Use this tag for questions related to DistributedCache; a facility provided by the Map-Reduce framework to cache files (text, archives, jars etc.) needed by applications.
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 urls are already present on the FileSystem at the path specified by the url and are accessible by every machine in the cluster.
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. In older version of Hadoop Map/Reduce users could optionally ask for symlinks to be created in the working directory of the child task. In the current version symlinks are always created. If the URL does not have a fragment the name of the file or directory will be used. If multiple files or directories map to the same link name, the last one added, will be used. All others will not even be downloaded.
DistributedCache tracks modification timestamps of the cache files. Clearly the cache files should not be modified by the application or
