First design walkthrough (URL shortener) in HLD - Architecture Diagram

System Overview - First design walkthrough (URL shortener)

This system creates short, easy-to-share links that redirect to long URLs. It must handle many users, provide fast redirects, and ensure short links are unique and reliable.

Architecture Diagram

User
  |
  v
Load Balancer
  |
  v
API Gateway
  |
  v
URL Shortener Service
  |           |
  v           v
Cache       Database

Components

User

client

Sends requests to shorten URLs or access short links

Load Balancer

load_balancer

Distributes incoming user requests evenly to API Gateway instances

API Gateway

api_gateway

Handles routing, authentication, and request validation

URL Shortener Service

service

Generates short URLs, stores mappings, and handles redirects

Cache

cache

Stores frequently accessed short URL mappings for fast redirects

Database

database

Stores persistent mapping of short URLs to original long URLs

Request Flow - 11 Hops

User → Load Balancer

Load Balancer → API Gateway

API Gateway → URL Shortener Service

URL Shortener Service → Cache

Cache → URL Shortener Service

URL Shortener Service → Database

Database → URL Shortener Service

URL Shortener Service → Cache

URL Shortener Service → API Gateway

API Gateway → Load Balancer

Load Balancer → User

Failure Scenario

Component Fails:Database

Impact:New short URLs cannot be created and cache misses cannot be resolved, but cached redirects still work

Mitigation:Use database replication and failover to maintain availability; rely on cache for read requests during failure

Architecture Quiz - 3 Questions

Which component ensures user requests are distributed evenly to prevent overload?

A Load Balancer

B Cache

C API Gateway

D Database

Design Principle

This design uses caching to speed up frequent redirects and a load balancer to handle many users. The database stores permanent mappings, while the API Gateway manages requests. This separation improves scalability and reliability.

Practice

1. What is the primary purpose of a URL shortener system?

easy

A. To generate random passwords for users

B. To increase the length of URLs for security

C. To store user passwords securely

D. To convert long URLs into short, easy-to-share links

Solution

1. Step 1: Understand the function of a URL shortener

   A URL shortener takes a long URL and creates a shorter alias that redirects to the original URL.

2. Step 2: Identify the correct purpose among options

   Only To convert long URLs into short, easy-to-share links describes this function correctly; others describe unrelated tasks.

3. Final Answer:

   To convert long URLs into short, easy-to-share links -> Option D

4. Quick Check:

   URL shortener purpose = convert long URLs to short [OK]

Hint: Focus on what URL shorteners do: shorten URLs [OK]

Common Mistakes:

• Confusing URL shortening with security features
• Thinking it stores passwords
• Assuming it generates random data unrelated to URLs

2. Which component is essential for generating unique short codes in a URL shortener?

easy

A. Cache server

B. Load balancer

C. Short code generator

D. User authentication module

Solution

1. Step 1: Identify the role of each component

   Short code generator creates unique short strings; load balancer distributes traffic; cache stores data temporarily; authentication manages users.

2. Step 2: Match component to short code creation

   Only the short code generator is responsible for creating unique short URLs.

3. Final Answer:

   Short code generator -> Option C

4. Quick Check:

   Unique code creation = short code generator [OK]

Hint: Short codes come from the short code generator [OK]

Common Mistakes:

• Confusing load balancer with code generation
• Thinking cache generates codes
• Assuming authentication creates short URLs

3. Given a URL shortener system, what happens when a user clicks a short URL http://short.ly/abc123?

medium

A. The system looks up abc123 in storage and redirects to the original long URL

B. The system generates a new short code for abc123

C. The system deletes the short URL from the database

D. The system asks the user to enter the long URL manually

Solution

1. Step 1: Understand the redirection process

   When a short URL is accessed, the system finds the original URL mapped to the short code and redirects the user.

2. Step 2: Evaluate each option

   The system looks up abc123 in storage and redirects to the original long URL correctly describes this lookup and redirect. Others describe incorrect or unrelated actions.

3. Final Answer:

   The system looks up abc123 in storage and redirects to the original long URL -> Option A

4. Quick Check:

   Click short URL = lookup and redirect [OK]

Hint: Clicking short URL triggers lookup and redirect [OK]

Common Mistakes:

• Thinking system creates new codes on click
• Assuming deletion happens on access
• Believing user must input original URL manually

4. In a URL shortener design, which issue arises if the short code generator does not check for duplicates?

medium

A. The system runs out of storage space quickly

B. Multiple long URLs map to the same short code causing incorrect redirects

C. Users cannot create new accounts

D. The system slows down due to too many cache hits

Solution

1. Step 1: Understand duplicate short code impact

   If duplicates exist, different long URLs share the same short code, causing wrong redirection.

2. Step 2: Analyze options for relevance

   Multiple long URLs map to the same short code causing incorrect redirects correctly identifies this problem. Others describe unrelated issues.

3. Final Answer:

   Multiple long URLs map to the same short code causing incorrect redirects -> Option B

4. Quick Check:

   Duplicate codes cause wrong redirects [OK]

Hint: Duplicates cause wrong URL redirects [OK]

Common Mistakes:

• Confusing storage issues with duplicate codes
• Thinking duplicates affect user accounts
• Assuming cache hits cause this problem

5. To scale a URL shortener for millions of users, which design choice improves performance and availability?

hard

A. Use distributed storage with caching and load balancing

B. Store all data on a single server to simplify access

C. Generate short codes sequentially without randomness

D. Disable caching to ensure fresh data every time

Solution

1. Step 1: Identify scalability needs

   Millions of users require fast access, high availability, and fault tolerance.

2. Step 2: Evaluate design choices

   Distributed storage spreads data load; caching speeds up lookups; load balancing distributes traffic. This combination supports scale.

3. Step 3: Reject poor choices

   Single server limits scale; sequential codes risk collisions; disabling cache slows response.

4. Final Answer:

   Use distributed storage with caching and load balancing -> Option A

5. Quick Check:

   Scaling needs distributed storage + cache + load balancer [OK]

Hint: Scale with distributed storage, caching, and load balancing [OK]

Common Mistakes:

• Choosing single server for large scale
• Ignoring caching benefits
• Using sequential codes risking collisions