Design a URL Shortener System Interview Question

Designing a URL shortener (e.g. TinyURL or Bitly) is a common system design interview question.

It looks simple but touches on core topics like databases, scaling, and caching.

Interviewers expect you to clarify requirements, outline a clear architecture, and discuss trade-offs. This guide provides a step-by-step approach to design a URL shortener for beginners and intermediate candidates.

Step 1: Understanding Requirements

First, define what the system should do and the constraints it must meet:

Functional Requirements

Shorten URL: Convert a long URL into a shorter, unique URL.
Redirect: Given a short URL, redirect the user to the original long URL.
Custom alias (optional): Allow users to choose a custom short link (e.g. domain/alias).
Link expiration (optional): Option to set an expiration time after which the short URL is invalid.
Analytics (optional): Track click counts or other usage stats for short URLs.

Non-Functional Requirements

Scalability: Handle a large number of URLs and redirect requests.
High Availability: The service should be reliable (e.g. ~99.9% uptime).
Low Latency: Redirects should be fast (minimal delay).
Durability: Short links remain valid for years unless explicitly expired.
Security: Prevent abuse (spam, malicious URLs) and protect user data.

These requirements will guide our design decisions.

Learn more about functional vs non-functional requirements.

Step 2: High-Level System Design

At a high level, the URL shortener performs two operations: create a short URL and redirect to the long URL.

Key components:

Application Servers: Handle HTTP requests to shorten URLs and to redirect users. They contain the logic for generating short codes and looking up original URLs.
Database: Stores mappings from short codes to original URLs (plus metadata like expiration times).
Key Generation Service (KGS): A component that generates unique short codes so no two requests get the same code.
Cache: In-memory cache (e.g. Redis) to store frequently accessed mappings and reduce database load.
Load Balancer: Distributes incoming requests across multiple servers for scalability.

Learn more about high-level sytem design.

Step 3: Database Design

Database choice: A NoSQL key-value store is ideal (short code as the key, long URL as the value) for fast lookups and easy horizontal scaling. For example, a distributed store like Cassandra or DynamoDB can efficiently handle billions of mappings. (A SQL database can work initially, but might require sharding and tuning at large scale.)

Schema: We need to store each short code and its associated long URL, plus any metadata. For example:

Field	Description
ShortCode	The short URL identifier (key)
OriginalURL	The original long URL
ExpiresAt	Expiration time (optional)

This simple schema (a single mapping table) allows constant-time lookups by short code. Each entry is small, so even millions of records are manageable.

If the data grows very large, we can partition (shard) the database by key or use a distributed database that handles sharding automatically.

Step 4: URL Shortening Techniques

We need a method to generate unique short codes for new URLs:

Sequential IDs + Base62: Assign an incrementing ID to each new URL and encode it in Base62 (using characters 0–9, A–Z, a–z). This produces short, unique codes. In a distributed system, we might use a central service or allocate ID blocks to different servers to avoid conflicts.
Distributed Generation: Instead of one global counter, generate codes in a decentralized way. For example, create a random 6–8 character string for each URL (checking the database to avoid collisions), or derive the code by hashing the original URL. These approaches eliminate a single bottleneck, but we must handle the rare case of collisions (e.g. two URLs generating the same code).

Each approach has pros and cons, but all provide a huge address space for short URLs. Many large systems use a dedicated Key Generation Service (KGS) to manage code creation at scale.

Step 5: Scalability and Performance

Design the service to scale and respond quickly:

Horizontal Scaling: Run multiple application servers and use a load balancer to distribute requests. No single server becomes a bottleneck, and we can handle more traffic by adding servers.
Caching: Cache popular short→long URL mappings in memory. Since reads (redirects) far outnumber writes (new URLs), caching hot entries (e.g. with Redis) means most redirects can be served from memory, greatly reducing latency and database load.
Database Sharding: Split the URL mappings across multiple database nodes if needed. For example, partition by short code range or hash so each shard handles a subset of the data. This spreads the load and allows the system to scale as data grows.
Geo-Distribution: Deploy servers in multiple regions so users connect to the nearest data center. This lowers latency for global users and also provides redundancy (if one region is down, another can serve traffic).

Step 6: Reliability and Fault Tolerance

Ensure the service remains available despite failures:

Data Replication & Backups: Keep multiple copies of the data (e.g. replicate each entry to several database nodes across different servers/regions) and perform regular backups. If one storage node fails or data is corrupted, a replica or backup can be used to recover.
Redundant Systems: Run multiple instances of each component. For example, have several app servers behind the load balancer and set up the database in a clustered (primary-replica) configuration with failover. This way the system has no single point of failure.

Learn more about fault tolerance.

Step 7: Security and Abuse Prevention

Protect the service from misuse and attacks:

Rate Limiting: Limit how many new short URLs a user/IP can create per minute to prevent spam or abuse. Learn more about rate limiting.
Validation & Secure Links: Validate all submitted URLs (and reject known malicious links). Serve redirects only over HTTPS and use non-sequential, random codes so they cannot be guessed or crawled by attackers.

Step 8: Real-World Considerations & Best Practices

Additional features & UX: You may offer optional link expiration and custom aliases. Also, use a very short domain and show a user-friendly error page if a short URL is invalid or expired, to ensure a good user experience.

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Final Thoughts

When designing a URL shortener, remember these key points:

Clarify Requirements: Know what features are needed (custom aliases, stats, expiration) and the expected scale.
Scalability & Reliability: Build a simple core mapping system, then plan for growth by scaling out (more servers, sharded databases) and adding redundancy (replicated data, multiple instances for failover).
Security Measures: Implement rate limiting, validate inputs, and use HTTPS to guard against abuse and protect users.