Collections are often so large that we cannot perform index
construction efficiently on a single machine. This is
particularly true of the World Wide Web for which we need
large computer clusters to construct any reasonably large sized
web index. Therefore some search engines have started to use
distributed indexing approach for index construction [12, 13].
The result of the construction process is a distributed index
that is partitioned across several machines – either according
to term or according to document.