At our company, we leverage Apache Druid to provide our customers with real-time analytics tools for various use-cases, including in-flight analytics, reporting and building target audiences. The common challenge of these use-cases is counting distinct elements in real-time at scale. We've been using Druid to solve these problems for the past 5 years, and gained a lot of experience with it. In this talk, we will share some of the best practices and tips we've gathered over the years. We will cover the following topics: - Data modeling - Ingestion - Retention and deletion - Query optimization