How to Use Middleware Patterns Outside HTTP in Go

You implement middleware outside HTTP by creating a generic function wrapper that accepts a core handler and returns a new handler with added behavior. This pattern relies on Go's first-class functions and the ability to compose handlers through closures, just like http.Handler chains.

type Handler func(ctx context.Context) error

func LoggingMiddleware(next Handler) Handler {
    return func(ctx context.Context) error {
        log.Println("Request started")
        err := next(ctx)
        log.Println("Request finished")
        return err
    }
}

func AuthMiddleware(next Handler) Handler {
    return func(ctx context.Context) error {
        if !isAuthenticated(ctx) {
            return errors.New("unauthorized")
        }
        return next(ctx)
    }
}

func main() {
    coreHandler := func(ctx context.Context) error {
        // Business logic
        return nil
    }

    // Chain middleware: Auth -> Logging -> Core
    finalHandler := AuthMiddleware(LoggingMiddleware(coreHandler))
    finalHandler(context.Background())
}

Middleware is a way to add extra steps like logging or security checks before your main code runs. Think of it like a series of security guards at a building; each guard checks something specific before letting you pass to the next one. In Go, you achieve this by wrapping your main function inside other functions that run code before and after the inner function executes.