Requirements

Functional Requirements:

• Shorten: Convert a long URL into a unique, short alias (e.g., bit.ly/3abc123).
• Redirect: Clicking the short link redirects the user to the original long URL.
• Custom Aliases (Optional): Users can choose their own short string (e.g., bit.ly/my-promo).
• Expiration: Links can have a set lifespan before they stop working.

Non-Functional Requirements:

• High Availability: The service must never go down; if redirection fails, the link is broken.
• Low Latency: Redirection should happen in milliseconds (<100ms).
• Read-Heavy: Expect 100x more people to click links than to create them.

API Design

Create Short Link:

POST /api/v1/shorten

Request: { "long_url": "https://example.com", "expire_at": "optional_timestamp" }

Return: { "short_url": "https://short.ly" }.

Redirection:

GET /{short_code}

Action: The server looks up the code and returns an HTTP 301 (Permanent Redirect).

High-Level Design

Load Balancer: Distributes traffic to multiple web servers to prevent any single server from being overwhelmed.

Web/Application Servers: Handle the logic for generating short codes and managing redirects.

Database & Cache:

Database (NoSQL/SQL): Stores the mapping (Short Code → Long URL). NoSQL like Cassandra or

DynamoDB is often preferred for its massive scalability.

Cache (Redis): Stores the most popular links in memory. Since 20% of links usually get 80% of the traffic, caching these "hot" links makes redirection nearly instant

Detailed Component Design

Shortening Algorithm (Base62 Encoding):

To get a 7-character code, we use characters [0-9, a-z, A-Z] (62 total).

provides ~3.5 trillion unique combinations, which is plenty for years of use.

Unique ID Generation:

Instead of hashing (which can cause collisions), the system uses a Distributed ID Generator (like Twitter Snowflake) to give every new link a unique number.

Example: ID 200921 converts to Base62 code zn9edcu.

Database Schema:

short_id (Primary Key, String)

original_url (String)

created_at (Timestamp)

expiration_at (Timestamp)