Chain of Responsibility

Passes a request along a chain of handlers until one processes it.

Context

An HTTP request must pass through: authentication → rate-limiting → validation → logging → handler. Each step can either proceed or terminate.

Problem

If the steps are tightly coupled, you cannot reorder or reuse them.

Solution

Each handler only knows the next one; it decides whether to process, delegate, or both.

#Example in C#

public abstract class Middleware
{
    private Middleware? _next;
    public Middleware LinkWith(Middleware next) { _next = next; return next; }

    public virtual bool Handle(Request r) => _next?.Handle(r) ?? true;
}

public class AuthMiddleware : Middleware
{
    public override bool Handle(Request r)
    {
        if (r.User is null) { r.Response = "401"; return false; }
        return base.Handle(r);
    }
}
public class RateLimitMiddleware : Middleware
{
    public override bool Handle(Request r) =>
        r.Hits < 100 ? base.Handle(r) : (r.Response = "429") is not null && false;
}

When NOT to use it

When the chain always has the same steps in the same order: a sequential method is simpler.

Tradeoffs

Pro	Con
Reorder and reuse steps	Difficult to know who processed the request
Fits well with pipelines (HTTP, MediatR)	Risk of loops or unprocessed requests

#behavioral #gof #chain