Yet another take on Go middlewares

by Federico Arias

January 22, 2020

I stole this idea from Mat Ryer. It’s an implementation of middlewares that relies on an adapter function. This function decorates a group of middlewares when applied to an http.Handler.

func Adapt(middlewares ...Middleware) Middleware {
	return func(h http.HandlerFunc) http.HandlerFunc {
		for _, m := range middlewares {
			h = m(h)
		}
		return h
	}
}

Something I did differently, as you might have noticed above, is to name a Middleware type:

type Middleware func(http.HandlerFunc) http.HandlerFunc

Also, in Mat’s implementation, the middlewares are inseparablely linked to their handlers. I choose to abstract away this chain of middlewares instead. With this change, we make future modifications more effective.

How is that so? Well, suppose you had 30 endpoints, each with the same middleware chain. What happens if you want to add another middleware between some of them? you’ll have to change every one of them. What happens if you abstract the group of middlewares in one curried function?

You have to change it only once ©.

Also, this has the advantage of letting you change the middlewares at runtime with a couple lines of code. An example of this could be the case were you want to have a more detailed logging or some special CORS configuration in your development server, and a different one in your production machine.

	if os.Getenv("ENVIRONMENT") == "development" {
		// Let's you change it with one line of code
		myMiddlewares = Adapt(Log, Auth, CORS)
	}

Another nifty modification is to replace the Adapt function with a Chain method on the middleware function itself.

func (mw Middleware) Chain(next Middleware) Middleware {
	return func(h http.HandlerFunc) http.HandlerFunc {
		return next(mw(h))
	}
}

This also has the small benefit of applying the middlewares in the order in which they were defined, and makes it more readable, both in the definition and in the implementation.

	myMiddlewares = Logger().
		Chain(CORS()).
		Chain(Auth())