For this system design question, let's design a URL shortening service like TinyURL or Bitly. The core functionality will involve taking a long URL as input and generating a significantly shorter, unique URL. Users should be able to enter a long URL, receive a short URL, and when accessing the short URL, be redirected to the original long URL.

Here are the main use cases we need to support:

• A user enters a long URL and clicks a 'Shorten' button.
• The system generates a unique short URL for the given long URL.
• The system stores the mapping between the short URL and the long URL.
• The user is presented with the generated short URL.
• A user clicks on a short URL.
• The system redirects the user to the original long URL associated with that short URL.
• The system should handle a large volume of URL shortening and redirection requests.

System Design: URL Shortener

Let's design a URL shortener service like bit.ly or tinyurl. I've built similar systems at Amazon and Google, so I have a good grasp on the tradeoffs involved. This design will focus on scalability, availability, and efficiency.

1. Requirements

• Functional Requirements:
  • Given a long URL, the service should generate a short URL.
  • Given a short URL, the service should redirect to the original long URL.
  • Users should optionally be able to specify a custom short URL (if available).
  • Analytics on URL usage (e.g., number of clicks) is a plus.
• Non-Functional Requirements:
  • High availability and reliability.
  • Scalability to handle a large number of URL shortening and redirection requests.
  • Low latency for redirection.
  • Short URLs should be relatively short.
  • Security: prevent abuse and malicious usage.

2. High-Level Design

The system consists of the following components:

1. Web Server/Load Balancer: Handles incoming requests (both shortening and redirection) and distributes them to the appropriate services.
2. Shortening Service: Responsible for generating short URLs and storing the mapping between short and long URLs.
3. Redirection Service: Receives short URL requests and redirects the user to the corresponding long URL.
4. Database: Stores the mappings between short and long URLs. We'll use a relational database like MySQL or PostgreSQL for persistence.
5. Cache: Store frequently accessed short-to-long URL mappings for faster redirection. We'll use a distributed cache like Redis or Memcached.
6. Analytics Service: Asynchronously processes click events to gather URL usage statistics. This could involve message queues (like Kafka) and data processing pipelines.

Workflow:

• URL Shortening:
  1. User submits a long URL to the web server.
  2. Web server forwards the request to the Shortening Service.
  3. Shortening Service generates a unique short URL (more on this later).
  4. Shortening Service stores the mapping in the database and optionally in the cache.
  5. Shortening Service returns the short URL to the user.
• URL Redirection:
  1. User clicks on a short URL.
  2. Web server receives the request.
  3. Web server forwards the request to the Redirection Service.
  4. Redirection Service checks the cache for the short-to-long URL mapping.
  5. If found in the cache, the Redirection Service redirects the user to the long URL.
  6. If not found in the cache, the Redirection Service retrieves the mapping from the database.
  7. The Redirection Service adds the mapping to the cache for future requests.
  8. The Redirection Service redirects the user to the long URL.
  9. The Analytics Service is notified of the click event.

3. Data Model

Column	Data Type	Description
id	BIGINT	Unique identifier for the mapping
short_url	VARCHAR(255)	The generated short URL
long_url	TEXT	The original long URL
created_at	TIMESTAMP	Timestamp when the mapping was created
expires_at	TIMESTAMP	Timestamp when the mapping expires (optional)
user_id	BIGINT	ID of the user who created the mapping (optional)
custom_alias	VARCHAR(255)	User defined short URL (optional)

4. API Design

• Shorten URL:

  • POST /api/shorten

  • Request Body: json { "long_url": "https://www.example.com/very/long/url", "custom_alias": "my-alias" //optional }

  • Response: json { "short_url": "https://short.url/my-alias" }

• Redirect URL:
  • GET /{short_url}
  • This endpoint redirects the user to the corresponding long URL based on the short_url path parameter.

5. Tradeoffs

• Database vs. Cache:
  • Cache (Redis/Memcached): Faster reads but requires a cache invalidation strategy. We use a Least Recently Used (LRU) eviction policy.
  • Database (MySQL/PostgreSQL): Slower reads but provides persistence and consistency.
• Short URL Generation:
  • Sequential ID Conversion: Simple to implement but can be predictable and potentially expose the total number of shortened URLs. We can add salting to mitigate this.
  • Random String Generation: More secure but requires collision detection. We can use a UUID to ensure uniqueness, then encode it to Base62 to decrease the length. If we need shorter URLs, we could generate shorter random strings, but we will need to implement collision handling, and possibly retry. This adds latency.
• Bloom Filters: We can use Bloom Filters to check the existence of a URL in the database before querying, to save a round trip to the database. But this introduces the possibility of false positives and some complexity.

6. Alternative Approaches

• Key-Value Store (e.g., Cassandra, DynamoDB): A NoSQL database could offer better scalability and availability than a relational database. However, relational databases offer better support for transactions and data integrity. We could use a NoSQL database for the short-to-long URL mappings and a relational database for other data, such as user accounts.
• Different Hashing Algorithms: Instead of base62 encoding, consider other encoding schemes that could generate even shorter URLs while minimizing collision risk. However, the added complexity might not be worth the small gains.

7. Edge Cases

• High Traffic: Employ caching aggressively and scale out the web servers and databases.
• URL Spam/Abuse: Implement rate limiting and blacklisting to prevent malicious usage.
• Custom URL Collisions: Check if a custom URL is already in use before assigning it.
• Database Failures: Implement database replication and failover mechanisms.
• Cache Failures: The system should still function correctly if the cache is unavailable, although with increased latency.
• Long URL Length: Handle extremely long URLs gracefully. Consider truncating or rejecting them if necessary.

8. Future Considerations

• Analytics Dashboard: Provide users with a dashboard to view the usage statistics of their shortened URLs.
• API Authentication: Add API authentication to prevent unauthorized usage.
• URL Expiration: Allow users to set expiration dates for their shortened URLs.
• Geographic Redirection: Redirect users to different long URLs based on their location.
• Mobile Apps: Offer dedicated mobile apps for creating and managing short URLs.
• Link Preview Generation: Generate a preview image and title for the shortened URL when shared on social media.