Got it, let's tackle this OAuth 2.0 adaptation for your food delivery API system. The key here is splitting your API access into two clear tiers: third-party-only public endpoints, and user-authenticated endpoints that still require third-party validation—all without forcing users to log in immediately. Here's a practical, compliant solution:

1. Use Client Credentials Flow for Third-Party-Only APIs #

For your 10 public APIs (restaurant search, dish filtering, etc.), the Client Credentials Flow is perfect. This flow lets third-party apps authenticate themselves without any user involvement, which aligns exactly with your "no login required" requirement.

How it works:

• Third-party apps register on your developer platform to get a client_id and client_secret (for server-side apps) or just a client_id (for SPAs/mobile apps, paired with PKCE to avoid exposing secrets).
• The app sends a POST request to your token endpoint (/oauth/token) with grant type client_credentials, plus their client_id/client_secret.
• Your auth server validates the app's credentials and returns an app-only access token—this token doesn't contain any user data, just the app's identity and permissions for public APIs.
• Your public APIs only need to validate this token's signature, expiration, and associated app permissions to allow access.

2. Use Authorization Code Flow (with PKCE) for User-Authenticated APIs #

For the remaining 2-3 APIs (order placement, history lookup), use the Authorization Code Flow (with PKCE for mobile/SPAs to enhance security). The critical twist here is: don't trigger this flow until the user initiates an action that requires their identity (like clicking "Place Order").

How it works:

• When the user tries to perform a user-specific action, the third-party app redirects them to your auth server's authorization page. The request includes:
  • client_id (to identify the app)
  • redirect_uri (where the user is sent back after auth)
  • scope (e.g., order:write order:read to define what the app can do on the user's behalf)
  • state (a random string to prevent CSRF)
  • PKCE code challenge (for mobile/SPAs)
• The user logs in to your system (via email/password, social login, etc.) and grants the app permission to access their order data.
• Your auth server redirects the user back to the app with an authorization code.
• The app exchanges this code for a user-associated access token (which includes both the app's identity and the user's ID/claims) and a refresh token.
• This token is then used to call the user-specific APIs.

3. Unified Token Validation Logic #

Build a centralized validation layer (either in your API gateway or a shared auth service) to handle both types of tokens consistently:

• For public APIs: Validate that the token is a valid app-only token, hasn't expired, and the associated app has permission to access the endpoint. No user check is needed.
• For user-specific APIs: Validate that the token is a valid user-associated token, the app identity matches the token's client ID, the user exists and is in good standing, and the token's scope includes the required permission (e.g., order:write for placing orders).

4. Step-by-Step Integration Workflow #

• App Registration: Third-party devs sign up on your platform to get their credentials and define allowed redirect URIs.
• Public API Usage: App uses client credentials to get an app-only token, then uses this token to call all public endpoints freely—no user login required at this stage.
• Trigger User Auth: Only when the user attempts a restricted action (like checkout), the app initiates the Authorization Code Flow.
• Token Exchange: App gets the user-associated token and stores it (securely, e.g., in HttpOnly cookies for web apps or keystore for mobile).
• Restricted API Calls: App uses the user-associated token to call order/history APIs, which your backend validates for both app and user identity.
• Token Refresh: Use the refresh token to get a new user-associated access token when the old one expires—this avoids asking the user to log in repeatedly.

5. Critical Tips to Avoid Forced Login #

• Delay Auth Until Necessary: Never redirect users to login when they first open the app or browse restaurants. Only trigger the auth flow when they take an action that needs their identity.
• Clear Scope Definitions: Use scopes to distinguish between public and user-specific permissions (e.g., public:read vs user:order:write). This makes validation simpler and more secure.
• Support Guest Checkout (Optional): If your system allows guest orders, you can add a third flow: generate a temporary guest user token when the user starts checkout without logging in, then let them link their account later if they want.

内容的提问来源于stack exchange，提问作者Rhythm Shahriar