What is optimistic rollup?
A layer-2 design that assumes posted transaction batches are valid and allows anyone to challenge them during a dispute window.
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In this entry
A layer-2 design that assumes posted transaction batches are valid and allows anyone to challenge them during a dispute window.
It is the design behind several of the largest Ethereum layer-2 networks, and the alternative to validity rollups, which prove correctness cryptographically instead of assuming it. Both publish their data to the base chain. They differ in how they convince it that the results are right.
Because nothing is proved up front, execution is cheap. The cost is the challenge period, which forces a delay of several days on withdrawals back to the base chain before they finalize.
How it works
- A sequencer orders transactions and executes them, giving users a near-instant confirmation on the layer 2.
- Batches of transactions, plus the resulting state root, are posted to the base chain. The transaction data is published so anyone can independently recompute the correct result.
- The base chain accepts the posted state as valid without checking it. This is the optimistic assumption.
- During a challenge window, any observer who recomputes a different result can submit a fraud proof. The dispute is adjudicated on the base chain, and a false claim costs the proposer its bond.
- Once the window closes with no successful challenge, the state is final and withdrawals against it complete.
The security model therefore rests on at least one honest party watching and being able to submit a proof. That party needs the data, which is why publishing transaction data to the base chain matters more than any other component.
Bridges that offer instant exits are third parties fronting you the funds for a fee, not the protocol working faster. They take on the wait and charge for it, and they carry their own counterparty risk.
Example
Illustrative timeline for moving $1,000 back to Ethereum.
| Route | Time to funds | Cost | Who you rely on |
|---|---|---|---|
| Canonical bridge | The full challenge window, commonly around seven days | Base chain gas only | The protocol |
| Third-party fast bridge | Minutes | Gas plus a percentage fee | The bridge operator's solvency |
| Withdraw via a centralized exchange supporting the network | Exchange processing time | Exchange withdrawal fee | The exchange |
Read the withdrawal timing on the official bridge before you plan around it, since the window is set per network and stated in its own documentation.
Why it matters when you buy
If you buy an asset on an exchange and withdraw to a layer 2, getting it back to the base chain later is not instant, and that surprises people at the worst moment. Check whether your exchange supports deposits directly from that network, because that route usually avoids the wait entirely. Which networks each venue supports is recorded at the exchange directory, and the per-network measurements are on the chain rankings.
Related terms
- fraud proof — the challenge that enforces correctness
- rollup — the wider category
- zk rollup — the validity-proof alternative
- sequencer — who orders transactions
- canonical bridge — the protocol's own exit route
- data availability — what makes challenges possible
Questions
Why does withdrawing take days?
Because the challenge window has to stay open long enough for an honest observer to notice a bad state and prove it. Shortening it would weaken the security guarantee that makes the design work.
Is my money at risk during the challenge period?
The funds are on the base chain the whole time, committed to a pending withdrawal. The wait is a delay, not custody by a third party, provided you use the canonical bridge.
Are optimistic rollups less safe than validity rollups?
They rest on different assumptions. A validity rollup proves each batch mathematically. An optimistic rollup assumes correctness unless challenged, which requires someone to be watching with the data available.