This document discusses using Spark to build interactive audience analytics by summarizing log data. It proposes using HyperLogLog to estimate cardinalities and handle distinct counting. The document shows how to model impression and segment data as Spark DataFrames, extend the DataFrame DSL to support HyperLogLog aggregations, and use Spark as an in-memory SQL database to serve analytics queries in 1-2 seconds. Spark allows building a complicated analytics structure, caching data in memory, and handling queries faster than other options like Hive, Impala, or Druid.