Masst DocsURL Shortener
System design for a URL shortening service like bit.ly or TinyURL.
Problem Statement
Design a URL shortening service like bit.ly that:
- Shortens long URLs to short aliases
- Redirects short URLs to original URLs
- Handles high read traffic
- Provides analytics (optional)
Requirements
Functional
- Create short URL from long URL
- Redirect short URL to original
- Custom aliases (optional)
- Link expiration (optional)
Non-Functional
- High availability
- Low latency redirects
- Scalable to billions of URLs
Capacity Estimation
Assumptions:
- 100M URLs created per month
- 100:1 read/write ratio
- 10 billion redirects per month
- URL stored for 5 years
Write QPS:
100M / (30 × 24 × 3600) ≈ 40 URLs/second
Read QPS:
40 × 100 = 4,000 redirects/second
Peak: ~10,000 redirects/second
Storage (5 years):
100M × 12 × 5 = 6 billion URLs
6B × 500 bytes = 3 TBShort URL Generation
Approach 1: Hash + Truncate
MD5(long_url) = "d41d8cd98f00b204..."
Short URL = first 7 chars = "d41d8cd"Problem: Collisions possible
Approach 2: Base62 Encoding
Use counter or random ID, encode in base62:
Characters: [a-z, A-Z, 0-9] = 62 chars
7 characters = 62^7 = 3.5 trillion combinations
ID: 123456789
Base62: "8M0kX"Approach 3: Pre-generated Keys
Generate keys offline, store in key database:
┌───────────────┐ ┌───────────────┐
│ Key Generator │────►│ Key Database │
│ (offline) │ │ (unused keys)│
└───────────────┘ └───────────────┘
│
▼
┌───────────────┐
│ App Server │
│ (takes a key) │
└───────────────┘System Architecture
┌─────────────┐
│ CDN │
└──────┬──────┘
│
┌──────▼──────┐
│ Load │
│ Balancer │
└──────┬──────┘
│
┌─────────────────┼─────────────────┐
│ │ │
┌──────▼──────┐ ┌──────▼──────┐ ┌──────▼──────┐
│ App Server │ │ App Server │ │ App Server │
└──────┬──────┘ └──────┬──────┘ └──────┬──────┘
│ │ │
└─────────────────┼─────────────────┘
│
┌──────▼──────┐
│ Cache │
│ (Redis) │
└──────┬──────┘
│
┌──────▼──────┐
│ Database │
│ (sharded) │
└─────────────┘Database Schema
CREATE TABLE urls (
id BIGINT PRIMARY KEY,
short_code VARCHAR(7) UNIQUE,
original_url TEXT,
user_id BIGINT,
created_at TIMESTAMP,
expires_at TIMESTAMP,
click_count BIGINT DEFAULT 0
);
-- Index for lookups
CREATE INDEX idx_short_code ON urls(short_code);API Design
Create Short URL
POST /api/shorten
{
"url": "https://example.com/very/long/url",
"custom_alias": "mylink", // optional
"expires_at": "2025-12-31" // optional
}
Response:
{
"short_url": "https://short.ly/abc123",
"original_url": "https://example.com/very/long/url"
}Redirect
GET /abc123
Response: 301 Redirect to original URLCaching Strategy
Read path:
1. Check cache (Redis)
2. If miss, query database
3. Store in cache
4. Return and redirect
Cache settings:
- TTL: 24 hours (or based on popularity)
- Hot URLs: Keep in cache longerKey Design Decisions
| Decision | Choice | Reason |
|---|---|---|
| URL length | 7 characters | 3.5T combinations |
| Encoding | Base62 | URL-safe characters |
| Database | NoSQL (DynamoDB) | Key-value access pattern |
| Cache | Redis | Fast lookups |
| Redirect code | 301 vs 302 | 301 for SEO, 302 for analytics |
Interview Tips
- Start with requirements and estimations
- Discuss key generation approaches
- Explain caching for read-heavy workload
- Mention 301 vs 302 redirect implications
- Discuss sharding strategy for database
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