Implementing the Chain of Responsibility

In this lesson, you will learn how to decouple senders and receivers by passing requests along a chain of handlers using Rust traits.

The Core Concept

The Chain of Responsibility pattern allows you to process a request through a series of "handlers." Each handler decides whether to process the request or pass it to the next link in the chain. This is a powerful way to organize complex business logic, such as an authentication pipeline or a multi-stage request validator, without cluttering your main application code.

In Rust, we implement this by defining a trait that specifies a common interface for every handler. Each handler holds a reference to the "next" item, usually wrapped in an Option<Box<dyn Handler>>.

A Practical Illustration

Imagine a request pipeline where we check for permissions, then logging, and finally data processing.

trait Handler {
    fn handle(&self, request: &str);
}

struct AuthHandler {
    next: Option<Box<dyn Handler>>,
}

impl Handler for AuthHandler {
    fn handle(&self, request: &str) {
        println!("Checking auth for: {}", request);
        if let Some(ref next_handler) = self.next {
            next_handler.handle(request);
        }
    }
}

By using Box<dyn Handler>, we leverage trait objects to allow for dynamic dispatch. This lets us build the chain at runtime. The next field acts as a pointer to the successor. If a handler finishes its work, it calls the handle method on its successor; if it decides the request is invalid, it simply stops the chain by not calling the next handler.

Why use this in Rust?

This pattern shines because it turns monolithic if-else blocks or massive match statements into modular, testable components. Each struct implementing the Handler trait is responsible for exactly one piece of logic. This aligns with our focus on Architecture-First Trait Design, where behavior is encapsulated and easily extendable. You can swap, remove, or reorder handlers without touching the logic of the other segments, keeping your system clean and maintainable.

Key Takeaway: The Chain of Responsibility pattern uses trait objects to route requests through a modular sequence, allowing each handler to either process a request or delegate it to the next step in the pipeline.