User Authentication Flow Class Diagram Template

This class diagram template visualizes the structural relationships behind a complete user authentication flow, including the classes, attributes, and methods involved in login, session handling, and logout. It maps out key entities such as User, AuthService, SessionManager, TokenStore, and CredentialValidator, showing how they interact through associations, dependencies, and method calls. By laying out the object-oriented architecture of authentication, the diagram helps teams understand not just the sequence of events but the responsibilities each component holds within the system.

## When to Use This Template

This template is especially valuable during the design phase of a web application, API, or mobile backend where secure user authentication is required. Use it when onboarding new developers who need to understand how login logic is structured, when conducting security audits to verify that session tokens are properly managed and invalidated on logout, or when refactoring legacy authentication code into a cleaner, more modular architecture. It is also a strong communication tool for presenting system design to non-technical stakeholders or during code review discussions.

## Common Mistakes to Avoid

One frequent mistake is conflating the class diagram with a sequence diagram — this template focuses on structure and relationships, not the step-by-step runtime flow. Avoid overloading a single class like AuthService with too many responsibilities; the diagram should reveal if your design violates the Single Responsibility Principle. Another pitfall is omitting the SessionToken or RefreshToken classes entirely, which obscures how session persistence and expiry are handled. Finally, failing to show the relationship between the User class and role or permission classes can leave security logic undocumented, making it harder to enforce access control policies consistently across the codebase.