The Apache Lucene project has recently released Solr 4, which included major performance improvements, NoSQL capabilities, ready for the cloud and much more.
Solr 4.0 now offers near real-time search, meaning that new content can be pushed to the user’s search results in almost real time. It does not have to be written on a disk somewhere first, which is the case for almost every other search engine.
The new version integrates with the Apache Zookeeper platform, a rock solid project for distributed coordination. Zookeeper holds the Solr configuration and understands the topology with hosts, collections, shards and replicated searchers. This practically means no single point of failure.
Other interesting improvements are a fully integrated spellchecker module, smaller index sizes, new facet types, join functionality and updateable documents.