Rollup
A Layer 2 scaling technique that executes transactions off-chain and posts batched data or proofs to the Layer 1.
A rollup moves execution off the base chain (Layer 1), processes many transactions, then posts a summary back to L1 to anchor security. The two main types are Optimistic Rollups, which assume transactions are valid and rely on a challenge period (fraud proofs) to catch invalid ones, and ZK-Rollups, which generate a cryptographic validity proof for every batch that L1 can verify instantly.
Optimistic Rollups (Arbitrum, Optimism, Base) are simpler to implement and EVM-compatible, but have 7-day withdrawal delays for native bridges. ZK-Rollups (zkSync, Starknet, Polygon zkEVM) offer near-instant finality and lower L1 calldata costs, but are more complex to build and historically less EVM-compatible.
EIP-4844 (Proto-Danksharding), activated on Ethereum in March 2024, introduced 'blobs' — cheap, temporary data storage for rollups — reducing L2 transaction costs by up to 10× overnight.