system-design

System Design

Overview

Principles for designing systems that handle scale, remain available, and perform well under load.

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Scalability Fundamentals

Vertical vs Horizontal Scaling

Vertical Scaling (Scale Up):
┌─────────────────────┐
│  Bigger Server      │
│  - More CPU         │
│  - More RAM         │
│  - Faster disk      │
└─────────────────────┘
Limit: Hardware ceiling

Horizontal Scaling (Scale Out):
┌──────┐ ┌──────┐ ┌──────┐ ┌──────┐
│Server│ │Server│ │Server│ │Server│
└──────┘ └──────┘ └──────┘ └──────┘
         ↑
    Load Balancer
Limit: Coordination complexity

Stateless Services

// ❌ Stateful - stores session in memory
class BadService {
  private sessions = new Map();

  login(userId: string) {
    this.sessions.set(userId, { loggedIn: true });
  }
}

// ✅ Stateless - external session store
class GoodService {
  constructor(private sessionStore: Redis) {}

  async login(userId: string) {
    await this.sessionStore.set(`session:${userId}`, { loggedIn: true });
  }
}

---

Load Balancing

Strategies

Strategy	Description	Use Case
Round Robin	Cycle through servers	Equal capacity servers
Weighted RR	Based on server capacity	Mixed capacity
Least Connections	Route to least busy	Long-lived connections
IP Hash	Same IP → same server	Session stickiness
URL Hash	Same URL → same server	Cache optimization

Health Checks

# Kubernetes-style health checks
livenessProbe:
  httpGet:
    path: /health/live
    port: 8080
  initialDelaySeconds: 3
  periodSeconds: 10

readinessProbe:
  httpGet:
    path: /health/ready
    port: 8080
  initialDelaySeconds: 5
  periodSeconds: 5

// Health check endpoints
app.get('/health/live', (req, res) => {
  // Am I running?
  res.status(200).json({ status: 'alive' });
});

app.get('/health/ready', async (req, res) => {
  // Can I serve traffic?
  const dbOk = await checkDatabase();
  const cacheOk = await checkCache();

  if (dbOk && cacheOk) {
    res.status(200).json({ status: 'ready' });
  } else {
    res.status(503).json({ status: 'not ready', db: dbOk, cache: cacheOk });
  }
});