Design the Creator Marketplace

A quick gloss on the vocabulary first, because the whole design hangs on it. Robux is Roblox's closed-loop virtual currency. A player buys Robux with real money, spends it inside the platform, and it never leaves as Robux. The catalog is the marketplace of user-generated items that creators build in Roblox Studio, an avatar hat, a shirt, a game pass that unlocks something inside an experience. When a player buys one, Roblox takes a marketplace fee and the creator keeps the rest as Earned Robux. DevEx, short for Developer Exchange, is the one door out. A creator past a threshold of Earned Robux cashes it out to US dollars at a published rate. So the job has three parts that all have to stay in lockstep. Move the Robux correctly, hand the buyer the thing they paid for, and eventually pay the creator real money.

Name the hard part up front, because the interviewer will steer straight at it. A buy is several writes that must all land together or not at all: debit the buyer's wallet, credit the creator's Earned Robux, credit the platform fee, and grant the item into the buyer's inventory. If the network drops and the client retries, the buyer must not be charged twice or handed two copies. And a single item can go viral, a limited-stock accessory during a drop, so thousands of buys a second can pile onto one item's stock counter and one creator's balance. The reported Roblox prompt names this directly: a Robux ledger handling millions of transactions a minute, with idempotency, cancellation of scheduled payments, and consistency under peak traffic. We build the correct ledger first, then earn our way to the hot creator and the payout pipeline.

The Roblox angle

Robux is a closed-loop currency, and that closure is the whole design. Money enters as a Robux purchase, moves between wallets on every buy, and leaves only through DevEx. So the ledger is the single source of truth for a real economy, one that paid creators over a billion dollars in the year to March 2025. Two moves carry this round.