Article
An optimistic rollup is a Layer-2 construction that batches transactions, executes them off-chain, and posts the resulting state roots to a settlement layer alongside the compressed call-data needed to reconstruct the batch. The construction is optimistic: state transitions are presumed correct unless challenged.
Validity is asserted, not proven. The rollup defers verification to a fraud-proof window, exchanging instant finality for cheaper computation.
Construction
A sequencer collects user transactions, orders them into a batch, and executes the batch
against the latest known state. The new state root, together with the batch
calldata, is published to the settlement layer in a single transaction.
Anyone with access to the published data may locally re-execute the batch and verify
that the posted state root is correct.
Fraud-Proof Window
The settlement layer accepts the posted state root provisionally and opens a challenge window — typically 7 days. During this window, any participant who detects an invalid state transition may submit a fraud proof. The settlement contract executes the disputed step on-chain; if it disagrees with the rollup's claimed result, the offending state root is reverted and the proposer is slashed.
Withdrawals
A withdrawal from an optimistic rollup is a state transition that releases assets on the settlement layer. Because the settlement contract cannot trust the rollup's claimed state until the fraud-proof window has closed, native withdrawals require waiting for the window to expire. Liquidity providers commonly offer fast-exit bridges that pay the user immediately and assume the wait themselves.
Fig. 04 — Settlement, rollup, and proof. Three primary forms, three roles.
Trade-offs
The optimistic construction is cheap because the settlement layer never re-executes the batch in the absence of dispute. The cost is latency: native withdrawals must wait the full fraud-proof window, and the system depends on the existence of at least one honest verifier with the data and resources to challenge fraudulent state.
Compare with ZK Rollups, where validity is proven rather than asserted, and withdrawals are limited only by proof generation time.