OAuth 2.0 flow in HLD - Scalability & System Analysis

Scalability Analysis - OAuth 2.0 flow

Growth Table: OAuth 2.0 Flow Scaling

Users	Requests per Second	Token Issuance	Database Load	Network Traffic	Latency
100 users	~10-20 RPS	Low token requests	Single DB instance handles well	Low bandwidth usage	Low latency
10,000 users	~1,000-2,000 RPS	Moderate token requests, refreshes	DB load increases, possible CPU spikes	Moderate bandwidth usage	Latency may increase slightly
1,000,000 users	~100,000 RPS	High token issuance and validation	DB bottleneck likely, connection limits hit	High bandwidth, possible network congestion	Latency noticeable without caching
100,000,000 users	~10,000,000 RPS	Very high token traffic, refresh storms	Single DB impossible, sharding needed	Very high bandwidth, CDN and edge caching needed	Latency critical, must optimize globally

First Bottleneck

The first bottleneck in OAuth 2.0 flow scaling is the database that stores tokens, authorization codes, and user sessions. As user count and requests grow, the DB faces high read/write load, connection limits, and latency issues. Token validation and refresh operations increase DB pressure quickly.

Scaling Solutions

• Database Scaling: Use read replicas to offload read queries, connection pooling to manage DB connections, and sharding to distribute data across multiple DB instances.
• Caching: Cache access tokens and user session data in fast in-memory stores like Redis to reduce DB hits.
• Horizontal Scaling: Add more authorization servers behind load balancers to handle increased request volume.
• Token Design: Use JWT tokens to reduce DB lookups by embedding claims in tokens, validated cryptographically.
• CDN and Edge: For public OAuth endpoints, use CDN to reduce latency and bandwidth usage globally.
• Rate Limiting: Implement rate limiting to prevent abuse and reduce load spikes.

Back-of-Envelope Cost Analysis

• At 1M users with 100K RPS, DB must handle ~100K token validations/refreshes per second.
• Assuming 1 token validation = 1 DB read, 1 refresh = 1 write + 1 read, DB QPS can exceed 200K.
• Single PostgreSQL instance handles ~5K QPS, so need ~40+ replicas or sharding.
• Network bandwidth: 100K RPS * 1KB token size = ~100MB/s (~800Mbps), requires 1Gbps+ network.
• Memory for caching tokens: 1M active tokens * 1KB = ~1GB RAM minimum.

Interview Tip

When discussing OAuth 2.0 scalability, start by explaining the flow and components. Identify the database as the first bottleneck due to token storage and validation. Then discuss caching and horizontal scaling of authorization servers. Mention token design improvements like JWT to reduce DB load. Finally, cover network and rate limiting strategies. Structure your answer from small scale to large scale with clear bottlenecks and solutions.

Self Check Question

Your database handles 1000 QPS for token validation. Traffic grows 10x to 10,000 QPS. What do you do first and why?

Key Result

The database storing tokens and sessions is the first bottleneck as OAuth 2.0 traffic grows; caching tokens and horizontally scaling authorization servers are key to scaling.

Practice

1. What is the main purpose of the OAuth 2.0 Authorization Code flow?

easy

A. To replace user authentication with biometric data

B. To store user passwords safely on the server

C. To allow apps to access user data securely without sharing passwords

D. To encrypt all user data before sending it

Solution

1. Step 1: Understand OAuth 2.0 Authorization Code flow

   This flow lets apps get permission to access user data without needing the user's password directly.

2. Step 2: Identify the main goal

   The goal is to keep user credentials safe while allowing apps to act on behalf of the user.

3. Final Answer:

   To allow apps to access user data securely without sharing passwords -> Option C

4. Quick Check:

   OAuth 2.0 Authorization Code flow = Secure app access without passwords [OK]

Hint: Remember: OAuth avoids sharing passwords directly [OK]

Common Mistakes:

• Confusing OAuth with password storage
• Thinking OAuth encrypts all user data
• Assuming OAuth replaces authentication methods

2. Which of the following is the correct sequence in the OAuth 2.0 Authorization Code flow?

easy

A. User logs in -> App receives access token -> App requests authorization code

B. App sends password -> User grants access -> App receives token

C. App directly receives access token -> User logs in -> Access granted

D. User logs in -> App gets authorization code -> App exchanges code for access token

Solution

1. Step 1: Recall the OAuth 2.0 Authorization Code flow steps

   User logs in and authorizes the app, which then receives an authorization code.

2. Step 2: Understand token exchange

   The app exchanges the authorization code for an access token to access user data.

3. Final Answer:

   User logs in -> App gets authorization code -> App exchanges code for access token -> Option D

4. Quick Check:

   Authorization Code flow sequence = Login, code, token [OK]

Hint: Authorization code comes before access token [OK]

Common Mistakes:

• Thinking app gets token before authorization code
• Confusing password sending with OAuth flow
• Mixing order of login and token receipt

3. Given this OAuth 2.0 flow snippet, what is the expected output after exchanging the authorization code?

POST /token
Headers: Content-Type: application/x-www-form-urlencoded
Body: grant_type=authorization_code&code=abc123&redirect_uri=https://app.com/callback&client_id=client123&client_secret=secretXYZ

Response:
{
  "access_token": "token456",
  "token_type": "Bearer",
  "expires_in": 3600
}

medium

A. An access token 'token456' valid for 3600 seconds

B. An error because client_secret is missing

C. A refresh token instead of access token

D. No token, only a confirmation message

Solution

1. Step 1: Analyze the token request

   The app sends a POST request with grant_type authorization_code and includes the code and client credentials.

2. Step 2: Interpret the response

   The response contains an access_token 'token456', token_type 'Bearer', and expires_in 3600 seconds, meaning the token is valid for one hour.

3. Final Answer:

   An access token 'token456' valid for 3600 seconds -> Option A

4. Quick Check:

   Token response includes access_token and expiry [OK]

Hint: Token response includes access_token and expiry time [OK]

Common Mistakes:

• Expecting refresh token instead of access token
• Missing client_secret causes error
• Confusing confirmation message with token response

4. A developer implements OAuth 2.0 Authorization Code flow but the app never receives an access token after exchanging the code. What is the most likely cause?

medium

A. The app forgot to include the client_id in the authorization request

B. The redirect URI used in the token request does not match the one registered

C. The user did not enter their password correctly

D. The access token expired before the request

Solution

1. Step 1: Identify common OAuth token exchange errors

   One frequent error is a mismatch in redirect URI between authorization and token requests.

2. Step 2: Understand impact of redirect URI mismatch

   If the redirect URI differs, the authorization server rejects the token request, so no access token is returned.

3. Final Answer:

   The redirect URI used in the token request does not match the one registered -> Option B

4. Quick Check:

   Redirect URI mismatch blocks token issuance [OK]

Hint: Check redirect URI matches exactly in all requests [OK]

Common Mistakes:

• Blaming user password errors for token issues
• Ignoring client_id in authorization request
• Assuming token expires before it is issued

5. You are designing a system that uses OAuth 2.0 Authorization Code flow for a mobile app and a web app. Which approach best secures the access tokens and refresh tokens?

hard

A. Use Authorization Code flow with PKCE for the mobile app and standard Authorization Code flow for the web app

B. Use Implicit flow for both apps to simplify token handling

C. Store access tokens in local storage for both apps to allow easy access

D. Send access tokens directly in URL query parameters for both apps

Solution

1. Step 1: Understand security needs for mobile and web apps

   Mobile apps cannot keep client secrets safe, so PKCE (Proof Key for Code Exchange) enhances security for Authorization Code flow.

2. Step 2: Choose best practices for token storage and flow

   Web apps can use standard Authorization Code flow with client secret. Implicit flow is deprecated due to security risks. Storing tokens in local storage or URLs exposes them to attacks.

3. Final Answer:

   Use Authorization Code flow with PKCE for the mobile app and standard Authorization Code flow for the web app -> Option A

4. Quick Check:

   PKCE secures mobile OAuth; standard flow suits web apps [OK]

Hint: Use PKCE for mobile apps, standard flow for web apps [OK]

Common Mistakes:

• Using Implicit flow despite security risks
• Storing tokens insecurely in local storage
• Passing tokens in URL parameters