Optimism is an EVM equivalent, optimistic rollup protocol designed to scale Ethereum. Scaling Ethereum means increasing the number of useful transactions the Ethereum network can process. Optimistic rollup is a layer 2 scalability technique which increases the computation & storage capacity of Ethereum without sacrificing security or decentralization. EVM Equivalence is complete compliance with the state transition function described in the Ethereum yellow paper, the formal definition of the protocol.

Optimistic rollup works by bundling multiple transactions into a single transaction, which is then verified by a smart contract on the Ethereum network. This process is called “rolling up” because the individual transactions are combined into a larger transaction that is submitted to the Ethereum network. The term “optimistic” refers to the fact that the system assumes that transactions are valid unless proven otherwise, which allows for faster and more efficient processing of transactions.

Overall Architecture

op-node + op-geth

The rollup node can run either in validator or sequencer mode:

  1. validator (aka verifier): Similar to running an Ethereum node, it simulates L2 transactions locally, without rate limiting. It also lets the validator verify the work of the sequencer, by re-deriving output roots and comparing them against those submitted by the sequencer. In case of a mismatch, the validator can perform a fault proof.
  2. sequencer: The sequencer is a priviledged actor, which receives L2 transactions from L2 users, creates L2 blocks using them, which it then submits to data availability provider (via a batcher). It also submits output roots to L1. There is only one sequencer in the entire stack for now, and it's where people critisize that OP stack is not decenralized.

op-batcher

The batch submitter, also referred to as the batcher, is the entity submitting the L2 sequencer data to L1, to make it available for verifiers.

op-proposer

Proposer generates and submitting L2 Output checkpoints to the L2 output oracle contract on Ethereum. After finalization period has passed, this data enables withdrawals.

Both batcher and proposer submit states to L1. Why are they separated?

Batcher collect and submit tx data into L1 with a batch, while proposer submits the commitments (output roots) to the L2's state, which finalizes the view of L2 account states. They are decoupled so that they can work in parallel for efficiency.

contracts-bedrock

Various contracts for L2 to interact with the L1: