Why is there no way to add an API handler function that runs on every request?

In designing Dropshot, we’ve tried to avoid a few problems we found with frameworks we used in the past. Many (most?) web frameworks, whether in Rust or another language, let you specify a chain of handlers for each route. You can usually specify some handlers that run before or after every request, regardless of the route. We found that after years of evolving a complex API server using this approach, it can get quite hard to follow the control flow for a particular request and to understand the implicit dependencies between different handlers within the chain. This made it time-consuming and error-prone to work on these API servers. (For more details, see the discussion in issue 58.)

With Dropshot, we wanted to try something different: if the primary purpose of these handlers is to share code between handlers, what if we rely instead on existing mechanisms — i.e., function calls. The big risk is that it’s easy for someone to accidentally forget some important function call, like the one that authenticates or authorizes a user. We haven’t gotten far enough in a complex implementation to need this yet, but the plan is to create a pattern of utility functions that return typed values. For example, where in Node.js you might add an early authentication handler that fills in request.auth, with Dropshot you’d have an authentication function that returns an AuthzContext struct. Then anything that needs authentication consumes the AuthzContext as a function argument. As an author of a handler, you know if you’ve got an AuthzContext available and, if not, how to get one (call the utility function). This composes, too: you can have an authorization function that returns an AuthnContext, and the utility function that returns one can consume the AuthzContext. Then anything that requires authorization can consume just the AuthnContext, and you know it’s been authenticated and authorized (possibly with details in that structure).

It’s early, and we may find we need richer facilities in the framework. But we’re hopeful this approach will make it faster and smoother to iterate on complex API servers. If you pick up Dropshot and try this out, let us know how it goes!