Requirements

Functional Requirements:

1. Create short URL - given a long URL, generate a unique short alias
2. Redirect - given a short URL, return the original long URL and redirect the user
3. Custom alias - allow users to pick a readable alias
4. Expiration - support optional TTL so links expire
5. Delete/update - let the owner remove or repoint a short link
6. Analytics - track click counts per short URL

Non-Functional Requirements:

1. High availability - the redirect must never fail even if a server dies
2. Low redirect latency - sub-100ms; the write path can be slower, the read path must be fast
3. Horizontal scalability - handle millions of reads/day by adding more servers

Capacity Estimation

• 100M new URLs/day (writes)
• Read:write ratio = 100:1 (typical for URL shorteners)
• Long URL ≈ 500 bytes avg; each DB record ≈ 1KB with metadata

Traffic / QPS

• Writes: 100M / 86400 = 1158 w/s
• Reads: 115741 r/s
• Peak: 348K r/s

Storage

• 100M * 1KB = 100 GB/day
• 5-year retention: 100 GB * 365 * 5 = 180TB

Cache

• Cache hot URLS: 20M * 500B = 10GB

API Design

POST /api/urls

Request:

{

"longUrl" : "string (required)",

"customAlias" : "string (optional)",

"expiresAt" : "datetime (optional)"

}

Response: 201 -> {"shortUrl"}

GET /api/urls/{shortURL}

Response: 302 -> {LongUrl}

DELETE /api/urls/{shortURL}

GET /api/urls/{shortUlr}/stats

PUT /api/urls/

High-Level Design

This architecture consists of the following components:

1. The system client itself - the primary user who wants to get a short link to their long URL

2. CDN - to speed up operation

3. Load Balancer - to protect the system from overload and ensure continuous server availability

4. API Gateway - a single entry point, added for routing and system security

5. Rate Limiter - a service that checks whether the short link has expired

6. Shortener Service - a service for creating, editing, and deleting short URLs

7. Redirect Service - a service responsible for redirecting a short URL to the original long URL

8. Shortener Service and Redirect Service - both use Cache to quickly find URLs; if they are not in the cache, they attempt to work with them through the Database

9. ID Generation Service - is used to generate a hash of identifiers for working with short URLs.

10. Analytics Service - an analytics service that tracks clicks on short URLs. The Shortener Service and Redirect Service send data to it

11. Database — the source of truth for all short_code → long_url mappings; absorbs all writes; the cache sits in front of it to serve the read-heavy traffic

Database Design

shortUrl:

• id
• created_at
• user_id
• original_url
• short_url

Detailed Component Design

Deep dive into 2-3 key components. Explain how they work, how they scale, discuss tradeoffs, capacity, and any relevant algorithms or data structures.