In November of 2016 Netflix successfully launched its new Download feature, allowing users to download and play content offline on their mobile devices. This feature required us to change our previously stateless distributed licensing service to be real time and stateful. In a matter of months we needed to create a brand new stateful service that could evolve with rapid feature requirement iterations, while also being able to scale to millions of members using the feature across the globe.

This talk describes how we achieved these goals with the use of a Cassandra-backed event sourcing architecture. We describe our event store implementation, including the use of data versioning and snapshotting to provide flexibility and scale. We will cover what we learned along the way, and what we could have done better. Finally, we will review some improvements and extensions that we are planning to address going forward. Attendees will take home some compelling reasons to consider event sourcing for their architectures: it’s flexibility to adapt to changing business requirements, the relevance to distributed and scalable microservice architecture, and the means to replay a timeline of events and determine current or potential state.