Context

At companies like Stripe, engineers often work in codebases that must evolve without becoming fragile. Design patterns matter because they provide proven ways to structure code for change, testing, and collaboration.

Core Question

Explain the role of design patterns in building maintainable software.

Address these points:

1. What design patterns are and why they exist.
2. How they improve maintainability through abstraction, decoupling, and clearer responsibilities.
3. When patterns help versus when they add unnecessary complexity.

Scope Guidance

The interviewer expects a practical software engineering explanation rather than a memorized catalog of patterns. You should discuss trade-offs, give 2-3 concrete examples such as Strategy, Factory, or Observer, and connect them to maintainability concerns like extensibility, testability, readability, and change isolation.