ZK-Rollup and Optimistic Rollup are both Layer-2 scaling technologies that take transactions off-chain, compute the resulting state there, and submit a commitment to this new state back to the blockchain. The key difference between them is the type of proof each protocol relies on. Optimistic Rollup relies on fraud proofs: a new state is assumed to be valid, unless proof to the contrary (a proof of fraud) is submitted to the blockchain within some timeframe. ZK-Rollup relies on validity proofs: a new state will always be presented to the blockchain with proof that it is in fact valid. For additional background, check out StarkWareâ€™s __comparison of validity proofs vs. fraud proofs.__

ZK-Rollup has a few important advantages over Optimistic Rollup:

Security: In ZK-Rollup, the system is always in a valid state. In Optimistic Rollup, an invalid state may exist, certainly until a fraud proof is submitted, and possibly indefinitely. Specifically, a 51% attack in OR can result in theft of funds, whereas in ZK-Rollup it could only allow for the reverting of transactions.

Capital Efficiency: ZK-Rollup is 1000X more capital efficient than Optimistic Rollup.

Scale: Optimistic Rollup consumes blockchain resources that grow linearly with the volume of transactions. ZK-Rollup can be designed (and so we did with StarkEX!) to consume exponentially less blockchain resources. StarkEx consumes an almost fixed amount of blockchain resources: they grow logarithmically with the volume of transactions, thus allowing for massive transaction throughput.