Rollup (optimistic and ZK)

A rollup is a type of layer 2 network that executes transactions outside a base blockchain, bundles ("rolls up") many of them together and posts the compressed data and results back to the base chain, usually Ethereum. Because many transactions share the cost of one base-layer posting, fees are much lower.

There are two main kinds. Optimistic rollups, such as Arbitrum, Optimism and Base, assume posted results are correct but allow anyone to challenge them with a fraud proof during a dispute window, which is why withdrawals to Ethereum through the official bridge can take about a week. ZK (zero-knowledge) rollups, such as zkSync, Starknet and Linea, post a cryptographic validity proof with each batch, allowing faster finality.

Ethereum's Dencun upgrade in March 2024 added "blobs", a cheaper type of data space for rollups, which cut layer-2 fees sharply. Most rollups still rely on a single company-run sequencer and upgradeable contracts.