The copy-listing generator classes are responsible for creating the list of files/directories to be copied from source. They examine the contents of the source paths (files/directories, including wildcards), and record all paths that need copying into a SequenceFile for consumption by the DistCp Hadoop Job. The main classes in this module include:
CopyListing: The interface that should be implemented by any copy-listing generator implementation. Also provides the factory method by which the concrete CopyListing implementation is chosen.
SimpleCopyListing: An implementation of CopyListing that accepts multiple source paths (files/directories), and recursively lists all of the individual files and directories under each for copy.
GlobbedCopyListing: Another implementation of CopyListing that expands wildcards in the source paths.
FileBasedCopyListing: An implementation of CopyListing that reads the source path list from a specified file.
Based on whether a source file list is specified in the DistCpOptions, the source listing is generated in one of the following ways:
If there is no source file list, the GlobbedCopyListing is used. All wildcards are expanded, and all of the expansions are forwarded to the SimpleCopyListing, which in turn constructs the listing (via recursive descent of each path).
If a source file list is specified, the FileBasedCopyListing is used. Source paths are read from the specified file, and then forwarded to the GlobbedCopyListing. The listing is then constructed as described above.
You can customize the method by which the copy-listing is constructed by providing a custom implementation of the CopyListing interface. The behaviour of DistCp differs here from the legacy DistCp, in how paths are considered for copy.
The legacy implementation only lists those paths that must definitely be copied on to the target. E.g., if a file already exists at the target (and -overwrite isn’t specified), the file is not even considered in the MapReduce copy job. Determining this during setup (i.e. before the MapReduce Job) involves file size and checksum comparisons that are potentially time consuming.
DistCp postpones such checks until the MapReduce job, thus reducing setup time. Performance is enhanced further since these checks are parallelized across multiple maps.
