/**
* Licensed to the Apache Software Foundation (ASF) under one
* or more contributor license agreements. See the NOTICE file
* distributed with this work for additional information
* regarding copyright ownership. The ASF licenses this file
* to you under the Apache License, Version 2.0 (the
* "License"); you may not use this file except in compliance
* with the License. You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package org.apache.hadoop.filecache;
import org.apache.hadoop.classification.InterfaceAudience;
import org.apache.hadoop.classification.InterfaceStability;
import org.apache.hadoop.conf.Configuration;
import org.apache.hadoop.fs.FileSystem;
import org.apache.hadoop.fs.Path;
import org.apache.hadoop.mapreduce.Job;
/**
* Distribute application-specific large, read-only files efficiently.
*
* <p><code>DistributedCache</code> is a facility provided by the Map-Reduce
* framework to cache files (text, archives, jars etc.) needed by applications.
* </p>
*
* <p>Applications specify the files, via urls (hdfs:// or http://) to be cached
* via the {@link org.apache.hadoop.mapred.JobConf}. The
* <code>DistributedCache</code> assumes that the files specified via urls are
* already present on the {@link FileSystem} at the path specified by the url
* and are accessible by every machine in the cluster.</p>
*
* <p>The framework will copy the necessary files on to the slave node before
* any tasks for the job are executed on that node. Its efficiency stems from
* the fact that the files are only copied once per job and the ability to
* cache archives which are un-archived on the slaves.</p>
*
* <p><code>DistributedCache</code> can be used to distribute simple, read-only
* data/text files and/or more complex types such as archives, jars etc.
* Archives (zip, tar and tgz/tar.gz files) are un-archived at the slave nodes.
* Jars may be optionally added to the classpath of the tasks, a rudimentary
* software distribution mechanism. Files have execution permissions.
* Optionally users can also direct it to symlink the distributed cache file(s)
* into the working directory of the task.</p>
*
* <p><code>DistributedCache</code> tracks modification timestamps of the cache
* files. Clearly the cache files should not be modified by the application
* or externally while the job is executing.</p>
*
* <p>Here is an illustrative example on how to use the
* <code>DistributedCache</code>:</p>
* <p><blockquote><pre>
* // Setting up the cache for the application
*
* 1. Copy the requisite files to the <code>FileSystem</code>:
*
* $ bin/hadoop fs -copyFromLocal lookup.dat /myapp/lookup.dat
* $ bin/hadoop fs -copyFromLocal map.zip /myapp/map.zip
* $ bin/hadoop fs -copyFromLocal mylib.jar /myapp/mylib.jar
* $ bin/hadoop fs -copyFromLocal mytar.tar /myapp/mytar.tar
* $ bin/hadoop fs -copyFromLocal mytgz.tgz /myapp/mytgz.tgz
* $ bin/hadoop fs -copyFromLocal mytargz.tar.gz /myapp/mytargz.tar.gz
*
* 2. Setup the application's <code>JobConf</code>:
*
* JobConf job = new JobConf();
* DistributedCache.addCacheFile(new URI("/myapp/lookup.dat#lookup.dat"),
* job);
* DistributedCache.addCacheArchive(new URI("/myapp/map.zip", job);
* DistributedCache.addFileToClassPath(new Path("/myapp/mylib.jar"), job);
* DistributedCache.addCacheArchive(new URI("/myapp/mytar.tar", job);
* DistributedCache.addCacheArchive(new URI("/myapp/mytgz.tgz", job);
* DistributedCache.addCacheArchive(new URI("/myapp/mytargz.tar.gz", job);
*
* 3. Use the cached files in the {@link org.apache.hadoop.mapred.Mapper}
* or {@link org.apache.hadoop.mapred.Reducer}:
*
* public static class MapClass extends MapReduceBase
* implements Mapper<K, V, K, V> {
*
* private Path[] localArchives;
* private Path[] localFiles;
*
* public void configure(JobConf job) {
* // Get the cached archives/files
* localArchives = DistributedCache.getLocalCacheArchives(job);
* localFiles = DistributedCache.getLocalCacheFiles(job);
* }
*
* public void map(K key, V value,
* OutputCollector<K, V> output, Reporter reporter)
* throws IOException {
* // Use data from the cached archives/files here
* // ...
* // ...
* output.collect(k, v);
* }
* }
*
* </pre></blockquote></p>
*
* It is also very common to use the DistributedCache by using
* {@link org.apache.hadoop.util.GenericOptionsParser}.
*
* This class includes methods that should be used by users
* (specifically those mentioned in the example above, as well
* as {@link DistributedCache#addArchiveToClassPath(Path, Configuration)}),
* as well as methods intended for use by the MapReduce framework
* (e.g., {@link org.apache.hadoop.mapred.JobClient}).
*
* @see org.apache.hadoop.mapred.JobConf
* @see org.apache.hadoop.mapred.JobClient
* @see org.apache.hadoop.mapreduce.Job
*/
@InterfaceAudience.Public
@InterfaceStability.Stable
public class DistributedCache extends
org.apache.hadoop.mapreduce.filecache.DistributedCache {
//
}