DistributedCache (Hadoop 0.20.2-CDH3B4 API)
Date: 2011-02-21

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## org.apache.hadoop.filecache
Class DistributedCache

    java.lang.Object
      org.apache.hadoop.filecache.DistributedCache

    public class DistributedCache

extends Object

Distribute application-specific large, read-only files efficiently.

`DistributedCache` is a facility provided by the Map-Reduce framework to cache files (text, archives, jars etc.) needed by applications.

Applications specify the files, via urls (hdfs:// or http://) to be cached via the `JobConf`. The `DistributedCache` assumes that the files specified via hdfs:// urls are already present on the `FileSystem` at the path specified by the url.

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.

`DistributedCache` 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.

`DistributedCache` 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.

Here is an illustrative example on how to use the `DistributedCache`:

>          // Setting up the cache for the application

>          1. Copy the requisite files to the FileSystem:

>          $ 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 JobConf:

>          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 Mapper
>          or 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);
>            }
>          }

It is also very common to use the DistributedCache by using `GenericOptionsParser`. This class includes methods that should be used by users (specifically those mentioned in the example above, as well as `addArchiveToClassPath(Path, Configuration)`), as well as methods intended for use by the MapReduce framework (e.g., `JobClient`). For implementation details, see `TrackerDistributedCacheManager` and `TaskDistributedCacheManager`.

See Also:
  `TrackerDistributedCacheManager`, `TaskDistributedCacheManager`, `JobConf`, `JobClient`

Field Summary
`static String` | `CACHE_ARCHIVES` Warning: `CACHE_ARCHIVES` is not a *public* constant.
`static String` | `CACHE_ARCHIVES_SIZES` Warning: `CACHE_ARCHIVES_SIZES` is not a *public* constant.
`static String` | `CACHE_ARCHIVES_TIMESTAMPS` Warning: `CACHE_ARCHIVES_TIMESTAMPS` is not a *public* constant.
`static String` | `CACHE_FILES` Warning: `CACHE_FILES` is not a *public* constant.
`static String` | `CACHE_FILES_SIZES` Warning: `CACHE_FILES_SIZES` is not a *public* constant.
`static String` | `CACHE_FILES_TIMESTAMPS` Warning: `CACHE_FILES_TIMESTAMPS` is not a *public* constant.
`static String` | `CACHE_LOCALARCHIVES` Warning: `CACHE_LOCALARCHIVES` is not a *public* constant.
`static String` | `CACHE_LOCALFILES` Warning: `CACHE_LOCALFILES` is not a *public* constant.
`static String` | `CACHE_SYMLINK` Warning: `CACHE_SYMLINK` is not a *public* constant.

Constructor Summary
`DistributedCache()`

Method Summary
`static void` | `addArchiveToClassPath(Path archive, Configuration conf)` Add an archive path to the current set of classpath entries.
`static void` | `addArchiveToClassPath(Path archive, Configuration conf, FileSystem fs)` Add an archive path to the current set of classpath entries.
`static void` | `addCacheArchive(URI uri, Configuration conf)` Add a archives to be localized to the conf.
`static void` | `addCacheFile(URI uri, Configuration conf)` Add a file to be localized to the conf.
`static void` | `addFileToClassPath(Path file, Configuration conf)` Add a file path to the current set of classpath entries.
`static void` | `addFileToClassPath(Path file, Configuration conf, FileSystem fs)` Add a file path to the current set of classpath entries.
`static void` | `addLocalArchives(Configuration conf, String str)` Add a archive that has been localized to the conf.
`static void` | `addLocalFiles(Configuration conf, String str)` Add a file that has been localized to the conf..
`static boolean` | `checkURIs(URI[] uriFiles, URI[] uriArchives)` This method checks if there is a conflict in the fragment names of the uris.
`static void` | `createAllSymlink(Configuration conf, File jobCacheDir, File workDir)` Deprecated. Internal to MapReduce framework. Use DistributedCacheManager instead.
`static void` | `createSymlink(Configuration conf)` This method allows you to create symlinks in the current working directory of the task to all the cache files/archives.
`static Path[]` | `getArchiveClassPaths(Configuration conf)` Get the archive entries in classpath as an array of Path.
`static long[]` | `getArchiveTimestamps(Configuration conf)` Get the timestamps of the archives.
`static URI[]` | `getCacheArchives(Configuration conf)` Get cache archives set in the Configuration.
`static URI[]` | `getCacheFiles(Configuration conf)` Get cache files set in the Configuration.
`static Path[]` | `getFileClassPaths(Configuration conf)` Get the file entries in classpath as an array of Path.
`static FileStatus` | `getFileStatus(Configuration conf, URI cache)` Returns `FileStatus` of a given cache file on hdfs.
`static long[]` | `getFileTimestamps(Configuration conf)` Get the timestamps of the files.
`static Path[]` | `getLocalCacheArchives(Configuration conf)` Return the path array of the localized caches.
`static Path[]` | `getLocalCacheFiles(Configuration conf)` Return the path array of the localized files.
`static boolean` | `getSymlink(Configuration conf)` This method checks to see if symlinks are to be create for the localized cache files in the current working directory Used by internal DistributedCache code.
`static long` | `getTimestamp(Configuration conf, URI cache)` Returns mtime of a given cache file on hdfs.
`static void` | `setArchiveTimestamps(Configuration conf, String timestamps)` This is to check the timestamp of the archives to be localized.
`static void` | `setCacheArchives(URI[] archives, Configuration conf)` Set the configuration with the given set of archives.
`static void` | `setCacheFiles(URI[] files, Configuration conf)` Set the configuration with the given set of files.
`static void` | `setFileTimestamps(Configuration conf, String timestamps)` This is to check the timestamp of the files to be localized.
`static void` | `setLocalArchives(Configuration conf, String str)` Set the conf to contain the location for localized archives.
`static void` | `setLocalFiles(Configuration conf, String str)` Set the conf to contain the location for localized files.

Methods inherited from class java.lang.Object
`clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait`

Field Detail

### CACHE_FILES_SIZES

    public static final String CACHE_FILES_SIZES

  Warning: `CACHE_FILES_SIZES` is not a *public* constant.

  See Also:
    Constant Field Values

### CACHE_ARCHIVES_SIZES

    public static final String CACHE_ARCHIVES_SIZES

  Warning: `CACHE_ARCHIVES_SIZES` is not a *public* constant.

  See Also:
    Constant Field Values

### CACHE_ARCHIVES_TIMESTAMPS

    public static final String CACHE_ARCHIVES_TIMESTAMPS

  Warning: `CACHE_ARCHIVES_TIMESTAMPS` is not a *public* constant.

  See Also:
    Constant Field Values

### CACHE_FILES_TIMESTAMPS

    public static final String CACHE_FILES_TIMESTAMPS

  Warning: `CACHE_FILES_TIMESTAMPS` is not a *public* constant.

  See Also:
    Constant Field Values

### CACHE_ARCHIVES

    public static final String CACHE_ARCHIVES

  Warning: `CACHE_ARCHIVES` is not a *public* constant.

  See Also:
    Constant Field Values

### CACHE_FILES

    public static final String CACHE_FILES

  Warning: `CACHE_FILES` is not a *public* constant.

  See Also:
    Constant Field Values

### CACHE_LOCALARCHIVES

    public static final String CACHE_LOCALARCHIVES

  Warning: `CACHE_LOCALARCHIVES` is not a *public* constant.

  See Also:
    Constant Field Values

### CACHE_LOCALFILES

    public static final String CACHE_LOCALFILES

  Warning: `CACHE_LOCALFILES` is not a *public* constant.

  See Also:
    Constant Field Values

### CACHE_SYMLINK

    public static final String CACHE_SYMLINK

  Warning: `CACHE_SYMLINK` is not a *public* constant.

  See Also:
    Constant Field Values

Constructor Detail

### DistributedCache

    public DistributedCache()

Method Detail

### getFileStatus

    public static FileStatus getFileStatus(Configuration conf,
                                           URI cache)
                                    throws IOException

  Returns `FileStatus` of a given cache file on hdfs.

  Parameters:
    `conf` - configuration
    `cache` - cache file
  Returns:
    `FileStatus` of a given cache file on hdfs
  Throws:
    `IOException`

### getTimestamp

    public static long getTimestamp(Configuration conf,
                                    URI cache)
                             throws IOException

  Returns mtime of a given cache file on hdfs. Internal to MapReduce.

  Parameters:
    `conf` - configuration
    `cache` - cache file
  Returns:
    mtime of a given cache file on hdfs
  Throws:
    `IOException`

### createAllSymlink

    public static void createAllSymlink(Configuration conf,
                                        File jobCacheDir,
                                        File workDir)
                                 throws IOException

  Deprecated. Internal to MapReduce framework. Use DistributedCacheManager instead.

  This method create symlinks for all files in a given dir in another directory

  Parameters:
    `conf` - the configuration
    `jobCacheDir` - the target directory for creating symlinks
    `workDir` - the directory in which the symlinks are created
  Throws:
    `IOException`

### setCacheArchives

    public static void setCacheArchives(URI[] archives,
                                        Configuration conf)

  Set the configuration with the given set of archives. Intended to be used by user code.

  Parameters:
    `archives` - The list of archives that need to be localized
    `conf` - Configuration which will be changed

### setCacheFiles

    public static void setCacheFiles(URI[] files,
                                     Configuration conf)

  Set the configuration with the given set of files. Intended to be used by user code.

  Parameters:
    `files` - The list of files that need to be localized
    `conf` - Configuration which will be changed

### getCacheArchives

    public static URI[] getCacheArchives(Configuration conf)
                                  throws IOException

  Get cache archives set in the Configuration. Used by internal DistributedCache and MapReduce code.

  Parameters:
    `conf` - The configuration which contains the archives
  Returns:
    An array of the caches set in the Configuration
  Throws:
    `IOException`

### getCacheFiles

    public static URI[] getCacheFiles(Configuration conf)
                               throws IOException

  Get cache files set in the Configuration. Used by internal DistributedCache and MapReduce code.

  Parameters:
    `conf` - The configuration which contains the files
  Returns:
    Am array of the files set in the Configuration
  Throws:
    `IOException`

### getLocalCacheArchives

    public static Path[] getLocalCacheArchives(Configuration conf)
                                        throws IOException

  Return the path array of the localized caches. Intended to be used by user code.

  Parameters:
    `conf` - Configuration that contains the localized archives
  Returns:
    A path array of localized caches
  Throws:
    `IOException`

### getLocalCacheFiles

    public static Path[] getLocalCacheFiles(Configuration conf)
                                     throws IOException

  Return the path array of the localized files. Intended to be used by user code.

  Parameters:
    `conf` - Configuration that contains the localized files
  Returns:
    A path array of localized files
  Throws:
    `IOException`

### getArchiveTimestamps

    public static long[] getArchiveTimestamps(Configuration conf)

  Get the timestamps of the archives. Used by internal DistributedCache and MapReduce code.

  Parameters:
    `conf` - The configuration which stored the timestamps
  Returns:
    a long array of timestamps
  Throws:
    `IOException`

### getFileTimestamps

    public static long[] getFileTimestamps(Configuration conf)

  Get the timestamps of the files. Used by internal DistributedCache and MapReduce code.

  Parameters:
    `conf` - The configuration which stored the timestamps
  Returns:
    a long array of timestamps
  Throws:
    `IOException`

### setArchiveTimestamps

    public static void setArchiveTimestamps(Configuration conf,
                                            String timestamps)

  This is to check the timestamp of the archives to be localized. Used by internal MapReduce code.

  Parameters:
    `conf` - Configuration which stores the timestamp's
    `timestamps` - comma separated list of timestamps of archives. The order should be the same as the order in which the archives are added.

### setFileTimestamps

    public static void setFileTimestamps(Configuration conf,
                                         String timestamps)

  This is to check the timestamp of the files to be localized. Used by internal MapReduce code.

  Parameters:
    `conf` - Configuration which stores the timestamp's
    `timestamps` - comma separated list of timestamps of files. The order should be the same as the order in which the files are added.

### setLocalArchives

    public static void setLocalArchives(Configuration conf,
                                        String str)

  Set the conf to contain the location for localized archives. Used by internal DistributedCache code.

  Parameters:
    `conf` - The conf to modify to contain the localized caches
    `str` - a comma separated list of local archives

### setLocalFiles

    public static void setLocalFiles(Configuration conf,
                                     String str)

  Set the conf to contain the location for localized files. Used by internal DistributedCache code.

  Parameters:
    `conf` - The conf to modify to contain the localized caches
    `str` - a comma separated list of local files

### addLocalArchives

    public static void addLocalArchives(Configuration conf,
                                        String str)

  Add a archive that has been localized to the conf. Used by internal DistributedCache code.

  Parameters:
    `conf` - The conf to modify to contain the localized caches
    `str` - a comma separated list of local archives

### addLocalFiles

    public static void addLocalFiles(Configuration conf,
                                     String str)

  Add a file that has been localized to the conf.. Used by internal DistributedCache code.

  Parameters:
    `conf` - The conf to modify to contain the localized caches
    `str` - a comma separated list of local files

### addCacheArchive

    public static void addCacheArchive(URI uri,
                                       Configuration conf)

  Add a archives to be localized to the conf. Intended to be used by user code.

  Parameters:
    `uri` - The uri of the cache to be localized
    `conf` - Configuration to add the cache to

### addCacheFile

    public static void addCacheFile(URI uri,
                                    Configuration conf)

  Add a file to be localized to the conf. Intended to be used by user code.

  Parameters:
    `uri` - The uri of the cache to be localized
    `conf` - Configuration to add the cache to

### addFileToClassPath

    public static void addFileToClassPath(Path file,
                                          Configuration conf)
                                   throws IOException

  Add a file path to the current set of classpath entries. It adds the file to cache as well. Intended to be used by user code.

  Parameters:
    `file` - Path of the file to be added
    `conf` - Configuration that contains the classpath setting
  Throws:
    `IOException`

### addFileToClassPath

    public static void addFileToClassPath(Path file,
                                          Configuration conf,
                                          FileSystem fs)
                                   throws IOException

  Add a file path to the current set of classpath entries. It adds the file to cache as well. Intended to be used by user code.

  Parameters:
    `file` - Path of the file to be added
    `conf` - Configuration that contains the classpath setting
    `fs` - FileSystem with respect to which `archivefile` should be interpreted.
  Throws:
    `IOException`

### getFileClassPaths

    public static Path[] getFileClassPaths(Configuration conf)

  Get the file entries in classpath as an array of Path. Used by internal DistributedCache code.

  Parameters:
    `conf` - Configuration that contains the classpath setting

### addArchiveToClassPath

    public static void addArchiveToClassPath(Path archive,
                                             Configuration conf)
                                      throws IOException

  Add an archive path to the current set of classpath entries. It adds the archive to cache as well. Intended to be used by user code.

  Parameters:
    `archive` - Path of the archive to be added
    `conf` - Configuration that contains the classpath setting
  Throws:
    `IOException`

### addArchiveToClassPath

    public static void addArchiveToClassPath(Path archive,
                                             Configuration conf,
                                             FileSystem fs)
                                      throws IOException

  Add an archive path to the current set of classpath entries. It adds the archive to cache as well. Intended to be used by user code.

  Parameters:
    `archive` - Path of the archive to be added
    `conf` - Configuration that contains the classpath setting
    `fs` - FileSystem with respect to which `archive` should be interpreted.
  Throws:
    `IOException`

### getArchiveClassPaths

    public static Path[] getArchiveClassPaths(Configuration conf)

  Get the archive entries in classpath as an array of Path. Used by internal DistributedCache code.

  Parameters:
    `conf` - Configuration that contains the classpath setting

### createSymlink

    public static void createSymlink(Configuration conf)

  This method allows you to create symlinks in the current working directory of the task to all the cache files/archives. Intended to be used by user code.

  Parameters:
    `conf` - the jobconf

### getSymlink

    public static boolean getSymlink(Configuration conf)

  This method checks to see if symlinks are to be create for the localized cache files in the current working directory Used by internal DistributedCache code.

  Parameters:
    `conf` - the jobconf
  Returns:
    true if symlinks are to be created- else return false

### checkURIs

    public static boolean checkURIs(URI[] uriFiles,
                                    URI[] uriArchives)

  This method checks if there is a conflict in the fragment names of the uris. Also makes sure that each uri has a fragment. It is only to be called if you want to create symlinks for the various archives and files. May be used by user code.

  Parameters:
    `uriFiles` - The uri array of urifiles
    `uriArchives` - the uri array of uri archives

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