Running Hadoop MapReduce on Alluxio

This guide describes how to get Alluxio running with Apache Hadoop MapReduce, so that you can easily run your MapReduce programs with files stored on Alluxio.

Initial Setup

The prerequisite for this guide includes

Prepare the Alluxio client jar

For the MapReduce applications to communicate with Alluxio service, it is required to have the Alluxio client jar on their classpaths. We recommend you to download this client jar from Alluxio download page.

Alternatively, advanced users can choose to compile this client jar from the source code. You can run the following command in the Alluxio directory:

$ mvn install -P<HADOOP_PROFILE> -DskipTests

Available Hadoop profiles include hadoop-1, hadoop-2.2, hadoop-2.3hadoop-2.8 for different distributions of Hadoop. You can further set a specific Hadoop release version to compile. For example,

$ mvn install -Phadoop-2.7 -Dhadoop.version=2.7.1 -DskipTests

After the compilation succeeds, the new Alluxio client jar can be found at /<PATH_TO_ALLUXIO>/core/client/runtime/target/alluxio-core-client-runtime-1.6.0-SNAPSHOT-jar-with-dependencies.jar. This is the jar that you should use for the rest of this guide.

Configuring Hadoop

Add the following three properties to the core-site.xml file in conf directory of your Hadoop installation:

<property>
  <name>fs.alluxio.impl</name>
  <value>alluxio.hadoop.FileSystem</value>
  <description>The Alluxio FileSystem (Hadoop 1.x and 2.x)</description>
</property>
<property>
  <name>fs.alluxio-ft.impl</name>
  <value>alluxio.hadoop.FaultTolerantFileSystem</value>
  <description>The Alluxio FileSystem (Hadoop 1.x and 2.x) with fault tolerant support</description>
</property>
<property>
  <name>fs.AbstractFileSystem.alluxio.impl</name>
  <value>alluxio.hadoop.AlluxioFileSystem</value>
  <description>The Alluxio AbstractFileSystem (Hadoop 2.x)</description>
</property>

This will allow your MapReduce jobs to recognize URIs with Alluxio scheme (i.e., alluxio://) in their input and output files.

Optionally, modify $HADOOP_CLASSPATH by changing hadoop-env.sh also in conf directory to have:

$ export HADOOP_CLASSPATH=/<PATH_TO_ALLUXIO>/core/client/runtime/target/alluxio-core-client-runtime-1.6.0-SNAPSHOT-jar-with-dependencies.jar:${HADOOP_CLASSPATH}

This ensures Alluxio client jar available for the MapReduce job client that creates and submits jobs to interact with URIs with Alluxio scheme.

Distributing the Alluxio Client Jar

In order for the MapReduce applications to be able to read and write files in Alluxio, the Alluxio client jar must be distributed on the classpath of the application across different nodes.

This guide on how to include 3rd party libraries from Cloudera describes several ways to distribute the jars. From that guide, the recommended way to distributed the Alluxio client jar is to use the distributed cache, via the -libjars command line option. Another way to distribute the client jar is to manually distribute it to all the Hadoop nodes. Below are instructions for the two main alternatives:

1.Using the -libjars command line option. You can use the -libjars command line option when using hadoop jar ..., specifying /<PATH_TO_ALLUXIO>/core/client/runtime/target/alluxio-core-client-runtime-1.6.0-SNAPSHOT-jar-with-dependencies.jar as the argument of -libjars. Hadoop will place the jar in the Hadoop DistributedCache, making it available to all the nodes. For example, the following command adds the Alluxio client jar to the -libjars option:

$ bin/hadoop jar libexec/share/hadoop/mapreduce/hadoop-mapreduce-examples-2.7.3.jar wordcount -libjars /<PATH_TO_ALLUXIO>/core/client/runtime/target/alluxio-core-client-runtime-1.6.0-SNAPSHOT-jar-with-dependencies.jar <INPUT FILES> <OUTPUT DIRECTORY>

2.Distributing the client jars to all nodes manually. To install Alluxio on each node, place the client jar /<PATH_TO_ALLUXIO>/core/client/runtime/target/alluxio-core-client-runtime-1.6.0-SNAPSHOT-jar-with-dependencies.jar in the $HADOOP_HOME/lib (may be $HADOOP_HOME/share/hadoop/common/lib for different versions of Hadoop) directory of every MapReduce node, and then restart Hadoop. Alternatively, add this jar to mapreduce.application.classpath system property for your Hadoop deployment to ensure this jar is on the classpath. Note that the jars must be installed again for each update to a new release. On the other hand, when the jar is already on every node, then the -libjars command line option is not needed.

Running Hadoop wordcount with Alluxio Locally

For simplicity, we will assume a pseudo-distributed Hadoop cluster, started by running (depends on the hadoop version, you might need to replace ./bin with ./sbin.):

$ cd $HADOOP_HOME
$ bin/stop-all.sh
$ bin/start-all.sh

Start Alluxio locally:

$ bin/alluxio-start.sh local

You can add a sample file to Alluxio to run wordcount on. From your Alluxio directory:

$ bin/alluxio fs copyFromLocal LICENSE /wordcount/input.txt

This command will copy the LICENSE file into the Alluxio namespace with the path /wordcount/input.txt.

Now we can run a MapReduce job (using Hadoop 2.7.3 as example) for wordcount.

$ bin/hadoop jar libexec/share/hadoop/mapreduce/hadoop-mapreduce-examples-2.7.3.jar wordcount -libjars /<PATH_TO_ALLUXIO>/core/client/runtime/target/alluxio-core-client-runtime-1.6.0-SNAPSHOT-jar-with-dependencies.jar alluxio://localhost:19998/wordcount/input.txt alluxio://localhost:19998/wordcount/output

After this job completes, the result of the wordcount will be in the /wordcount/output directory in Alluxio. You can see the resulting files by running:

$ bin/alluxio fs ls /wordcount/output
$ bin/alluxio fs cat /wordcount/output/part-r-00000
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