Systa: Learn System Design

systa teaches system design the way it finally clicks — by letting you *play* with the ideas instead of only reading about them. Most resources hand you a static diagram and hope it sticks. systa hands you a live simulator. Read a short, focused lesson, then open the simulator beside it and watch a load balancer spread traffic, a cache fill and evict, a message queue back up, or a DNS query walk the hierarchy — all reacting as you turn the dials. Change the algorithm, add servers, raise the traffic, trigger a failure, and see the trade-off happen in front of you instead of imagining it. All lessons, simulators and case studies are bundled in the app and work offline. No account, no sign-in. WHAT'S INSIDE • 103 lessons across 13 structured courses — Foundations of Distributed Systems, Scaling & Load Balancing, APIs & Communication, Protocols & Wire Formats, Data Storage & Replication, Caching & CDNs, Messaging & Queues, Consistency & Consensus, Architecture & Operations, Cloud & Operations, Reliability & Resilience, Security, and AI & LLM Systems. • 33 interactive simulators — load balancing, caching, rate limiting, queues, replication, sharding, consistent hashing, CDN, circuit breaker, API gateway, DNS resolution, TLS handshake, B-tree, LSM-tree, bloom filter, consensus, autoscaling, backpressure, connection pooling, cache stampede, erasure coding, saga vs 2PC, failure detection, delivery semantics, and AI/LLM ones (tokenization, embeddings, attention, RAG, agents). Each animates the concept and lets you change the parameters live. • 46 case studies with design boards — work through how real systems are built: URL shortener, news feed, chat, Instagram, YouTube, search autocomplete, notifications, rate limiter, and many more — requirements first, numbers second, architecture last. • A design playground — drag components onto a canvas, wire them together, run traffic through your own design, and watch where the bottleneck appears. Export your diagram as an image to share. • Interview prep — 88 real system design interview questions, from warm-ups to hard ones. Answer out loud, then compare against a 30-second answer, a full model answer, trade-offs, follow-ups and red flags. Tick the points you hit; shaky answers come back for review. • Capacity calculators — back-of-the-envelope math done right: QPS → server count, storage sizing, bandwidth & egress, and cache memory & hit rate. Every number shows its working. • Practice that sticks — multiple-choice quizzes with an explanation for every answer, and spaced-repetition flashcards. • Progress tracking — daily streaks, levels, XP, and achievements that keep you moving. DESIGNED TO READ WELL A calm, paper-like reading surface you can tune to taste: light and dark modes, selectable accent colors and base tones, serif or sans body text, and adjustable text size — comfortable for long study sessions. BUILT FOR INTERVIEWS — AND CURIOSITY Whether you're preparing for a system design interview, leveling up as an engineer, or just want to finally understand how large systems fit together, systa turns vague intuition into concrete reasoning: clarify the requirements, estimate the scale, sketch the design, and defend the trade-offs out loud. FREE & PRO Start free — the first lessons of Foundations, the Load Balancer and Cache simulators, the URL Shortener and News Feed case studies, some flashcard decks, and two playground runs are open to everyone. The free version shows ads; watch a short ad to unlock more lessons, simulators, case studies and decks for 24 hours, or an extra playground run each week. A single one-time Pro purchase unlocks everything and removes all ads: all 103 lessons, 33 simulators and 46 case studies, unlimited playground runs, quizzes, flashcards, the 88 interview questions and the capacity calculators. No subscription, ever — buy once and keep it. Download systa and make system design click.