The off-chain VM is where applications live and state changes are executed it serves as the upper layer or "layer 2" for an optimistic rollup.Īs optimistic rollups are designed to run programs either written or compiled for the EVM, the off-chain VM incorporates many EVM design specs. Off-chain virtual machine (VM): Although contracts managing the optimistic rollup protocol run on Ethereum, the rollup protocol performs computation and state storage on another virtual machine separate from the Ethereum Virtual Machine. In this sense, Ethereum serves as the base layer or "layer 1" for optimistic rollups. This includes contracts that store rollup blocks, monitor state updates on the rollup, and track user deposits. On-chain contracts: The optimistic rollups's operation is controlled by smart contracts running on Ethereum. The architecture of an optimistic rollup comprises the following parts: Like the Ethereum blockchain, this transaction record is immutable and forms the "optimistic rollup chain." Optimistic rollups process transactions off the main Ethereum chain, but post off-chain transactions (in batches) to an on-chain rollup contract. Each optimistic rollup is managed by a set of smart contracts deployed on the Ethereum network. Optimistic rollups are off-chain scaling solutions built to operate on top of Ethereum. How do optimistic rollups interact with Ethereum? Others can continue to build on an unconfirmed rollup block, but with a caveat: transaction results will be reversed if based on an incorrectly executed transaction published previously.
If the rollup batch remains unchallenged (i.e., all transactions are correctly executed) after the challenge period elapses, it is deemed valid and accepted on Ethereum. The other effect of a successful fraud proof is that the sequencer responsible for including the incorrectly executed transaction in a block receives a penalty. If the fraud proof succeeds, the rollup protocol re-executes the transaction(s) and updates the rollup's state accordingly. After a rollup batch is submitted on Ethereum, there's a time window (called a challenge period) during which anyone can challenge the results of a rollup transaction by computing a fraud proof.
Optimistic rollups instead rely on a fraud-proving scheme to detect cases where transactions are not calculated correctly. This separates optimistic rollups from zero-knowledge rollups that publish cryptographic proofs of validity for off-chain transactions. Optimistic rollups are considered “optimistic” because they assume off-chain transactions are valid and don't publish proofs of validity for transaction batches posted on-chain. Optimistic rollups also use compression techniques to reduce the amount of data posted on Ethereum. This approach enables spreading fixed costs across multiple transactions in each batch, reducing fees for end-users. Optimistic rollup operators bundle multiple off-chain transactions together in large batches before submitting to Ethereum. Optimistic rollups execute transactions outside of Ethereum, but post transaction data to Mainnet as calldata. What is an optimistic rollup?Īn optimistic rollup is an approach to scaling Ethereum that involves moving computation and state storage off-chain. You should have read and understood our pages on Ethereum scaling and layer 2. Optimistic rollups also write transactions to Ethereum as calldata, reducing gas costs for users. Unlike other scaling solutions, such as sidechains, optimistic rollups derive security from Mainnet by publishing transaction results on-chain, or plasma chains, which also verify transactions on Ethereum with fraud proofs, but store transaction data elsewhere.Īs computation is the slow, expensive part of using Ethereum, optimistic rollups can offer up to 10-100x improvements in scalability.
They reduce computation on the main Ethereum chain by processing transactions off-chain, offering significant improvements in processing speeds. Optimistic rollups are layer 2 (L2) protocols designed to extend the throughput of Ethereum's base layer.
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