Litmus
System design practice that judges your decisions, not your diagram. Each problem gives you real requirements and an empty canvas. Every box you add gets a verdict: right-sized, overkill, or about to break, with the reason why.
Pass, and a Deep Dive raises the bar, just like an interviewer would: more traffic, users worldwide, a database node that dies. Your design has to keep up. Stuck? Get a hint first, then watch the reference design built box by box.
Try Litmus