Skip to content

CodingLevel 2

Correctness review

Easy

Medium

Hard

Continue Read the guide View as list

Spotting correctness bugsFinding the edge case that crashes the system.3 min Memory safetyDangling pointers and use-after-free crashes.3 min Dates, time zones & encodingWhy 2:30 AM can happen twice in one night, and why emojis break string lengths.3 min Database bugs in codeWhy two simultaneous clicks on "Purchase" can sell the same concert ticket twice.3 min Numeric & money bugsWhy 0.1 + 0.2 doesn't equal 0.3, and why odometers roll over to 0.3 min Frontend bugs (React)Why your React interval is counting 0, 0, 0, 0 instead of 1, 2, 3.3 min API contracts & compatibilityWhy renaming a database column breaks mobile apps halfway across the world.3 min Paging a collection that keeps growingWhen rows are being inserted while you page, counting from the start of the list stops meaning anything, so point at a row instead of counting to one.7 min Reviewing AI-generated codeWhy code that "looks perfectly correct" can subtly destroy your app.3 min Structured loggingWhy `console.log("User " + id + " failed")` is impossible to search at scale.3 min Errors a client can actually act onAn error response is a set of instructions for a program that cannot ask you a follow-up question.7 min Precision vs recall in product termsOne dial, the threshold, trades blocking good customers against letting bad ones through. There is no setting that does neither.4 min Insecure deserializationWhen reading data turns into running code.3 min Server-side request forgery (SSRF)Turning your application into a proxy to attack the internal network.3 min Accessibility basics: keyboard, contrast, announceThe same form can be flawless with a mouse and unusable with a keyboard, a screen reader, or in grayscale, so you check it through those lenses.4 min Cache evictionA small shelf with room for only a few things, when something new arrives, something old has to leave. The policy is the rule for choosing who goes.4 min DNS lookup hot loopWhy a simple loop can accidentally DDOS your DNS provider.3 min File i/o: buffered vs directBuffered I/O routes reads and writes through the kernel page cache so repeats are fast; direct I/O skips the cache and talks straight to the disk.6 min Flushing to disk (fsync and the page cache)A successful write() only reaches the page cache, your data is durable only after fsync pushes those dirty pages down to the physical disk and waits for the device to confirm.5 min GRPC missing deadlinesWhy a fast microservice can take down your whole company if it forgets to set a timer.3 min HTTP Keep-Alive churnWhy opening and closing network connections is the slowest thing your server does.3 min HTTP streaming (server-sent events)How ChatGPT types its answers one word at a time over a single HTTP connection.3 min ML inference serving (dynamic batching)How to maximize expensive GPUs by forcing impatient HTTP requests to carpool.3 min Model evaluation in PyTorchSwitch the model out of training mode, freeze the gradients, then count how often it was right.3 min Prototype pollutionWrite to __proto__ once and you've quietly added a property to every object in the program.4 min Socket short writeswrite() may accept fewer bytes than you gave it, you must loop until everything is sent, advancing past what was already written.4 min SQL index designAn index is the book's table of contents, it lets the database jump straight to a row instead of reading every page.4 min Streaming a downloadPour the river through a cup, a sip at a time, never try to hold the whole river at once.3 min TCP socket framingTCP hands you a river of bytes, not a stack of letters. To read your messages back out, you have to mark where each one ends.4 min The PyTorch training loopEvery model you train is the same four lines on repeat: guess, measure how wrong, find which way is downhill, take a step.4 min UDP recv buffer truncationA datagram is all-or-nothing per read, too small a buffer means the tail is dropped on the floor, not saved for later.3 min Unbounded HTTP readsWhy trusting `readAll()` on a network socket is a fast track to an Out of Memory crash.3 min Uploading a big file in partsCut a giant file into chunks, send each one on its own, then ask the server to glue them back together.4 min Verifying backups (checksum + test restore)A backup you have never restored is a hope, not a backup, pin it down with a checksum and an actual test restore.5 min WebSocket partial frame reassemblyA socket hands you bytes, not messages, so you keep a buffer and only deliver a message once every byte of it has actually shown up.5 min