HomeAway, a leading online vacation rental marketplace, uses MongoDB to store and analyze user interaction data from billions of sessions across multiple devices. The company employs a KeyRing system to identify and link together related user sessions, constructing a complete picture of each user's experience. To process the vast amounts of data, HomeAway utilizes a Kafka pipeline that persists links between KeyRings and KeyChains in MongoDB, leveraging sharding for horizontal scaling and optimizing performance with pre-splitting of sharding keys and selected cloud hardware.