OP Mainnet is an optimistic rollup. When you submit a transaction, the transaction's data is written to the Ethereum mainnet, but the transaction itself is executed by a single computer, the sequencer (instead of thousands of computers as Ethereum does it). This significantly reduces the transaction cost, while a challenge mechanism ensures the sequencer is honest.
Ethereum has limited capacity
which can generally be understood due to constraints of the blockchain trilemma
. To have decentralization we need to enable as many people as possible to run a node, which means that the number of transactions that can be processed per minute is limited by the capacity of standard hardware. This limits our ability to scale, unless we accept the security implication of not having every node process every transaction.
Solutions that build on top on Ethereum are called layer 2 or L2 (with Ethereum itself called layer 1 or L1). OP Mainnet uses a type of solution called an optimistic rollup
post all transactions on layer 1, so data integrity and availability are provided by Ethereum.
Optimistic Rollups use economic incentives to ensure the data processing, done off chain, is done correctly. The sequencer node posts the merkle root of the blockchain state on L1 (called the state root). Other nodes, called verifiers, can issue fault challengesif they believe the state root is incorrect. In the case of a fault challenge part of the transaction is executed on L1 to verify which is the correct state root.
Sequencers that post correct state roots, and verifiers that challenge incorrect ones, are rewarded for their honesty. Sequencers that post incorrect state roots are penalized for dishonesty. Verifiers that challenge correct results, which could be used as a denial of service attack, are penalized. If a state is not challenged for the challenge period (seven days on OP Mainnet), it is assumed to be correct. As long as there is at least one honest verifier, the state will end up being the correct one - and the economic incentives are aligned with honesty.
We have a bridge
that allows users to deposit into OP Mainnet and withdraw from it using this mechanism. A withdrawal requires you to waiting the challenge period (until the blockchain state becomes indisputable), but faster withdrawals are available from third party bridges
that run their own verifiers so they know
the state submitted is correct.