I learned about extensions when reading the hyper docs. But they also pop up in lots of other Rust web libraries, like http, tonic, and actix-web. So they must be really useful, if so many libraries offer them. But I personally had no idea what they were or how to use them. Today I'm going to explain what extensions really are (a set of values, keyed by type), and how you can use them to pass data between different parts of your web servers (e.g. middleware, routers, and handler functions). I'll give you a real-world example from the gRPC server I'm building at my job.