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Questions answered, and a design doc for every template

Short answers to the questions that come up when a system fails, each with the arithmetic and a problem in the lab where you can watch it. Then design docs: the context, goals and non-goals, the numbers that size the system, why each component is there, the alternatives, and what fails under load.

Why does…

Why does a cold cache overload a database?A 90% hit rate means the database is sized for a tenth of the reads. Flush the cache and it gets all of them at once.Watch it
Why do retries make an outage worse?A slow dependency plus a timeout and two retries means three times the load, exactly when it can least take it.Watch it
Why can p99 latency improve while the system is failing?Latency percentiles count the requests that finished. When the slow ones fail instead, the survivors look fast.Watch it
Why does one server fall over on launch day?A server has a fixed number of request slots and CPUs. Past that line, requests queue, and queues only grow.Watch it
Why does an LLM provider quota break a chat app that has plenty of servers?Tokens per minute is a ceiling on the provider’s side. Replicas on your side cannot raise it; they just queue behind it faster.Watch it
Why does a bot flood reach the database when there is a load balancer in front?A load balancer spreads traffic; it does not refuse any. Without a filter at the edge, every bot request is served like a user’s.Watch it

Design docs

Interview deep dives

The full interview, end to end

The classic system design interviews at full depth: clarifying questions, capacity estimates, API, data model, the design, deep dives with the trade-offs, the follow-ups interviewers ask, common mistakes and what a strong answer covers at each level. Each one comes with a build-from-scratch problem in the lab, tested case by case.

Solved designs · 10 credits each, once

How the real products are put together

Instagram, Netflix, Slack, ChatGPT, a coding agent platform and more, as working designs that meet their goals, each with a design doc: every component and why it is there, the numbers, what breaks and how to fix it. Unlock one for 10 credits (new accounts start with 50) and it stays open for you.