Idempotency in CQRS and Event Sourcing — Part 2: commands, projections and outbox

In HTTP, idempotency is straightforward: store the key, return the cache. In a CQRS/Event Sourcing system, it's more subtle. The command may be idempotent, but what about the event it generates? The projection consuming it? Idempotency must cross the entire stack. If you haven't read part 1, it cove