001 /**
002 * Licensed to the Apache Software Foundation (ASF) under one
003 * or more contributor license agreements. See the NOTICE file
004 * distributed with this work for additional information
005 * regarding copyright ownership. The ASF licenses this file
006 * to you under the Apache License, Version 2.0 (the
007 * "License"); you may not use this file except in compliance
008 * with the License. You may obtain a copy of the License at
009 *
010 * http://www.apache.org/licenses/LICENSE-2.0
011 *
012 * Unless required by applicable law or agreed to in writing, software
013 * distributed under the License is distributed on an "AS IS" BASIS,
014 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
015 * See the License for the specific language governing permissions and
016 * limitations under the License.
017 */
018
019 package org.apache.hadoop.filecache;
020
021 import org.apache.hadoop.classification.InterfaceAudience;
022 import org.apache.hadoop.classification.InterfaceStability;
023 import org.apache.hadoop.conf.Configuration;
024 import org.apache.hadoop.fs.FileSystem;
025 import org.apache.hadoop.fs.Path;
026 import org.apache.hadoop.mapreduce.Job;
027
028 /**
029 * Distribute application-specific large, read-only files efficiently.
030 *
031 * <p><code>DistributedCache</code> is a facility provided by the Map-Reduce
032 * framework to cache files (text, archives, jars etc.) needed by applications.
033 * </p>
034 *
035 * <p>Applications specify the files, via urls (hdfs:// or http://) to be cached
036 * via the {@link org.apache.hadoop.mapred.JobConf}. The
037 * <code>DistributedCache</code> assumes that the files specified via urls are
038 * already present on the {@link FileSystem} at the path specified by the url
039 * and are accessible by every machine in the cluster.</p>
040 *
041 * <p>The framework will copy the necessary files on to the slave node before
042 * any tasks for the job are executed on that node. Its efficiency stems from
043 * the fact that the files are only copied once per job and the ability to
044 * cache archives which are un-archived on the slaves.</p>
045 *
046 * <p><code>DistributedCache</code> can be used to distribute simple, read-only
047 * data/text files and/or more complex types such as archives, jars etc.
048 * Archives (zip, tar and tgz/tar.gz files) are un-archived at the slave nodes.
049 * Jars may be optionally added to the classpath of the tasks, a rudimentary
050 * software distribution mechanism. Files have execution permissions.
051 * Optionally users can also direct it to symlink the distributed cache file(s)
052 * into the working directory of the task.</p>
053 *
054 * <p><code>DistributedCache</code> tracks modification timestamps of the cache
055 * files. Clearly the cache files should not be modified by the application
056 * or externally while the job is executing.</p>
057 *
058 * <p>Here is an illustrative example on how to use the
059 * <code>DistributedCache</code>:</p>
060 * <p><blockquote><pre>
061 * // Setting up the cache for the application
062 *
063 * 1. Copy the requisite files to the <code>FileSystem</code>:
064 *
065 * $ bin/hadoop fs -copyFromLocal lookup.dat /myapp/lookup.dat
066 * $ bin/hadoop fs -copyFromLocal map.zip /myapp/map.zip
067 * $ bin/hadoop fs -copyFromLocal mylib.jar /myapp/mylib.jar
068 * $ bin/hadoop fs -copyFromLocal mytar.tar /myapp/mytar.tar
069 * $ bin/hadoop fs -copyFromLocal mytgz.tgz /myapp/mytgz.tgz
070 * $ bin/hadoop fs -copyFromLocal mytargz.tar.gz /myapp/mytargz.tar.gz
071 *
072 * 2. Setup the application's <code>JobConf</code>:
073 *
074 * JobConf job = new JobConf();
075 * DistributedCache.addCacheFile(new URI("/myapp/lookup.dat#lookup.dat"),
076 * job);
077 * DistributedCache.addCacheArchive(new URI("/myapp/map.zip", job);
078 * DistributedCache.addFileToClassPath(new Path("/myapp/mylib.jar"), job);
079 * DistributedCache.addCacheArchive(new URI("/myapp/mytar.tar", job);
080 * DistributedCache.addCacheArchive(new URI("/myapp/mytgz.tgz", job);
081 * DistributedCache.addCacheArchive(new URI("/myapp/mytargz.tar.gz", job);
082 *
083 * 3. Use the cached files in the {@link org.apache.hadoop.mapred.Mapper}
084 * or {@link org.apache.hadoop.mapred.Reducer}:
085 *
086 * public static class MapClass extends MapReduceBase
087 * implements Mapper<K, V, K, V> {
088 *
089 * private Path[] localArchives;
090 * private Path[] localFiles;
091 *
092 * public void configure(JobConf job) {
093 * // Get the cached archives/files
094 * localArchives = DistributedCache.getLocalCacheArchives(job);
095 * localFiles = DistributedCache.getLocalCacheFiles(job);
096 * }
097 *
098 * public void map(K key, V value,
099 * OutputCollector<K, V> output, Reporter reporter)
100 * throws IOException {
101 * // Use data from the cached archives/files here
102 * // ...
103 * // ...
104 * output.collect(k, v);
105 * }
106 * }
107 *
108 * </pre></blockquote></p>
109 *
110 * It is also very common to use the DistributedCache by using
111 * {@link org.apache.hadoop.util.GenericOptionsParser}.
112 *
113 * This class includes methods that should be used by users
114 * (specifically those mentioned in the example above, as well
115 * as {@link DistributedCache#addArchiveToClassPath(Path, Configuration)}),
116 * as well as methods intended for use by the MapReduce framework
117 * (e.g., {@link org.apache.hadoop.mapred.JobClient}).
118 *
119 * @see org.apache.hadoop.mapred.JobConf
120 * @see org.apache.hadoop.mapred.JobClient
121 * @see org.apache.hadoop.mapreduce.Job
122 */
123 @InterfaceAudience.Public
124 @InterfaceStability.Stable
125 public class DistributedCache extends
126 org.apache.hadoop.mapreduce.filecache.DistributedCache {
127 //
128 }