Authentication vs authorization in HLD - Scaling Approaches Compared

Scalability Analysis - Authentication vs authorization

Growth Table: Authentication vs Authorization

Users	Authentication Load	Authorization Load	System Changes
100 users	Simple login checks, single auth server	Basic role checks, simple permission store	Monolithic auth service, in-memory session store
10,000 users	Increased login requests, session management	More roles and permissions, caching needed	Introduce load balancer, distributed session cache
1,000,000 users	High concurrent logins, token issuance	Complex policies, attribute-based access control	Horizontal scaling of auth servers, token-based auth (JWT), distributed permission store
100,000,000 users	Massive login traffic, multi-factor auth	Dynamic authorization, fine-grained policies	Global distributed auth clusters, CDN for static auth assets, real-time policy evaluation services

First Bottleneck

The first bottleneck is usually the authentication service during peak login bursts. This is because it must verify credentials, generate tokens, and manage sessions, which are CPU and I/O intensive operations. Authorization checks are generally lighter and cached, so they become bottlenecks later as policies grow complex.

Scaling Solutions

• Authentication: Use horizontal scaling with load balancers to handle login requests. Implement token-based authentication (e.g., JWT) to reduce session state. Use caching for session validation.
• Authorization: Cache permission data to reduce database hits. Use distributed caches like Redis. For complex policies, use policy evaluation engines that scale horizontally.
• General: Employ CDNs for static auth assets (e.g., login pages). Use rate limiting and throttling to protect auth endpoints.

Back-of-Envelope Cost Analysis

Assuming 1 million users with 10% logging in daily:

• Login requests per second: ~1,000,000 * 10% / (24*3600) ≈ 1.2 QPS
• Authorization checks per second: ~1,000,000 * 10 / (24*3600) ≈ 116 QPS
• Storage: User credentials and tokens ~100MB - 1GB depending on retention
• Bandwidth: Login requests small (~1KB), authorization checks smaller (~0.5KB), total ~10-20 MB/s

Interview Tip

Start by clearly defining authentication and authorization. Discuss their differences and dependencies. Then explain how each scales differently. Highlight bottlenecks and propose targeted solutions. Use real numbers and simple analogies like "authentication is like checking ID at a door, authorization is like checking if you can enter a room inside." Keep answers structured and focused.

Self Check

Your authentication database handles 1000 QPS. Traffic grows 10x to 10,000 QPS. What do you do first?

Answer: Introduce horizontal scaling by adding more authentication servers behind a load balancer and implement token-based stateless authentication (e.g., JWT) to reduce database load. Also, add caching layers for session validation to reduce repeated DB hits.

Key Result

Authentication services become bottlenecks first due to login verification load; scaling requires horizontal servers, token-based stateless sessions, and caching, while authorization scales with caching and distributed policy evaluation.