Rollup

A rollup bundles many transactions together, executes them outside the base chain, and publishes the results plus enough data to verify them back to the layer 1. Because the data is on the base chain, anyone can reconstruct the rollup's state and challenge or prove its correctness. This is what distinguishes rollups from sidechains, which run their own separate security.

There are two main designs. Optimistic rollups such as Arbitrum and Optimism assume batches are valid and allow a challenge window, usually seven days, during which anyone can submit proof of fraud. Zero-knowledge rollups such as zkSync and Starknet generate a cryptographic proof that every batch is correct, so there is no challenge window and withdrawals can be faster, at the cost of heavier computation.

Rollups are the center of Ethereum's scaling plan. Together they handle far more transactions than Ethereum mainnet itself, with fees typically under a cent after the 2024 Dencun upgrade. The open issues are decentralizing the sequencers that order transactions and making it easy to move between dozens of separate rollups.