URL Shortener (like bit.ly)¶

Quick Reference Guide for System Design Interviews

Problem Statement¶

Design a URL shortening service that creates short aliases for long URLs. Users should be able to create short URLs and be redirected to the original URL when accessing the short link.

Requirements¶

Functional Requirements¶

Shorten a long URL to a short URL
Redirect short URL to original URL
Optional: Custom short URLs
Optional: Expiration time
Optional: Analytics (click count, location, referrer)

Non-Functional Requirements¶

Availability: 99.99% (redirects are critical)
Latency: < 100ms for redirect
Scalability: Handle 100M URLs, 10B redirects/month
Durability: URLs should never be lost

Back of Envelope Estimation¶

Capacity Estimation

High-Level Architecture¶

URL Shortener Architecture

Short URL Generation¶

Approach 1: Base62 Encoding of Counter¶

Counter-Based Generation

Approach 2: Hash-Based¶

Hash-Based Generation

Approach 3: Pre-Generated Keys (Recommended)¶

Key Generation Service

Database Schema¶

-- URL Mapping Table
CREATE TABLE url_mappings (
    short_url       VARCHAR(10) PRIMARY KEY,
    long_url        TEXT NOT NULL,
    user_id         UUID,
    created_at      TIMESTAMP NOT NULL,
    expires_at      TIMESTAMP,
    click_count     BIGINT DEFAULT 0,

    INDEX idx_user_id (user_id),
    INDEX idx_created_at (created_at)
);

-- For custom URLs
CREATE TABLE custom_urls (
    custom_alias    VARCHAR(50) PRIMARY KEY,
    short_url       VARCHAR(10) REFERENCES url_mappings(short_url),
    user_id         UUID NOT NULL,
    created_at      TIMESTAMP NOT NULL
);

-- Analytics (separate table or time-series DB)
CREATE TABLE click_events (
    id              UUID PRIMARY KEY,
    short_url       VARCHAR(10),
    timestamp       TIMESTAMP NOT NULL,
    ip_address      INET,
    user_agent      TEXT,
    referrer        TEXT,
    country         VARCHAR(2),

    INDEX idx_short_url_time (short_url, timestamp)
);