What is an optimistic rollup?
- Author
- CRYPTO PORT Editorial
- Published
- Updated
- Reading time
- 5 min
In short
An optimistic rollup accepts posted results as valid by default and lets anyone challenge them with a fraud proof. Skipping per-batch proofs keeps it cheap, but exits to the main chain must wait out the challenge window.
Key points
- Results are provisionally accepted as correct
- Anyone can submit a fraud proof to contest them
- Exits wait out the challenge period
- Existing contracts port over with little change
Definition
A rollup design that treats posted results as valid unless a fraud proof is submitted within a defined window.
When the operator posts a result, nothing verifies it at that moment. Instead a window opens during which anyone can dispute it and submit a proof of fraud. A successful challenge reverts the result and costs the dishonest party its bond.
Security therefore rests on at least one honest party watching and willing to challenge. Given that assumption, no proof is needed per batch, which keeps costs low. Arbitrum and Optimism use this design.
For users the visible consequence is exits. Because nothing finalises on the main chain until the challenge window closes, withdrawals through the canonical bridge typically take several days. Third-party liquidity providers offer instant exits instead, at the cost of a fee and exposure to that provider.
Watch out for
- · Canonical-bridge withdrawals can take days to finalise
- · Instant-exit services substitute the provider's credit risk for that wait