Is there any good open source or free realtime aggregation engine available today with possible java APIs. Looking something close to OLAP like system.
Oops I should have been more specific with my question. Am looking to analyze the product sales data with respect to vendor, location, type etc. (although for now data volume is not that much but expect it to grow in a year or so) and need to generate reports. After googling found Pentaho Mondrian Analysis Services and Apache Hive. Need to find out which one will be better. If you know of any other product, please post it here.