Mock system design interview at Discord

A live 45-minute round on a question reported in Discord 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 Slack

Real-time team messaging: channels, DMs, presence, history and search for millions of concurrent 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 Discord round is reported to look like

45–60 minutes, in one or two architecture sessions. The problem is one Discord has faced or might, often real-time chat or messaging, and sometimes a general one such as a URL shortener.

What it weighs

Requirements: the questions asked, and how the answers shape the design
Tradeoffs with reasons, fitted to what the problem actually needs
Simplicity: the simplest design that works, and whether it still holds at 2x and 10x
On real-time prompts: long-lived connections, fan-out to very large servers and reconnecting after a drop

The common mistake

Designing in silence. Asking questions and checking in with the interviewer is part of what is assessed and knowledge of the domain is not, so ask about an unfamiliar area rather than guessing.

From public interview guides and candidate reports, and Discord’s own interview guidance where it publishes any. Not endorsed by Discord. 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 7 questions this round draws from

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

Design SlackReal-time team messaging: channels, DMs, presence, history and search for millions of concurrent users.
drawn
Design a Matchmaking SystemGroup online game players into fair, low-latency matches within seconds, at millions of concurrent players.
Design RedditServe communities of posts with deeply nested comment threads, take tens of thousands of votes a second, rank by hot and best, and keep a 50,000-comment thread fast while it is still growing.
Design ZoomReal-time audio, video and screen share for 2 to 1,000 people per meeting, 300 M participants a day, under 150 ms mouth to ear.
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 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.
Design DiscordCommunity chat and voice: servers with a million members, years of history, roles and presence, for 20 M concurrent users.

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