System Design

A URL shortener turns https://example.com/a/very/long/path?campaign=autumn into something like sho.rt/6ZY3LCH, and sends anyone who opens the short link to the …

A product manager says PAY-204: next Friday's campaign may double checkout traffic for three hours. The beginner mistake is asking, "How many servers do we need…

An order database's primary data center suffers a catastrophic, unrecoverable event — not a transient node failure, an actual disaster. Two numbers now determin…

A product-page personalization API needs to answer in under 100ms for a good user experience. A shopper in Sydney hits it, and the request has to reach the orig…

A multi-tenant analytics platform partitions incoming events by tenant_id, hashed onto 16 shards — a design that looked perfectly balanced in every load test. T…

A URL shortener's redirect endpoint, GET /r/{code}, looks purely read-heavy — billions of redirects against a handful of short codes created per second. But eve…

An order service runs in two regions, us-east and eu-west. In one design, both regions serve live customer traffic right now, splitting the load. In another, us…

A permission check in front of a wire-transfer approval endpoint times out. What should happen next? One engineer's instinct says "don't block the transfer over…

A payment service keeps processing requests with zero dropped transactions while one of its three nodes crashes mid-request — that's fault tolerance. A differen…

A product page shows the item, its price, and an "add to cart" button — the core path — plus a "customers also viewed" recommendation strip powered by a separat…

A checkout API runs three replicas across two availability zones behind a load balancer — on paper, no single point of failure. Then one AZ has a networking inc…

An order-ingestion API writes to an in-memory queue, a worker pool drains it, and a payment provider on the other end gets slow. The queue keeps accepting every…

A shopping cart write to cart-77 needs to land on enough replicas that a later read is guaranteed to see it — but "enough" is a number you choose, and the numbe…

A 4-node session cache scales to 5 nodes to handle more traffic. With hash(key) % N, that single node addition changes almost every key's target node, and the c…

A user changes their display name, refreshes the page a second later from a phone on a different network, and sees the old name. Nothing crashed. No request fai…

A checkout API can wait until every step finishes, or accept an order and complete work later. The first path gives the caller an immediate final answer but cou…

A shopping cart must remember items, so the product cannot be literally stateless. The useful design question is where that state lives. Keep the cart inside on…

An order API can return responses all day and still charge the wrong amount. It can be temporarily unreachable while every stored order remains safe. It can fai…

A dashboard can look “fast” while users still wait, and a load test can report huge requests-per-second while p99 checkout times explode. Latency is how long on…

You press Enter on https://shop.example/products/42. A moment later, a product page appears. That small action crosses naming, transport security, HTTP, edge in…

A support ticket that needs docs, a tool call, and a model reply does not need twelve equal "frameworks." It needs an orchestration lane, a retrieval lane, a se…

A travel checkout may call a public weather API, your own booking API, and a partner airline API. All three could use REST, but they should not share the same c…

Checkout slows while product pages remain healthy. CPU is moderate, but the orders table shows rising lock waits and writes queue behind one hot index. Adding a…

A prompt is only one part of a production AI system. Engineers also need vocabulary for execution loops, tool connections, model access, cost, evaluation, safet…

This guide is for developers who can build a frontend, API, and database but have not yet operated the whole request path. By the end, you can trace one checkou…

A product page is easy to cache until a price changes: the CDN still has the old response, one app instance has an older in-process value, and Redis has already…

This guide is for backend engineers who know HTTP and database transactions but need to decide where six microservice patterns fit. By the end, you can trace on…

A product page's cache key expires at 2:00:00pm. In the same instant, 5,000 concurrent requests check the cache, all get a miss, and all 5,000 independently que…