Rollups
Rollups
Rollups execute transactions off-chain and post compressed transaction data plus state roots to Ethereum L1. L1 validators don't re-execute transactions — they only verify a proof or rely on a challenge mechanism. This is the dominant Ethereum scaling architecture.
Optimistic Rollups
Assume transactions are valid by default. Anyone can submit a fraud proof within a challenge window (~7 days) to dispute an invalid state root.
- Fraud proofs — challenger re-executes the disputed transaction on-chain; if the sequencer cheated, the state root is reverted and the sequencer is slashed
- Withdrawal delay — users must wait out the challenge window before L1 finality (~7 days)
- Proving cost — near-zero during normal operation; only expensive if fraud occurs
Major optimistic rollups: optimism (OP Mainnet), base, Arbitrum One, Arbitrum Nova.
ZK Rollups
Use cryptographic validity proofs (SNARKs or STARKs) to prove every batch of transactions is correct. No challenge window — L1 accepts state transitions as soon as the proof verifies.
- Validity proofs — math guarantees correctness; no trust in sequencer required for finality
- Fast withdrawals — finalized as soon as proof is verified on L1 (minutes to hours)
- Proving cost — expensive to generate; requires specialized hardware or proving networks
- EVM compatibility — harder to achieve; zkEVM projects (zkSync Era, Polygon zkEVM, StarkNet) have varying levels of bytecode equivalence
Fraud Proofs vs Validity Proofs
| Property | Optimistic | ZK |
|---|---|---|
| Withdrawal delay | ~7 days | Minutes |
| L1 cost (normal) | Low | Higher (proof verification) |
| EVM equivalence | Full | Partial to near-full |
| Trust assumption | 1-of-N honest challenger | Cryptographic |
| Proving hardware | None (normal case) | Required |
Maturity / Staging
L2BEAT classifies rollups by decentralization stage:
- Stage 0 — state roots published on L1, data available off-chain, node software is public, and a proof system exists — but a centralized operator can override it (training wheels on)
- Stage 1 — working proof system live, at least 5 external fraud proof submitters, 7-day exit window enforced, Security Council has ≥8 members with ≥50% external (non-team) members
- Stage 2 — fully permissionless fraud proofs, 30-day exit window, Security Council authority limited to bugs that are adjudicable on-chain only
As of early 2026, Arbitrum, OP Mainnet, and Base are Stage 1. Most ZK rollups remain Stage 0 due to centralized provers.
Related
op-stack · ethereum-l2s · data-availability · optimism