Lucene 4 Cookbook

Using the DirectoryReader to open index in Near Real-Time

First of all, let's cover the basics. The DirectoryReader attribute that we are already familiar with actually allows you to open an index with IndexWriter with the option to include uncommitted changes. This gives you a point-in-time snapshot of the index, including any updates that are not committed yet. In a typical search application, IndexSearcher would need to be reopened periodically anyway to expose recent updates. This feature provides an option to immediately expose index updates without waiting for IndexWriter to commit first. Potentially, the search application may have to maintain multiple IndexSearchers as depicted in the following diagram:

One use case for this feature is ...