I'm assuming this is primarily used for analytics ? Metamarkets another ad company open sourced Druid for a similar purpose. I'm curious as to the differences in these solutions.
This is a library to read and write a data format that is optimized to give access to granular events and actors within an event stream. For example this could be used to trail all of the events generated by one entity (credit card, cookie, email, account and so on) over a dataset. At that point you can choose what you want to do with it: extract features for ML, train ML directly on raw data, run arbitrary queries for outliers and anomaly detection and what have you.