Mock system design interview at Netflix

A live 45-minute round on a question reported in Netflix interviews, run by an AI interviewer that asks one thing at a time, probes what you say, looks at your whiteboard, and weighs what that round is reported to weigh. It ends with a debrief: a score, what was strong, what was missing.

your question

Design YouTube

Upload, transcode, and stream video to a billion viewers: a processing pipeline on one side, a CDN problem on the other.

You will not see the requirements, the numbers or the design before you start. That is the point.

Walk through it first

What the Netflix round is reported to look like

45–60 minutes, often on streaming delivery, the data platform behind it, or a high-volume service such as search or ad events.

What it weighs

Availability first: what keeps playing when a dependency fails
Content delivery: CDN placement, caching and the cost of bytes moved
Event pipelines at very high volume, and what has to be exact versus approximate
Operating it: resilience testing, regional failover and observability

The common mistake

Spending the hour on the metadata service. The bytes, the CDN and what happens when a region goes down are what the interviewer wants to hear about.

From public interview guides and candidate reports, and Netflix’s own interview guidance where it publishes any. Not endorsed by Netflix. The interviewer is an AI running in that style; it is practice, not a prediction.

Start the round

Timed · 45 minutesInterview conditions. The AI interviewer owns the clock; the AI Mentor is off.With the AI Mentor · untimedNo clock, and an AI Mentor beside you to ask when you are stuck.

The interviewer’s opening question is free. Answering it is part of Pro. See an example round and its debrief →

The 11 questions this round draws from

Each is reported in Netflix interviews. Pick one to sit, or read its reference design and walk through a strong answer first.

Design YouTubeUpload, transcode, and stream video to a billion viewers: a processing pipeline on one side, a CDN problem on the other.
drawn
Design Search AutocompleteSuggest the top ten completions after every keystroke, in under fifty milliseconds, from what the world searched for in the last hour, without suggesting anything that gets you in the newspaper.
Design an Ad Click AggregatorCount a million ad clicks a second, show advertisers a dashboard that is seconds behind, stop a campaign the moment its budget runs out, and be exactly right when the invoice is cut.
Design NetflixStream a fixed catalog to 40 M people at once, 200 Tbps at peak, starting in under two seconds, from cache servers placed inside the viewers’ own internet providers.
Design AWS Step FunctionsA managed workflow engine that runs 200 M state-machine executions a day, some lasting a year, applying every transition exactly once.
Design a Recommendation SystemThe “recommended for you” video home feed: 50 personalised videos picked from billions for 500 M daily users in under 200 ms.
Design a Code Deployment SystemBuild each merge once into a signed artifact, roll it out in stages to 50,000 hosts in 10 regions, and roll back automatically on regression.
Design TwitchIngest 100 k live broadcasts, transcode them in real time, and deliver each about three seconds behind live, to three viewers or 500 k.
Design a CDNA multi-tenant CDN for 30 M customer domains: 300 PoPs, 50 M requests a second served mostly from cache, and global purges in seconds.
Design an API GatewayOne front door for public and partner APIs: authenticate, limit and route 1 M requests a second to 300 services, adding under 10 ms.
Design an A/B Testing PlatformRun 500 experiments at once on 300 M users: sticky variants in microseconds, exposure logging, and confidence intervals teams can trust.

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