Interview practice: try designing this yourself before reading. Go to the exercise ↓

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 TB

Short 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:

System Architecture

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 URL

Caching 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 longer

Key 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

Practice

Try it yourself

Design a URL shortener like bit.ly that turns long URLs into 7-character links and redirects visitors.

Your design should cover

Create a short link for a long URL, optionally with a custom alias and an expiry
Redirect a short link to the original URL with low latency
About 100 million new links a month, with reads far outnumbering writes (around 100:1)
Links must never collide, and redirects should stay available during partial failures
Basic click analytics per link

Your answer

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URL Shortener