Optimism is the first Ethereum L2 to develop an Optimistic Rollup solution compatible with the Ethereum Virtual Machine (EVM). Parallel to Ethereum, Optimism is able to process transactions at scale while inheriting the security of Ethereum.
Optimistic Rollup gets its name from the way transactions are processed. By default, transactions are optimistically assumed to be accurate and no calculations are performed. It relies on anti-fraud mechanisms to ensure the legitimacy of transactions. Validators (anyone running an Optimism "full node") are able to challenge the validity of transaction proposals submitted by sequencers, which Rollup will then check by performing computations on transactions on Ethereum. Transaction proposals can be challenged during the challenge period, which Optimism sets to 1 week, before being finalized and submitted to the Ethereum mainnet.
If the proposal turns out to be fraudulent, the validators are rewarded and the sequencer is punished. Rewards are taken from the bound ERC-20 token pool that the sequencer needs to bind. In the case of Optimism, the required token is ETH. This creates an incentive mechanism where validators are incentivized to catch bad actors and sequencers are incentivized to only submit accurate transaction proposals. Fraudulent proposals are removed and correct proposals are finalized. The transaction itself is not reverted, only the fraudulent proposal result of the transaction is removed.
Optimism has 4 main benefits: EVM equivalence, data safety, speed, and cost.
Optimism is one of the most EVM-compatible chains, and its focus is to go a step further and be EVM-equivalent. Optimism is able to support any Ethereum application using its Optimistic Virtual Machine (OVM), which is an EVM-compatible virtual machine. Developers are able to deploy any Ethereum-based dApp onto Optimism with few architectural changes. This allows decentralized applications (dApps) built on Ethereum to be seamlessly integrated on Optimism.
Unlike sidechains like Ronin (which Axie infinity employs) which have their own security while running independently, rollups like Optimism get their security from the Ethereum mainnet. Transactions are processed on Optimism, but the transaction's data is written and stored on Ethereum. This allows Optimism to inherit the security of Ethereum while maintaining scalability.
Computation is the slow and expensive part of using Ethereum. By not counting transactions by default, Optimism is able to achieve 10-100x scalability improvements depending on the nature of the transactions. Optimism features near-instant trade finality, enabling users to check the results of their trades almost instantly. Optimism transactions are also cheap, at a fraction of the cost of Ethereum transactions. Currently, Optimism’s base transfer fee is $1.66, while Ethereum’s base transfer fee is $8.77 (see real-time fees here). The Optimism team estimates that they have collectively saved over $335 million in gas fees across transactions processed on the network.