Mock system design interview at TikTok

A live 45-minute round on a question reported in TikTok 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 a News Feed

The home timeline of Twitter, Instagram or Facebook: posts from people you follow, ranked, fresh, and fast for hundreds of millions of users.

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

Walk through it first

What the TikTok round is reported to look like

45–60 minutes, mostly in experienced-hire loops, and product-shaped around TikTok itself: short-video upload and processing, delivery through a CDN, the For You feed and its recommendations, or notifications.

What it weighs

Scale reasoning for a global audience of over a billion users
Video at scale: transcoding, storage and CDN delivery, including sudden viral spikes
How the feed is chosen: candidate generation and ranking, not only storage and serving
Latency across regions, and how data is replicated between them

The common mistake

Listing components and waiting to be steered. Candidates describe the round as loosely structured, so take the direction yourself and spend the time on the real bottleneck rather than a tour of every box.

From public interview guides and candidate reports, and TikTok’s own interview guidance where it publishes any. Not endorsed by TikTok. 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 TikTok interviews. Pick one to sit, or read its reference design and walk through a strong answer first.

Design a News FeedThe home timeline of Twitter, Instagram or Facebook: posts from people you follow, ranked, fresh, and fast for hundreds of millions of users.
drawn
Design YouTubeUpload, transcode, and stream video to a billion viewers: a processing pipeline on one side, a CDN problem on the other.
Design Live CommentsComments and reactions on live video that reach viewers within two seconds, at 50 M concurrent viewers and 5 M on one stream.
Design a Top-K Heavy Hitters ServiceRank the top 1,000 videos by views over the last minute, hour, day and all time, from a million views a second.
Design SpotifyStream 100 M tracks to 600 M listeners with sound in 200 ms, offline downloads, regional licensing, and every play counted for royalties.
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 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 Post SearchKeyword search over a billion users’ posts, searchable seconds after posting, sorted by recency or likes, hiding every post the searcher may not see.
Design an Ad Serving SystemPick the best of 10 M active ads for 5 M requests a second in under 100 ms, within every campaign’s targeting, budget and frequency caps.
Design TikTokA full-screen feed of short videos for 1 B daily users, where the next clip starts instantly and every swipe reshapes what comes next.
Design a Content Moderation SystemCatch spam, nudity, hate and illegal media in 4 billion daily posts and comments before they spread, with 15,000 reviewers for the hard cases.

Other companies: Anthropic · Google · Meta · Amazon · OpenAI · Microsoft · Uber · DoorDash