https://cs.lozic.me/areas/systems-design/Recent content in Systems-Design on CS TheoremsHugoen-usFri, 27 Aug 2027 12:00:00 +0100https://cs.lozic.me/posts/t066-the-cap-theorem-and-pacelc/Fri, 27 Aug 2027 12:00:00 +0100https://cs.lozic.me/posts/t066-the-cap-theorem-and-pacelc/<h2 id="symptom">Symptom</h2> <p>Someone in a design review says &ldquo;we&rsquo;re AP, not CP,&rdquo; and the room nods.</p> <p>Nobody asks which consistency, which availability, or what happens when there is no partition, which is 99.9% of the time. The phrase has done the work of a decision without containing one.</p>https://cs.lozic.me/posts/t089-conways-law/Fri, 23 Oct 2026 12:00:00 +0200https://cs.lozic.me/posts/t089-conways-law/<h2 id="symptom">Symptom</h2> <p>The service has four layers. Nobody designed four layers. There were four teams.</p> <p>Or the version that arrives in a postmortem: the outage happened at the boundary between two services, in the retry logic, because each side assumed the other owned the deduplication. Both teams were right about their own component and neither had ever been in a room with the other.</p>https://cs.lozic.me/posts/t091-hyrums-law-and-postels-principle/Fri, 11 Sep 2026 12:00:00 +0200https://cs.lozic.me/posts/t091-hyrums-law-and-postels-principle/<h2 id="symptom">Symptom</h2> <p>You fix a typo in an error message. Four days later a customer escalates: their alerting pipeline greps for the old string and has gone quiet.</p> <p>Or you make a function faster, and a test suite that had been green for two years starts failing intermittently — not because the function is wrong, but because it used to be slow enough to hide a race.</p>