a distributed data store for logs
要求能够将PII信息隐藏，能够做top k exceptions，能够做快速查询，比如按各种不同的fi
Our goal is to build a real-time aggregation system. WebServer logs event to our system using the following function call: log(ad_id, user_id, event_type). At this moment we plan to have only 2 types of events: "ad was shown" and "ad was clicked on".