Latency, Throughput, Availability, Reliability & p99 Explained | Module 1 Lesson 4

Latency, throughput, p99, availability, reliability, scalability — these are the five terms every system design conversation uses. In this lesson we lock them all down with concrete numbers, the highway analogy, the famous "nines" availability table, and the Friday-morning banking scenario that shows why "available" and "reliable" are NOT the same thing. Module 1 Lesson 4 of System Design from Scratch — a free system design course built for engineers who only know basic programming. 📚 WHAT YOU'LL LEARN IN THIS LESSON • LATENCY — what it really measures (the whole round trip, not just the network leg) • THROUGHPUT — and why it's independent of latency (1-lane vs 5-lane highway example) • Why AVERAGE latency lies (the "99 fast, 1 slow" trap) • PERCENTILES explained — p50, p95, p99, p99.9 — translated into real users per hour • TAIL LATENCY — what the "tail" means and why p99 is what production engineers track • AVAILABILITY — the famous "nines" table (99% / 99.9% / 99.99% / 99.999%) and real downtime per year • AVAILABILITY vs RELIABILITY — and why available-but-unreliable (the $0 banking app) is worse than being down • SCALABILITY — what it means and the difference between vertical and horizontal 📚 ABOUT THIS COURSE System Design from Scratch is a free, in-depth system design course built for engineers with only basic programming. Aimed at system design interview prep AND building real intuition for SLOs, latency budgets, and the kind of numbers production engineers actually argue about. 👋 ABOUT ME I'm Aaveg — an engineer at a YC-backed startup for the last 4 years. Building this course because the explanations I wanted as a learner didn't exist in one place. 🔗 FIND ME ELSEWHERE LinkedIn → https://www.linkedin.com/in/aaveggupta/ Instagram → https://www.instagram.com/iamaaveg/ ☕ SUPPORT THE COURSE Every lesson is and will always be 100% free. If something here helped and you want to fuel the next one: https://buymeacoffee.com/aavegcodes 🔔 SUBSCRIBE for more system design lessons. #systemdesign #latency #p99