What is an optimistic rollup?
An optimistic rollup is a Layer 2 scaling solution designed to improve Ethereum's transaction speed and reduce gas fees. It works by bundling multiple transactions together and processing them off-chain while assuming they are valid unless proven otherwise—hence the term "optimistic."
By operating on top of Ethereum's main chain (Layer 1), optimistic rollups increase network efficiency while maintaining security through fraud proofs. If a fraudulent transaction is detected, the system allows users to challenge and revert it.
How it works
Optimistic rollups function in a few key steps:
Transaction bundling – Multiple transactions are grouped together and executed off-chain.
Submission to Layer 1 – A compressed version of these transactions is sent to Ethereum's mainnet.
Optimistic assumption – The rollup assumes all transactions are valid and posts them without immediate verification.
Fraud detection period – A challenge window (typically 7 days) allows users to dispute fraudulent transactions by providing proof.
Finalization – If no fraud is detected, the transactions become final and irreversible. If fraud is found, the invalid transaction is removed, and the offender may be penalized.
This approach allows Ethereum to handle more transactions without overloading the main blockchain.
Optimistic rollups vs. ZK-rollups
Optimistic rollups are often compared to zero-knowledge rollups (ZK-rollups). While both scale Ethereum, they differ in verification methods:
Optimistic rollups assume transactions are valid unless disputed, using fraud proofs.
ZK-rollups use cryptographic proofs to verify transactions before posting them to Layer 1.
Optimistic rollups are easier to implement and support smart contracts more flexibly, but they have longer withdrawal times due to the fraud challenge period.
Challenges and considerations
Withdrawal delays – Users must wait through the challenge period (usually a week) before accessing funds on Ethereum's main chain.
Fraud proof efficiency – The security model relies on at least one honest participant to detect fraud.
Adoption and security risks – As a newer technology, optimistic rollups must prove their robustness over time.
