System Design Fundamentals Every Developer Should Know

System design interviews at companies like Google, Amazon, and Flipkart can feel intimidating. But here's the thing—they're testing a skill you'll actually use daily as a senior engineer. Let me break down the concepts that matter.

The Building Blocks

Every large system uses these components in some combination:

Load Balancers

Distribute traffic across multiple servers. Think of it like having multiple billing counters at Big Bazaar during a sale.

User Requests → Load Balancer → Server 1
                             → Server 2
                             → Server 3

Common algorithms:

• Round Robin: Requests go to each server in turn
• Least Connections: Send to server with fewest active connections
• IP Hash: Same user always hits same server (useful for sessions)

Caching

The single biggest performance improvement you can make. A cache hit is ~100x faster than a database query.

User → Cache (Redis) → Cache Hit? Return data
                    → Cache Miss? Query DB → Store in Cache → Return

Cache invalidation strategies:

• TTL (Time to Live): Data expires after set time
• Write-through: Update cache when DB updates
• Write-behind: Update cache first, DB asynchronously

Databases

SQL (PostgreSQL, MySQL)

• ACID compliant
• Complex queries with JOINs
• Best for: Banking, inventory, anything needing transactions

NoSQL (MongoDB, Cassandra)

• Flexible schema
• Horizontal scaling
• Best for: Logs, analytics, rapidly changing data

Message Queues

When you can't process requests immediately:

User Order → Queue (RabbitMQ/Kafka) → Order Processing Service
                                   → Email Service
                                   → Analytics Service

Designing for Scale

Horizontal vs Vertical Scaling

Vertical: Bigger server (limited by hardware) Horizontal: More servers (preferred for web apps)

Database Sharding

Split data across multiple databases:

• By user ID: Users 1-1M on DB1, 1M-2M on DB2
• By geography: India users on Mumbai cluster, US on Virginia

CDN (Content Delivery Network)

Serve static assets from servers closest to users. Cloudflare, AWS CloudFront, or Akamai.

Real-World Example: URL Shortener

Let's design something like bit.ly:

Requirements:

• Shorten long URLs
• Redirect shortened URLs
• Handle 100M URLs, 1000 redirects/second

High-Level Design:

User → API Gateway → URL Service → Database
                               → Cache (hot URLs)

URL Generation:

1. Take MD5 hash of original URL
2. Encode first 7 chars in base62
3. Check for collisions
4. Store mapping in DB

Storage Calculations:

• 100M URLs × 500 bytes = 50GB (fits in single DB with indexing)
• 1000 redirects/second = cache the top 20% (Pareto principle)

Interview Framework

When given any system design question:

1. Clarify requirements (5 mins)

   • Who are the users?
   • What scale are we talking?
   • What features are must-have vs nice-to-have?

2. High-level design (10 mins)

   • Draw main components
   • Show data flow

3. Deep dive (15 mins)

   • Database schema
   • API design
   • Scaling considerations

4. Trade-offs and alternatives (5 mins)

   • What could fail?
   • How would you monitor?

Resources for Further Learning

• "Designing Data-Intensive Applications" by Martin Kleppmann
• System Design Primer on GitHub
• Real company engineering blogs (Uber, Netflix, Spotify)

The goal isn't to memorize solutions—it's to develop intuition for how systems work together. Build small projects that use these patterns, and the interview becomes a conversation about things you've actually done.