Solr 5.0 will be the next major release of Solr, represented by the SVN "branch" named trunk. There is currently no planned release timeframe.
Development is still going strong on the 4.x version. Most changes are applied to trunk and then backported to branch_4x, so the two branches are very similar. As of May 2013, there is no compelling reason to create branch_5x and move towards a release of version 5.0. Because trunk now requires a minimum of Java 7 and branch_4x must still compile and run on Java 6, the two branches will start diverging quickly.
This list of new features and major changes for the 5.0 release is incomplete and subject to change:
Solr will no longer be a webapp. It will be a standalone application that embeds the servlet container.
One list of major motivations from Mark Miller: https://mail-archives.apache.org/mod_mbox/lucene-dev/201305.mbox/%3C807476C6-E4C3-4E7E-9F67-2BECB63990DE@gmail.com%3E
- Netty has been mentioned as a possibility, but nothing has been finalized.
- Solr will require Java 1.7 or later.
Once released, this page may be updated if there are any errata for the release documentation that users should be aware of.