Please see github for a more updated version of this doc https://github.com/sakaiproject/sakai/tree/master/search
The search implementation in Sakai has been rewritten backed with Elasticsearch. Elasticsearch is a multi-tenant, cloud friendly, search service written on top of Lucene. It has a JSON based REST api, and has been proven to scale extremely well with large datasets in clustered environments. Elasticsearch (ES) can run as a standalone server or embedded. The current implementation in Sakai embeds ES into Sakai. It would not be an huge effort to allow ES to run standalone and integrate with Sakai this way as well, but that work has not be completed to date.