OAuth 2.0 Authorization Class Diagram Template

This class diagram template maps out the key entities, attributes, and relationships involved in the OAuth 2.0 Authorization Code Grant flow. It captures the core classes — such as Client, Authorization Server, Resource Server, User, Authorization Code, and Access Token — along with their properties and the associations between them. By visualizing how these components interact at a structural level, the diagram helps teams understand the roles each party plays before a single line of code is written. Developers can see at a glance how an authorization code is issued, exchanged for an access token, and ultimately used to access protected resources on behalf of a user.

## When to Use This Template

This template is most valuable during the design and documentation phases of building or integrating an OAuth 2.0-protected API. Security architects use it to communicate the trust boundaries between the client application, the authorization server, and the resource server. Backend engineers reference it when implementing token issuance logic, while frontend developers use it to understand what data their application needs to store and pass between requests. It is also an excellent teaching aid for onboarding new team members to an existing OAuth implementation, making abstract security concepts concrete and navigable.

## Common Mistakes to Avoid

One frequent error is conflating the Authorization Server and the Resource Server into a single class. While some implementations combine these roles, the OAuth 2.0 specification treats them as distinct entities, and your diagram should reflect that separation to remain accurate and reusable. Another mistake is omitting the Refresh Token class entirely; since refresh tokens are a critical part of long-lived sessions, leaving them out creates an incomplete picture of the flow. Developers also sometimes model the authorization code as a simple string attribute on the Client class rather than as its own entity with a lifecycle — expiry time, single-use constraint, and PKCE code challenge. Representing it as a first-class object makes the security constraints explicit and easier to enforce during implementation. Finally, avoid drawing associations without labeling multiplicity; showing whether a client can hold one or many tokens at a time is essential for understanding session management and preventing token leakage.