URL Shortener

Introduction

Build a URL shortener like bit.ly — encode long URLs to short codes, redirect on access, track click counts. Practice hashing/base62 encoding, Redis caching, rate limiting and high-read API design.

Informative example

Shorten and redirect:

@PostMapping("/shorten")
public ShortUrlResponse shorten(@Valid @RequestBody ShortenRequest req) {
    String code = encoder.encode(urlRepo.save(req.url()).getId());
    cache.put(code, req.url());  // Redis
    return new ShortUrlResponse("https://short.io/" + code);
}

@GetMapping("/{code}")
public ResponseEntity<Void> redirect(@PathVariable String code) {
    String url = cache.get(code).orElseGet(() -> urlRepo.findByCode(code));
    analytics.recordClick(code);
    return ResponseEntity.status(302).location(URI.create(url)).build();
}

Best practices

• Use base62 (a-zA-Z0-9) for short codes — 6 chars = 56 billion URLs.
• Cache hot URLs in Redis — redirects are read-heavy.
• Add rate limiting (Resilience4j) on the shorten endpoint.

Purpose of this lesson

Master URL Shortener so you can apply it confidently in production Java code, technical interviews, and code reviews.

Step-by-step explanation

1. Understand the core idea behind URL Shortener.
2. Walk through the runnable example and tweak it in the playground.
3. Apply the pattern in a small Spring Boot or CLI exercise of your own.
4. Re-read the common mistakes and interview Q&A to lock the concept in.

Interactive workflow diagram

Debugging tips

• Read the full stack trace — Java's exception messages name the offending class and line.
• Reproduce in the smallest possible main() method before fixing in the real app.
• Use IntelliJ's debugger breakpoints and 'Evaluate Expression' rather than scattering System.out.

Optimization strategies

• Redis cache for hot redirect codes — 99% of traffic is reads.
• Base62 encoding keeps URLs short and URL-safe.

Enterprise example

Teams at Netflix, Uber and Goldman Sachs apply URL Shortener daily — usually wrapped behind Spring Boot services with observability hooks (Micrometer + OpenTelemetry).

Interview questions & answers

Summary

In this lesson you learned URL Shortener — the concept, syntax, a runnable example, and the production pitfalls to avoid. Apply it in the playground before moving on.