What is Optimistic Rollup?

Optimistic Rollup is a Layer 2 scaling technique that executes transactions off-chain while posting transaction data to a Layer 1 blockchain (typically Ethereum) for data availability. The protocol operates on an "optimistic" assumption — transactions are presumed valid by default and no computation proof is required at the time of posting. Sequencers batch transactions, execute them off-chain, and submit the resulting state roots along with compressed transaction data to the L1. Only if someone disputes a state transition does the system invoke a fraud proof mechanism, where the disputed transaction is re-executed on the L1 to verify correctness. This challenge period typically lasts 7 days, during which any observer can submit a fraud proof to contest an invalid state root.

The optimistic approach provides significant throughput improvements over direct L1 execution, as computation happens off-chain while the L1 only needs to store transaction data and verify fraud proofs when disputes arise. This results in substantially lower gas costs per transaction compared to L1 execution. The main trade-off is the challenge period — withdrawals from the rollup to L1 require waiting for the dispute window to close, though third-party liquidity providers offer faster withdrawal services for a fee. Security derives directly from the L1: as long as at least one honest verifier monitors the chain and can submit fraud proofs, invalid state transitions will be caught and reverted. The compressed transaction data posted to L1 ensures that anyone can independently reconstruct the rollup state.

Optimistic Rollups are used by several major Ethereum Layer 2 networks, most notably Optimism (OP Mainnet) and Arbitrum, which together secure billions of dollars in value and process millions of transactions. Optimism developed the OP Stack, which has been adopted by multiple other rollups including Base (by Coinbase) and several other chains forming the Superchain ecosystem. Arbitrum, developed by Offchain Labs, uses an interactive multi-round fraud proof system called Arbitrum Nitro. The Optimistic Rollup approach was first proposed in 2019 and has become one of the two dominant L2 scaling paradigms alongside ZK Rollups, with its relative simplicity and EVM compatibility being key advantages.