Rollup

A rollup is a Layer 2 that executes transactions off-chain for speed and cost, then posts data or proofs to Layer 1, usually Ethereum, so it inherits L1 security. Execution is expensive on-chain. Data and verification are cheaper. Rollups move execution off-chain and leave enough on L1 that anyone can check correctness or rebuild state.

Optimistic rollups assume validity and use a challenge window, typically 7 days, for fraud proofs. ZK rollups post zero-knowledge proofs that the batch is correct, so there is no challenge wait. Both inherit L1 security. Even a malicious operator cannot trap user funds if users can exit to L1. Costs are often 10-100x lower than Ethereum mainnet, unlike sidechains with their own security.

Arbitrum, Optimism, Base, zkSync, and StarkNet are major rollups. Ethereum's scaling plan is rollup-centric, with L1 optimized for posting rollup data. The core insight: execution is expensive on-chain, but data storage and verification are relatively cheap.

Two types dominate: Optimistic rollups assume transactions are valid by default, using a challenge period (typically 7 days) where anyone can submit fraud proofs if they detect invalid state transitions. ZK rollups use zero-knowledge proofs to cryptographically prove transaction validity, requiring no challenge period because validity is mathematically guaranteed.

A rollup executes off-chain and posts data or proofs to Ethereum. Optimistic and ZK are the two main designs.