DROONGA

Droonga is a distributed full-text search engine, based on a stream oriented processing model. In many operations (searching, updating, grouping, and so on), Droonga processes various data by pipeline. As the result, Droonga has large potential around its flexibility and extensibility. Moreover, those features provide high availability for people who develop any data processing engine based on Droonga. You can process complex operations by mixing operations, and you can add custom operations to Droonga via plugins written as Ruby-scripts.