Rollup

A rollup is a Layer 2 scaling solution that executes transactions off-chain for speed and cost efficiency while posting transaction data or proofs to Layer 1 (typically Ethereum) for security inheritance. The core insight: execution is expensive on-chain, but data storage and verification are relatively cheap.

Rollups move execution off-chain while keeping enough data on Layer 1 that anyone can verify correctness or reconstruct the rollup state. 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. Both approaches inherit Layer 1 security: even if the rollup operators are malicious, users can exit to L1 with their assets.

Rollups achieve 10-100x lower transaction costs than Ethereum mainnet while maintaining trustless security, this is the key distinction from sidechains, which have independent security models. Arbitrum, Optimism, Base, zkSync, and StarkNet are major rollups. Ethereum's scaling roadmap centers on rollups, with the mainnet evolving to optimize for rollup data posting.