// GLOSSARY -- WEB3 INFRASTRUCTURE

What is Optimistic Rollup?

1 min read Updated

An optimistic rollup is a Layer 2 scaling solution that assumes transactions are valid by default and only runs computation if a fraud proof is submitted during a challenge period — typically 7 days.

WHY IT MATTERS

Optimistic rollups are 'optimistic' because they assume validity rather than proving it. Transactions are executed off-chain and results are posted to L1. If no one challenges the results within the challenge period (~7 days), they're considered final.

If someone detects an invalid state transition, they submit a fraud proof — demonstrating the error on L1. The invalid batch is reverted and the malicious sequencer is penalized.

The 7-day challenge period affects withdrawals to L1 — native withdrawals take a week. Third-party bridges offer faster exits for a fee by fronting the funds.

FREQUENTLY ASKED QUESTIONS

Why 7 days?
The challenge period must be long enough for anyone to detect and prove fraud. 7 days provides ample time for watchers to submit fraud proofs. Shorter periods increase security risk.
Has a fraud proof ever been submitted?
In production, major optimistic rollups haven't needed fraud proofs because sequencers have been honest. The mechanism exists as a deterrent. Permissionless fraud proofs are being enabled.
Which projects use optimistic rollups?
Arbitrum One, Optimism, Base, and other OP Stack chains. They're the most adopted L2 approach due to EVM compatibility and relatively simple technology.

FURTHER READING

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