Optimistic Rollup

Optimistic rollups treat transactions as valid by default and post state to Layer 1 without an immediate validity proof. Security is a challenge period, typically 7 days, when anyone can submit a fraud proof. A successful proof reverts the bad transaction and penalizes the sequencer. If nobody challenges, the state finalizes.

The design is simpler and cheaper than zero-knowledge proving, and closer to existing Ethereum tooling. Arbitrum and Optimism aim at EVM equivalence so apps deploy with little change. The cost is withdrawal delay. Moving assets from L2 to L1 waits out the window, unless you use a third-party bridge that takes fraud risk for a fee.

Arbitrum One and Optimism are the largest optimistic rollups, with billions in TVL. Newer designs add more proving systems and faster effective exits. Optimistic rollups won on simplicity. ZK rollups may take share as proving gets cheaper. If a fraud proof succeeds, the invalid transaction is reverted and the malicious sequencer is penalized.

If no challenge occurs during the window, the state becomes final. Recent developments include multiple proving systems and shorter effective withdrawal times through improved bridge designs. Optimistic rollups benefit from relative simplicity but may eventually cede ground to ZK rollups as zero-knowledge proof technology matures.

Optimism and Arbitrum assume batches are valid, then allow a challenge window. Withdrawals wait out that window unless a fast bridge is used.