System design practice, written by hand

github.com/elithrade/url-shortener

Highlights

• A URL shortener in ASP.NET Core (.NET 10) with Postgres and a Redis cache. I started it to stay sharp at writing code by hand, so I write the core logic myself and use AI as a reviewer and mentor.
• Handles code collisions, rate limiting with a token bucket, link expiry with a background sweep, click counting through a bounded queue with batched writes, and a cache that falls back to Postgres when Redis is down or slow.
• Load‑tested with k6 on one machine: 10,000 redirects a second at a p99 of 1.5 ms with the cache and 5.3 ms without. The tests also showed that an app started cold under full load builds a backlog of requests, so I am now adding a warm‑up step before the app reports ready.
• The in‑memory and Postgres stores pass one shared contract test suite, and most tests were mutation‑checked by breaking the code on purpose. Deployed to Azure Container Apps with Bicep and GitHub Actions. Each bug and experiment becomes a flashcard in LESSONS.md.

Stack