Import files with distributed cache with Amazon EMR - Amazon EMR
Supported file typesLocation of cached filesAccess cached files from streaming applicationsAccess cached files from streaming applications
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Import files with distributed cache with Amazon EMR

DistributedCache is a Hadoop feature that can boost efficiency when a map or a reduce task needs access to common data. If your cluster depends on existing applications or binaries that are not installed when the cluster is created, you can use DistributedCache to import these files. This feature lets a cluster node read the imported files from its local file system, instead of retrieving the files from other cluster nodes.

For more information, go to http://hadoop.apache.org/docs/stable/api/org/apache/hadoop/filecache/DistributedCache.html.

You invoke DistributedCache when you create the cluster. The files are cached just before starting the Hadoop job and the files remain cached for the duration of the job. You can cache files stored on any Hadoop-compatible file system, for example HDFS or Amazon S3. The default size of the file cache is 10GB. To change the size of the cache, reconfigure the Hadoop parameter, local.cache.size using the bootstrap action. For more information, see Create bootstrap actions to install additional software with an Amazon EMR cluster.

Supported file types

DistributedCache allows both single files and archives. Individual files are cached as read only. Executables and binary files have execution permissions set.

Archives are one or more files packaged using a utility, such as gzip. DistributedCache passes the compressed files to each core node and decompresses the archive as part of caching. DistributedCache supports the following compression formats:

  • zip

  • tgz

  • tar.gz

  • tar

  • jar

Location of cached files

DistributedCache copies files to core nodes only. If there are no core nodes in the cluster, DistributedCache copies the files to the primary node.

DistributedCache associates the cache files to the current working directory of the mapper and reducer using symlinks. A symlink is an alias to a file location, not the actual file location. The value of the parameter, yarn.nodemanager.local-dirs in yarn-site.xml, specifies the location of temporary files. Amazon EMR sets this parameter to /mnt/mapred, or some variation based on instance type and EMR version. For example, a setting may have /mnt/mapred and /mnt1/mapred because the instance type has two ephemeral volumes. Cache files are located in a subdirectory of the temporary file location at /mnt/mapred/taskTracker/archive.

If you cache a single file, DistributedCache puts the file in the archive directory. If you cache an archive, DistributedCache decompresses the file, creates a subdirectory in /archive with the same name as the archive file name. The individual files are located in the new subdirectory.

You can use DistributedCache only when using Streaming.

Access cached files from streaming applications

To access the cached files from your mapper or reducer applications, make sure that you have added the current working directory (./) into your application path and referenced the cached files as though they are present in the current working directory.

Access cached files from streaming applications

You can use the Amazon Web Services Management Console and the Amazon CLI to create clusters that use Distributed Cache.

Console
To specify distributed cache files with the new console

  1. Sign in to the Amazon Web Services Management Console, and open the Amazon EMR console at https://console.amazonaws.cn/emr.

  2. Under EMR on EC2 in the left navigation pane, choose Clusters, and then choose Create cluster.

  3. Under Steps, choose Add step. This opens the Add step dialog. In the Arguments field, include the files and archives to save to the cache. The size of the file (or total size of the files in an archive file) must be less than the allocated cache size.

    If you want to add an individual file to the distributed cache, specify -cacheFile, followed by the name and location of the file, the pound (#) sign, and the name you want to give the file when it's placed in the local cache. The following example demonstrates how to add an individual file to the distributed cache.

    -cacheFile \
s3://amzn-s3-demo-bucket/file-name#cache-file-name

    If you want to add an archive file to the distributed cache, enter -cacheArchive followed by the location of the files in Amazon S3, the pound (#) sign, and then the name you want to give the collection of files in the local cache. The following example demonstrates how to add an archive file to the distributed cache.

    -cacheArchive \
s3://amzn-s3-demo-bucket/archive-name#cache-archive-name

    Enter appropriate values in the other dialog fields. Options differ depending on the step type. To add your step and exit the dialog, choose Add step.

  4. Choose any other options that apply to your cluster.

  5. To launch your cluster, choose Create cluster.

CLI
To specify distributed cache files with the Amazon CLI

  • To submit a Streaming step when a cluster is created, type the create-cluster command with the --steps parameter. To specify distributed cache files using the Amazon CLI, specify the appropriate arguments when submitting a Streaming step.

    If you want to add an individual file to the distributed cache, specify -cacheFile, followed by the name and location of the file, the pound (#) sign, and the name you want to give the file when it's placed in the local cache.

    If you want to add an archive file to the distributed cache, enter -cacheArchive followed by the location of the files in Amazon S3, the pound (#) sign, and then the name you want to give the collection of files in the local cache. The following example demonstrates how to add an archive file to the distributed cache.

    For more information on using Amazon EMR commands in the Amazon CLI, see https://docs.amazonaws.cn/cli/latest/reference/emr.

Example 1

Type the following command to launch a cluster and submit a Streaming step that uses -cacheFile to add one file, sample_dataset_cached.dat, to the cache. 

aws emr create-cluster --name "Test cluster" --release-label emr-4.0.0 --applications Name=Hive Name=Pig --use-default-roles --ec2-attributes KeyName=myKey --instance-type m5.xlarge --instance-count 3 --steps Type=STREAMING,Name="Streaming program",ActionOnFailure=CONTINUE,Args=["--files","s3://my_bucket/my_mapper.py s3://my_bucket/my_reducer.py","-mapper","my_mapper.py","-reducer","my_reducer.py,"-input","s3://my_bucket/my_input","-output","s3://my_bucket/my_output", "-cacheFile","s3://my_bucket/sample_dataset.dat#sample_dataset_cached.dat"]

When you specify the instance count without using the --instance-groups parameter, a single primary node is launched, and the remaining instances are launched as core nodes. All nodes will use the instance type specified in the command.

If you have not previously created the default EMR service role and EC2 instance profile, type aws emr create-default-roles to create them before typing the create-cluster subcommand.

Example 2

The following command shows the creation of a streaming cluster and uses -cacheArchive to add an archive of files to the cache. 

aws emr create-cluster --name "Test cluster" --release-label emr-4.0.0 --applications Name=Hive Name=Pig --use-default-roles --ec2-attributes KeyName=myKey --instance-type m5.xlarge --instance-count 3 --steps Type=STREAMING,Name="Streaming program",ActionOnFailure=CONTINUE,Args=["--files","s3://my_bucket/my_mapper.py s3://my_bucket/my_reducer.py","-mapper","my_mapper.py","-reducer","my_reducer.py,"-input","s3://my_bucket/my_input","-output","s3://my_bucket/my_output", "-cacheArchive","s3://my_bucket/sample_dataset.tgz#sample_dataset_cached"]

When you specify the instance count without using the --instance-groups parameter, a single primary node is launched, and the remaining instances are launched as core nodes. All nodes will use the instance type specified in the command.

If you have not previously created the default EMR service role and EC2 instance profile, type aws emr create-default-roles to create them before typing the create-cluster subcommand.