What is a rollup?
A rollup is a Layer 2 scaling solution that executes transactions off-chain (on a separate execution layer) but posts compressed transaction data and state commitments back to the Layer 1 chain (typically Ethereum) for security and data availability. Rollups inherit L1 security while delivering 10-100x throughput and 10-100x lower gas costs.
Two rollup families
- Optimistic rollups (Arbitrum, Optimism, Base): assume transactions are valid by default; allow a fraud-proof challenge window (~7 days) before withdrawals finalise. Mature ecosystem; longer withdrawal latency.
- ZK rollups (zkSync, Starknet, Polygon zkEVM, Linea, Scroll): use validity proofs (zero-knowledge proofs) to mathematically prove correctness of each batch. Faster finality, smaller calldata, but more compute-intensive.
EIP-4844 (Proto-Danksharding)
Ethereum’s March 2024 Dencun upgrade introduced blobs — a dedicated data availability layer for rollups that dramatically reduced L2 gas costs (often 90%+). Blobs are temporary (pruned after 18 days) but sufficient for L2 fraud/validity proof windows. This was the first concrete step toward full Danksharding.
Optimistic vs ZK rollups — and the legal pressure points
A rollup scales Ethereum by executing transactions off the main chain and posting compressed proofs or data back to it. The two families differ in how they prove correctness: optimistic rollups assume transactions are valid and allow a challenge window in which fraud can be disputed, while zero-knowledge (ZK) rollups post cryptographic validity proofs. The legal pressure points track the technology. Data availability — whether the underlying transaction data is actually published — affects whether users can independently exit. The sequencer that orders transactions can be a point of centralisation and censorship. And the bridge that locks assets into the rollup carries the same custody and liability questions as any cross-chain bridge. For a project building on a rollup, these are due-diligence items, not background detail.