system-design

name	system-design
description	Scalability, availability, and distributed systems design
domain	software-design
version	1.0.0
tags	["scalability","availability","distributed","caching","load-balancing","database"]
triggers	{"keywords":{"primary":["system design","scalability","distributed","architecture","high availability"],"secondary":["load balancer","caching","sharding","replication","cap theorem","microservices"]},"context_boost":["interview","scale","performance","infrastructure"],"context_penalty":["frontend","ui","mobile"],"priority":"high"}

System Design

Overview

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

---

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 });
  }
});

---

Caching Strategies

Cache Patterns

Cache-Aside (Lazy Loading):
┌────────┐    miss     ┌────────┐
│  App   │ ──────────→ │  Cache │
│        │ ←────────── │        │
└────────┘    null     └────────┘
    │
    │  read
    ↓
┌────────┐
│   DB   │  ──── write ──→ Cache
└────────┘

Write-Through:
App → Cache → DB (synchronous)

Write-Behind (Write-Back):
App → Cache → (async) → DB

// Cache-aside implementation
class CachedUserService {
  constructor(
    private cache: Redis,
    private db: Database
  ) {}

  async getUser(id: string): Promise<User> {
    // Try cache first
    const cached = await this.cache.get(`user:${id}`);
    if (cached) return JSON.parse(cached);

    // Cache miss - read from DB
    const user = await this.db.users.findById(id);
    if (user) {
      // Store in cache with TTL
      await this.cache.set(`user:${id}`, JSON.stringify(user), 'EX', 3600);
    }
    return user;
  }

  async updateUser(id: , : <>): <> {
     user =  ...(id, data);
    
     ..();
     user;
  }
}

Cache Invalidation

Strategy	Description	Complexity
TTL	Expire after time	Simple
Event-based	Invalidate on write	Medium
Version-based	Key includes version	Medium
Tag-based	Group related keys	Complex

---

Database Scaling

Read Replicas

                    ┌─────────────────┐
     Writes ──────→ │   Primary DB    │
                    └────────┬────────┘
                             │ replication
           ┌─────────────────┼─────────────────┐
           ↓                 ↓                 ↓
    ┌──────────┐      ┌──────────┐      ┌──────────┐
    │ Replica 1│      │ Replica 2│      │ Replica 3│
    └──────────┘      └──────────┘      └──────────┘
           ↑                 ↑                 ↑
           └─────── Reads ───┴────────────────┘

Sharding (Partitioning)

// Hash-based sharding
function getShard(userId: string, numShards: number): number {
  const hash = crypto.createHash('md5').update(userId).digest('hex');
  return parseInt(hash.slice(0, 8), 16) % numShards;
}

// Range-based sharding
function getShardByDate(date: Date): string {
  const year = date.getFullYear();
  const month = date.getMonth() + 1;
  return `orders_${year}_${month.toString().padStart(2, '0')}`;
}

// Consistent hashing for dynamic shards
class ConsistentHash {
  private ring: Map<number, string> = new Map();

  addNode(node: string) {
    for ( i = ; i < ; i++) { 
       hash = .();
      ..(hash, node);
    }
  }

  (: ):  {
     hash = .(key);
    
     ( [nodeHash, node]  [.....()].()) {
       (nodeHash >= hash)  node;
    }
     ..().().;
  }
}

---

Message Queues

Patterns

Point-to-Point:
Producer → Queue → Consumer

Pub/Sub:
              ┌─→ Subscriber 1
Publisher → Topic ─→ Subscriber 2
              └─→ Subscriber 3

Work Queue:
              ┌─→ Worker 1
Producer → Queue ─→ Worker 2  (competing consumers)
              └─→ Worker 3

Delivery Guarantees

Guarantee	Description	Implementation
At-most-once	May lose messages	Fire and forget
At-least-once	May duplicate	Ack after process
Exactly-once	No loss, no dupe	Idempotency + dedup

// Idempotent processing
async function processOrder(event: OrderEvent) {
  // Check if already processed
  const processed = await redis.get(`processed:${event.id}`);
  if (processed) {
    console.log(`Already processed ${event.id}`);
    return;
  }

  // Process the order
  await db.orders.create(event.order);

  // Mark as processed (with TTL for cleanup)
  await redis.set(`processed:${event.id}`, '1', 'EX', 86400 * 7);
}

---

High Availability

Redundancy Patterns

Active-Active:
┌────────┐     ┌────────┐
│Server A│ ←─→ │Server B│  Both handle traffic
└────────┘     └────────┘

Active-Passive:
┌────────┐     ┌────────┐
│ Active │ ──→ │Standby │  Failover on failure
└────────┘     └────────┘

Circuit Breaker

class CircuitBreaker {
  private failures = 0;
  private lastFailure: Date | null = null;
  private state: 'closed' | 'open' | 'half-open' = 'closed';

  constructor(
    private threshold: number = 5,
    private timeout: number = 30000
  ) {}

  async execute<T>(fn: () => Promise<T>): Promise<T> {
    if (this.state === 'open') {
      if (Date.now() - this.lastFailure!.getTime() > this.timeout) {
        this.state = 'half-open';
      } else {
        throw new Error('Circuit is open');
      }
    }

    try {
      const result = await ();
      .();
       result;
    }  (error) {
      .();
       error;
    }
  }

   () {
    . = ;
    . = ;
  }

   () {
    .++;
    . =  ();
     (. >= .) {
      . = ;
    }
  }
}

---

CAP Theorem

         Consistency
            /\
           /  \
          /    \
         /      \
        /   CA   \
       /──────────\
      /            \
     / CP        AP \
    /________________\
Partition          Availability
Tolerance

CA: Single node (RDBMS)
CP: MongoDB, HBase (may reject writes during partition)
AP: Cassandra, DynamoDB (eventual consistency)

Consistency Models

Model	Description	Example
Strong	Read sees latest write	RDBMS
Eventual	Eventually consistent	DNS, Cassandra
Causal	Respects causality	Chat apps
Read-your-writes	See your own writes	Social feeds

---

Rate Limiting

Algorithms

// Token Bucket
class TokenBucket {
  private tokens: number;
  private lastRefill: number;

  constructor(
    private capacity: number,
    private refillRate: number // tokens per second
  ) {
    this.tokens = capacity;
    this.lastRefill = Date.now();
  }

  consume(tokens: number = 1): boolean {
    this.refill();
    if (this.tokens >= tokens) {
      this.tokens -= tokens;
      return true;
    }
    return false;
  }

  private refill() {
    const now = Date.now();
    const elapsed = (now - this.lastRefill) / 1000;
    this.tokens = .(
      .,
      . + elapsed * .
    );
    . = now;
  }
}


  {
  () {}

   (: ): <> {
     now = .();
     windowStart = now - . * ;

     pipe = ..();
    pipe.(key, , windowStart);
    pipe.(key, now, );
    pipe.(key);
    pipe.(key, .);

     results =  pipe.();
     count = results[][]  ;

     count <= .;
  }
}

---

Common System Designs

System	Key Components
URL Shortener	Hash function, Redis cache, DB
Twitter Feed	Fan-out, Redis timeline, Kafka
Chat App	WebSocket, Presence, Message queue
E-commerce	Cart service, Inventory, Payment
Video Streaming	CDN, Chunking, Adaptive bitrate

---

Related Skills

• [[architecture-patterns]] - Microservices, event-driven
• [[database]] - Database optimization
• [[caching-implementation]] - Cache strategies
• [[reliability-engineering]] - SRE practices