StarkEx is a STARK-powered scalability engine for crypto exchanges developed by StarkWare. StarkEx uses cryptographic proofs to attest to the validity of a batch of transactions (such as trades and transfers) and updates a commitment to the state of the exchange on-chain. StarkEx allows exchanges to provide non-custodial trading at scale with high liquidity and lower costs. StarkEx currently supports ETH, ERC-20 and ERC-721 tokens, and can readily support tokens on other EVM-compatible blockchains. StarkEx is a mature platform that has been deployed on Ethereum mainnet since June 2020.
More information about the L2 design can be found in StarkWare's documentation. (Note: Some of the details described there may be specific to StarkWare's previous StarkEx system and may not apply to the dYdX system.)