Expert: policy for deletion of stale {@link IndexCommit index commits}.

Implement this interface, and pass it to one of the {@link IndexWriter} or {@link IndexReader} constructors, to customize when older {@link IndexCommit point-in-time commits} are deleted from the index directory. The default deletion policy is {@link KeepOnlyLastCommitDeletionPolicy}, which always removes old commits as soon as a new commit is done (this matches the behavior before 2.2).

One expected use case for this (and the reason why it was first created) is to work around problems with an index directory accessed via filesystems like NFS because NFS does not provide the "delete on last close" semantics that Lucene's "point in time" search normally relies on. By implementing a custom deletion policy, such as "a commit is only removed once it has been stale for more than X minutes", you can give your readers time to refresh to the new commit before {@link IndexWriter} removes the old commits. Note that doing so will increase the storage requirements of the index. See LUCENE-710 for details.