🔗 URL Shortener Kotlin API

This is a lightweight, production-ready MVP for a URL shortening service built with Kotlin, Spring Boot, and Redis.

It provides RESTful APIs to:

• Shorten a long URL into a unique, short hash.
• Retrieve the original long URL from the short hash.

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🚀 Tech Stack

Layer	Tech
Language	Kotlin (JVM)
Framework	Spring Boot
Storage	Redis (in-memory DB)
Validation	Jakarta Validation)
Build Tool	Gradle
Logging	SLF4J + LoggerFactory

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📦 Features

✅ Shorten any valid URL using POST /api/shorten
✅ Retrieve full URL using GET /api/{hash}
✅ Base62 encoding for compact URL hash
✅ URL format validation
✅ Redis-backed persistence
✅ Global exception handler for clean API responses
✅ Production-ready structure and logging

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🔧 Getting Started

🔁 Prerequisites

• Java 17+
• Kotlin 1.9+
• Redis server running locally (default: localhost:6379)
• Gradle

▶️ Run the App

./gradlew bootRun

API Endpoints

• Shorten URL
  • Request:

POST /api/shorten
Content-Type: application/json

{
  "url": "https://example.com"
}

• Response:

"https://short.ly/abc123"

• Resolve URL
  • Request:

  GET /api/abc123

• Response:

{
    "url": "https://example.com"
}

If hash not found:

"URL not found"

Base62 Encoding Logic

• Uses Redis' atomic increment (INCR) on a global url_counter.
• Encodes the numeric ID into Base62 using alphanumeric characters.
• Maps encoded key → original URL in Redis.

Production-Readiness & Scalability Discussion

This MVP follows clean architecture principles with a modular controller/service/DTO structure. It uses Spring Boot for scalable, production-ready backend services and Redis for fast and atomic ID mapping.

• Clean, maintainable Kotlin codebase with validation and logging.
• Stateless API supports horizontal scaling.
• Redis + Base62 ID generation ensures globally unique and compact identifiers.
• Validation with @Pattern and @NotBlank ensures input safety.
• Environment configuration can be externalized using application.properties/profiles, making it suitable for multi-environment deployment (dev/stage/prod).
• Logging using SLF4J adds observability.
• Global exception handler improves reliability and client error handling.
• Potential Bottlenecks & Solutions: Redis is a single point of failure in MVP; in production, use Redis Cluster or fallback caching layer.