gzip files as your generated
|
||||
output. Pass '-D mapreduce.output.fileoutputformat.compress=true -D
|
||||
mapreduce.output.fileoutputformat.compress.codec=org.apache.hadoop.io.compress.GzipCodec'
|
||||
as option to your streaming job.
|
||||
|
||||
** How do I provide my own input/output format with streaming?
|
||||
|
||||
You can specify your own custom class by packing them and putting the custom
|
||||
jar to \$\{HADOOP_CLASSPATH\}.
|
||||
|
||||
** How do I parse XML documents using streaming?
|
||||
|
||||
You can use the record reader StreamXmlRecordReader to process XML documents.
|
||||
|
||||
+---+
|
||||
hadoop jar hadoop-streaming-${project.version}.jar \
|
||||
-inputreader "StreamXmlRecord,begin=BEGIN_STRING,end=END_STRING" \
|
||||
(rest of the command)
|
||||
+---+
|
||||
|
||||
Anything found between BEGIN_STRING and END_STRING would be treated as one
|
||||
record for map tasks.
|
||||
|
||||
** How do I update counters in streaming applications?
|
||||
|
||||
A streaming process can use the stderr to emit counter information.
|
||||
<<<reporter:counter:\<group\>,\<counter\>,\<amount\>>>> should be sent to
|
||||
stderr to update the counter.
|
||||
|
||||
** How do I update status in streaming applications?
|
||||
|
||||
A streaming process can use the stderr to emit status information. To set a
|
||||
status, <<<reporter:status:\<message\>>>> should be sent to stderr.
|
||||
|
||||
** How do I get the Job variables in a streaming job's mapper/reducer?
|
||||
|
||||
See {{{./MapReduceTutorial.html#Configured_Parameters}
|
||||
Configured Parameters}}. During the execution of a streaming job, the names
|
||||
of the "mapred" parameters are transformed. The dots ( . ) become underscores
|
||||
( _ ). For example, mapreduce.job.id becomes mapreduce_job_id and
|
||||
mapreduce.job.jar becomes mapreduce_job_jar. In your code, use the parameter
|
||||
names with the underscores.
|
||||
@ -93,6 +93,7 @@
|
||||
<item name="Encrypted Shuffle" href="hadoop-mapreduce-client/hadoop-mapreduce-client-core/EncryptedShuffle.html"/>
|
||||
<item name="Pluggable Shuffle/Sort" href="hadoop-mapreduce-client/hadoop-mapreduce-client-core/PluggableShuffleAndPluggableSort.html"/>
|
||||
<item name="Distributed Cache Deploy" href="hadoop-mapreduce-client/hadoop-mapreduce-client-core/DistributedCacheDeploy.html"/>
|
||||
<item name="Hadoop Streaming" href="hadoop-mapreduce-client/hadoop-mapreduce-client-core/HadoopStreaming.html"/>
|
||||
<item name="Hadoop Archives" href="hadoop-mapreduce-client/hadoop-mapreduce-client-core/HadoopArchives.html"/>
|
||||
<item name="DistCp" href="hadoop-mapreduce-client/hadoop-mapreduce-client-core/DistCp.html"/>
|
||||
</menu>
|
||||
|
||||
Loading…
Reference in New Issue
Block a user