Chain of Responsibility

Intent

Pass a request through a sequence of handlers until one rejects it or the chain completes.

Problem

Validation and middleware often require reusable steps. One giant function makes ordering and reuse difficult.

Solution

Represent each step as a handler linked to the next handler.

TypeScript Implementation

Support tickets pass through authentication, title, and priority handlers.

npm run chain-of-responsibility

Trade-offs

• Great for middleware pipelines.
• Runtime flow is indirect.
• Missing or misordered handlers can change behavior.

Practical Perspective

Behavioral patterns are about distributing responsibilities between objects so workflows stay understandable as rules grow.

For Chain of Responsibility, the important question is not “can I draw the UML diagram?” but “what dependency or decision becomes easier to change after I introduce this pattern?” In production code, the pattern should make ownership clearer, reduce accidental coupling, and give tests a natural seam.

Real-World Use Cases

• Business rules that vary by tenant or product where Chain of Responsibility keeps responsibilities separated.
• Workflow orchestration where Chain of Responsibility keeps responsibilities separated.
• Event-driven UI or domain flows where Chain of Responsibility keeps responsibilities separated.
• Validation, authorization, pricing, routing, or lifecycle logic where Chain of Responsibility keeps responsibilities separated.

Decision Questions

• Which object owns the decision?
• Can a rule change without editing stable workflow code?
• Is runtime behavior explicit enough to debug?
• Use it when a request should pass through ordered handlers that may stop early.
• Make stop/continue behavior visible in the return type.

Design Checklist

• Start with the client code: define the interface you want callers to depend on.
• Keep concrete classes small and named after one responsibility.
• Make creation, selection, delegation, or notification rules explicit instead of hidden in conditionals.
• Prefer composition roots for wiring objects together.
• Document the reason for using the pattern so future contributors do not cargo-cult it.

Common Mistakes

• Adding the pattern before the code has a real variation point.
• Creating abstractions that only rename concrete classes.
• Hiding important runtime behavior so debugging becomes harder.
• Letting examples stay toy-sized without showing where the pattern boundary sits in real code.
• Forgetting tests for negative paths, invalid states, or fallback behavior.

Testing Guidance

• Test through the public abstraction, not private implementation details.
• Use fakes or test doubles for collaborators so the pattern seam is verified.
• Add one integration-style test proving the objects are wired correctly.
• Cover edge cases that motivated the pattern: missing strategy, rejected state transition, failed handler, invalid factory family, stale proxy cache, or similar.
• Keep tests named after behavior and business outcome rather than pattern terminology.

Refactoring Signals

• The pattern is useful when adding a new variation no longer requires editing stable caller code.
• It is probably overdesigned when every new class has only one trivial method and no independent reason to exist.
• If contributors cannot explain the runtime flow quickly, simplify the wiring or improve names.
• If tests must mock too many layers, the abstraction boundary is likely in the wrong place.