System requirements

Functional:

List functional requirements for the system (Ask the chat bot for hints if stuck.)...

URL Shortening: Users can input a long URL and receive a shortened version.

Redirection: When someone accesses the shortened URL, they are redirected to the original long URL.

Custom Aliases: Users can create custom short links instead of random strings.

Analytics: Track the number of clicks, geographic data, and referral sources for each short URL.

User Accounts (Optional): Allow users to manage their shortened URLs.

Expiration Dates (Optional): Set expiration dates for short URLs after which they become inactive.

Non-Functional:

List non-functional requirements for the system...

• Scalability: Handle a growing number of URL shortenings and redirections.
• Performance: Ensure quick response times for creating and accessing short URLs.
• Reliability: Maintain high availability so that short URLs are always accessible.
• Security: Prevent misuse, such as phishing or spam through short URLs.
• Usability: Simple and intuitive interface for users to create and manage short URLs.

Capacity estimation

Estimate the scale of the system you are going to design...

Assumptions:

• Users: 1 million active users.
• Short URLs per User: On average, 10 short URLs created per user per month.
• Total Short URLs: 10 million short URLs per month.
• Redirections: Each short URL is accessed 100 times on average per month.

Calculations:

• Total Short URLs Stored: Assuming the service runs for a year, 10 million × 12 = 120 million short URLs.
• Database Size:
  • Each URL entry might take around 200 bytes (including original URL, short code, user ID, timestamps).
  • Total storage: 120 million × 200 bytes ≈ 24 GB.
• Bandwidth Considerations:
  • Redirections: 120 million short URLs × 100 accesses = 12 billion redirections per year.
  • If each redirection involves transferring 1 KB of data (HTTP request/response), total bandwidth = 12 billion KB ≈ 12 TB per year.

API design

Define what APIs are expected from the system...

POST /shorten: Create a new short URL.

GET /{short_code}: Redirect to the original URL.

GET /analytics/{short_code}: Retrieve analytics for a short URL.

POST /custom: Create a short URL with a custom alias.

DELETE /{short_code}: Delete a short URL (for user accounts).

Request/Response Payloads:

Example: POST /shorten

Request:

Response:

Database design

Defining the system data model early on will clarify how data will flow among different components of the system. Also you could draw an ER diagram using the diagramming tool to enhance your design...

Database Data Models:

URLs Table:

Users Table (Optional):

High-level design

You should identify enough components that are needed to solve the actual problem from end to end. Also remember to draw a block diagram using the diagramming tool to augment your design. If you are unfamiliar with the tool, you can simply describe your design to the chat bot and ask it to generate a starter diagram for you to modify...

Components Needed:

• API Server: Handles incoming requests for creating and accessing short URLs.
• Database: Stores mappings between short codes and original URLs.
• Cache: Speeds up frequent redirections using a caching layer like Redis.
• Load Balancer: Distributes incoming traffic across multiple servers to ensure reliability.
• Unique ID Generator: Generates unique short codes, possibly using algorithms or services like UUID.
• Analytics Service: Tracks and stores click data for each short URL.
• CDN (Content Delivery Network): Optional, for faster delivery of static content if needed.
• Monitoring & Logging Tools: To track system health and debug issues.

Request flows

Explain how the request flows from end to end in your high level design. Also you could draw a sequence diagram using the diagramming tool to enhance your explanation...

End-to-End Request Flow:

To understand how requests flow through the URL Shortening Service, we'll explore two main scenarios:

1. Creating a Short URL
2. Redirecting to the Original URL

1.1. Creating a Short URL

Step-by-Step Flow:

1. User Input:
   • Client Side: The user accesses the service via a web or mobile app, enters a long URL into the input field, and clicks the "Shorten" button.
2. API Request:
   • Client to Server: A POST request is sent to the POST /shorten endpoint with the long URL and any optional parameters like a custom alias or expiration date.
3. Short Code Generation:
   • API Server: Receives the request and invokes the Short Code Generator to create a unique short code.
4. Database Storage:
   • API Server to Database: Stores the mapping between the short code and the original URL in the NoSQL Database.
5. Response to User:
   • Server to Client: Returns the shortened URL to the user, which they can copy and share.

1.2. Redirecting to the Original URL

Step-by-Step Flow:

1. User Clicks Short URL:
   • Client Side: Someone clicks on the short URL (e.g., https://short.ly/abc123).
2. API Request:
   • Client to Server: A GET request is sent to the GET /{short_code} endpoint.
3. Lookup in Cache:
   • API Server: Checks if the short code exists in the Cache (Redis). If found, retrieves the original URL quickly.
4. Database Lookup (if cache miss):
   • Cache Miss: If the short code isn't in the cache, the server queries the NoSQL Database for the original URL.
5. Update Cache:
   • API Server to Cache: After retrieving from the database, the original URL is added to the cache for future requests.
6. Redirect User:
   • Server to Client: The server responds with a 301/302 redirect to the original URL, sending the user to the desired destination.
7. Analytics Logging:
   • API Server to Message Queue: Logs the click event by sending it to the message queue for the Analytics Worker to process.

Detailed component design

Dig deeper into 2-3 components and explain in detail how they work. For example, how well does each component scale? Any relevant algorithm or data structure you like to use for a component? Also you could draw a diagram using the diagramming tool to enhance your design...

2.1. Short Code Generator

Function: Generates unique short codes for the original URLs.

Design:

• Algorithm:
  • Base62 Encoding: Uses a combination of uppercase letters, lowercase letters, and digits to create compact representations.
  • Incremental IDs: Assigns a unique incremental ID to each new URL, which is then converted to a base62 string.
• Collision Avoidance:
  • Ensures each generated short code is unique by checking against existing entries.
  • In case of a collision, regenerates the code or appends additional characters.

Scaling:

• Horizontal Scaling: Distributes the ID generation across multiple servers using techniques like sharding or pre-generating ID ranges.
• Data Structure: Utilizes a simple counter or unique identifier (e.g., auto-incrementing in the database) encapsulated within a service to maintain consistency.

Trade offs/Tech choices

Explain any trade offs you have made and why you made certain tech choices...

NoSQL vs. SQL:

• Pros of NoSQL: Highly scalable, handles large volumes of read/write operations efficiently, flexible schema.
• Cons of NoSQL: May lack the transactional guarantees of SQL databases.
• Decision: Chose NoSQL for scalability and performance, which are critical for handling high traffic in a URL shortening service.

Using a Cache:

• Pros: Speeds up redirection by reducing database lookups, handles high read traffic effectively.
• Cons: Potential for cache misses, requires cache invalidation strategies.
• Decision: Implemented a caching layer to enhance performance and handle high traffic efficiently.

Asynchronous Processing with Queues:

• Pros: Improves system responsiveness, manages background tasks without blocking main operations.
• Cons: Adds complexity to the system architecture, requires monitoring of queues and workers.
• Decision: Used message queues for analytics and notifications to keep the main services fast and responsive.

Short Code Generation:

• Pros: Simple algorithms (e.g., base62 encoding) are easy to implement and fast.
• Cons: Potential for collisions as the number of URLs grows, requires mechanisms to handle duplicates.
• Decision: Implemented a reliable short code generation strategy, possibly incorporating randomization and collision checks to ensure uniqueness.

Failure scenarios/bottlenecks

Try to discuss as many failure scenarios/bottlenecks as possible.

3.1. Short Code Collision

• Scenario: Two different long URLs are assigned the same short code, leading to incorrect redirections.
• Impact: Users may be redirected to the wrong URL, causing confusion and loss of trust.
• Mitigation:
  • Unique Generation: Implement a robust short code generation algorithm that minimizes collision chances, like using incremental IDs with base62 encoding.
  • Collision Checks: Ensure the system checks for existing short codes before assigning, regenerating if necessary.
  • Monitoring: Continuously monitor collision rates and adjust algorithms as needed.

3.2. Cache Failures

• Scenario: The caching layer (Redis) becomes unavailable or experiences downtime.
• Impact: Increased load on the database due to cache misses, leading to higher latency and potential performance degradation.
• Mitigation:
  • Redundancy: Implement Redis clustering with failover mechanisms to maintain cache availability.
  • Graceful Fallback: Ensure the API server can function correctly by falling back to database lookups if the cache is down, though with higher latency.
  • Monitoring: Continuously monitor cache health and performance, triggering alerts on failures.

3.3. Database Scalability Issues

• Scenario: The NoSQL database becomes a bottleneck due to high read/write traffic.
• Impact: Slower response times for creating and redirecting URLs, potentially causing service delays or downtime.
• Mitigation:
  • Horizontal Scaling: Distribute the database across multiple nodes to handle increased load.
  • Sharding: Partition the database to manage large datasets efficiently.
  • Read Replicas: Use read replicas to distribute read traffic and reduce load on the primary database.
  • Optimized Queries: Ensure that database queries are optimized for performance.

Future improvements

What are some future improvements you would make? How would you mitigate the failure scenario(s) you described above?

4.1. Enhanced Security Features

• Phishing Detection:
  • How: Implement mechanisms to detect and prevent malicious URLs from being shortened using machine learning algorithms to analyze URL content and flag suspicious patterns.
• Password-Protected Short URLs:
  • How: Allow users to protect their short URLs with passwords, adding an authentication layer before accessing the original URL.
• HTTPS Support:
  • How: Ensure all short URLs use HTTPS to secure redirection, protecting data integrity and user privacy.