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An Ethereum layer 2 (L2) rollup executes transactions away from Ethereum’s main network, groups them into batches, and publishes transaction data or a state update to Ethereum. Batching spreads the cost of publishing to layer 1 (L1) across many transactions, while Ethereum provides settlement and, in rollups, data availability.
How does a rollup work?
- Users submit transactions: They send transactions to an L2 system, where execution takes place outside Ethereum L1.
- The L2 executes and groups them: The system processes transactions and combines them into a batch.
- The batch is published to Ethereum: The L2 submits transaction data and/or a state update to an Ethereum contract. Publication enables independent participants to verify updates or reconstruct the rollup’s state, depending on the design.
Because multiple transactions share the L1 publication cost, a rollup can reduce the amount of transaction execution handled directly by Ethereum’s base layer. This does not mean every rollup uses the same proof system, data arrangement, or security assumptions. Ethereum.org’s Layer 2 overview describes Ethereum as the settlement and data-availability layer for Ethereum rollups.
What is the difference between optimistic and ZK rollups?
| Type | How correctness is established | What to understand |
|---|---|---|
| Optimistic rollup | A proposed update is treated as valid unless someone challenges it through a fraud-proof process during a challenge period. | Independent verification depends on the relevant data being available so participants can check the proposed state transition. Ethereum.org’s optimistic rollup documentation explains the fraud-proof approach. |
| Zero-knowledge (validity-proof) rollup | The system submits a cryptographic validity proof for a proposed state update; verification of that proof establishes that the transition is correct. | Transaction or state data is still published so participants can reconstruct and use the rollup state. The “ZK” label can be misleading: the key contrast here is validity proofs rather than fraud challenges. Ethereum.org’s ZK rollup documentation describes this model. |
Neither label alone gives a complete security rating. To assess a particular system, examine how it establishes correctness, where its data is available, how participants can verify or reconstruct state, and what its contracts and operators require for settlement and withdrawals.
Why does data availability matter?
Data availability means the information needed to verify a block or reconstruct a rollup’s state has been made available. Independent participants need relevant data to check updates, rebuild state, or interact with the chain. The publication and storage mechanism affects a system’s assumptions; availability for verification does not, by itself, guarantee that historical data will remain retrievable indefinitely. The duration and storage properties depend on the protocol and publication design. Ethereum.org’s data-availability documentation explains the role of this data.
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How is a rollup different from a validium?
A validium uses validity proofs but places transaction data availability somewhere other than Ethereum. That distinction shows why a validity proof alone does not define a rollup’s full architecture or security model. Ethereum.org’s Layer 2 overview discusses data availability and security as part of how to evaluate L2 systems. For any specific project, check its actual architecture rather than relying only on its chosen label.
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