A generation is analogous to versioning in a revision control system. In TrackingIndexWriter, when an index changes, a new generation is created and can be used to open the index in that particular point in time. TrackingIndexWriter is a wrapper class to IndexWriter. It provides the corresponding addDocument, updateDocument, and deleteDocument methods to keep a track of index changes. On each update, a long value is returned, reflecting the current index generation. This value can be used to acquire an IndexSearcher that includes all the updates up to this specific point (generation). This class is intended to run alongside with ControlledRealTimeReopenThread. The ControlledRealTimeReopenThread is a utility class that runs as a separate thread managing the periodic reopening of the IndexSearcher. It accepts the TrackingIndexWriter and SearcherManager in its constructor to initialize this object. The generation value returned from TrackingIndexWriter...