This document describes the evolution of Expedia's Test and Learn (TNL) platform over three versions (V0, V1, V2). It discusses problems with earlier versions like long query times and increasing data size. The new V2 architecture introduced Kafka for messaging, Cassandra for the database, and Column-oriented storage for improved scalability, responsiveness, and fault tolerance. Lessons learned include ensuring each system component can handle the workload and having pre-production environments to test changes.