In an event-driven, (micro-)services based architecture, we imagine a bunch of services with a single responsibility interacting with each other by emitting and reacting to domain events. Sounds nice! Here are some of the decisions we need to make to get there: When do we use API calls vs events for interactions? Do we need to build up local replicas from events when we need data from other services? How do we design events? When do we use event sourcing? In this lightning talk we'll tour through these questions and principles that can help you answer them.