Design a Real-Time Collaborative Grid

"Design an app in the style of reddit r/place" is a recurring Datadog onsite system design round, asked verbatim more than once. The shape is fixed. There is a shared pixel grid. Start with 1000x1000, so a million pixels. Any user can set one pixel to a color chosen from a small palette of fifteen. After placing, they must wait a per-user cooldown of five minutes before placing again. Everyone watching the canvas sees changes in near-real-time, and a brand-new viewer who just opened the page has to be handed the current state of all million pixels. The headline constraint is the audience. We serve and stream to millions of concurrent viewers at once.

Pin the vocabulary down before sketching boxes, because the design rotates on three nouns. A pixel is the cell at a grid coordinate, holding a color and, if we want attribution, who set it last and when. The grid is the full 1000x1000 array of those cells, the thing a new viewer downloads. A place event is the single action a user takes: set pixel (x, y) to color c, subject to the cooldown. Lock those three first. Pixel. Grid. Place event. Every later choice serves one of them.

The Datadog angle

Be honest with yourself, this is the generic real-time-systems curveball. It is not an observability design, and that is the curveball. Most of the Datadog design round is their actual product, metrics and alerting, where the interviewer built the thing. A canvas-painting app sits far from monitoring, deliberately. The round throws it to test whether you can reason about real-time fundamentals, websocket fan-out, rate limiting, snapshot-plus-delta, without the comfort of a domain you have studied. There is no product knowledge to lean on, so the signal is pure systems thinking. Keep that straight in your head. The interviewer wants to watch you derive fan-out and a cooldown limiter from first principles, with nothing memorized to fall back on.

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