1. Supported purpose: Casbin is primarily used for access control and permission management in applications, providing fine-grained access control based on authorization rules, while OAuth2 is mainly utilized for delegated authorization, enabling users to grant permissions to specific resources without disclosing their credentials. Casbin focuses on defining and enforcing access control policies, while OAuth2 deals with managing access tokens for resource requests.
2. Policy management: Casbin centralizes access control policies and enforces them based on policy rules defined by administrators, whereas OAuth2 manages authorization through the exchange of tokens between the resource owner, client, and authorization server, ensuring secure access to protected resources. Casbin's policy management revolves around defining rules for access control, whereas OAuth2's focus is on granting and utilizing access tokens for authorized actions.
3. Adaptability: Casbin is language-agnostic, making it versatile for integration into various programming languages and frameworks, allowing developers to implement fine-grained access control policies universally, whereas OAuth2 is a standardized authorization framework that mandates specific flows and token formats for authentication and authorization across different platforms and services. Casbin's adaptability makes it easier for developers to implement access control in diverse environments, while OAuth2 ensures consistency in authorization processes across systems.
4. Granularity of control: Casbin offers fine-grained access control through the use of access control models like RBAC, ABAC, and PBAC, enabling specific authorization for individual actions or resources based on defined policies, while OAuth2 provides broader delegated authorization through the issuance and validation of access tokens, granting broader access rights to clients based on the scopes requested during authorization flows. Casbin's control granularity allows for precise definition of access policies, whereas OAuth2 provides a more general level of authorization based on token scopes.
5. Authentication vs. Authorization: Casbin primarily focuses on authorization, determining what actions a user is allowed to perform based on defined policies and roles, while OAuth2 focuses on authentication and authorization, ensuring that the user is who they claim to be and granting them access to specific resources based on their permissions. Casbin handles authorization decisions based on access control policies, whereas OAuth2 encompasses both authentication and authorization processes during resource access.
6. Scope of application: Casbin is typically used within the application codebase to enforce access control policies and manage permissions for users or entities interacting with the system, while OAuth2 is commonly employed as a protocol for securing APIs, enabling third-party applications to access protected resources on behalf of users through token-based authorization. Casbin's application scope is primarily within the application logic for access control, whereas OAuth2's domain extends to secure API access and delegated authorization for external clients.

In Summary, Casbin and OAuth2 differ in their supported purposes, policy management approaches, adaptability, granularity of control, focus on authentication versus authorization, and scope of application in secure access management.