Watermill is a Go library for working efficiently with message streams. It is intended as a library for building event-driven applications, enabling event sourcing, CQRS, RPC over messages, sagas. Why? Lack of standard messaging library There are many third party and standard library tools which help to implement standardized RPC or HTTP communication in Golang. There are also multiple third party HTTP routers and frameworks. But when you want to build a message-driven application, there are no libraries which are infrastructure-agnostic and not opinionated.