Sequencer Client

The client's primary function is to receive transactions from users, order them into a sequence, and batch them for submission to the base layer (L1). This ordering is critical for determining the final state and preventing double-spends. The client may use algorithms like First-Come-First-Served (FCFS) or more complex mechanisms for MEV (Maximal Extractable Value) management.

The client maintains a local copy of the rollup's state (e.g., account balances, smart contract storage). It executes all transactions in the order it creates, updating this state to compute the new state root. This execution must be deterministic and identical to that performed by other nodes for the system to remain secure.

After creating a batch, the client must publish the transaction data to a data availability layer, typically the L1 Ethereum blockchain. This ensures the data is accessible for anyone to reconstruct the rollup state, a requirement for fraud proofs or validity proofs. Methods include calldata posting or using dedicated data availability committees (DACs).

In a ZK-Rollup, the sequencer client is responsible for generating a cryptographic validity proof (e.g., a ZK-SNARK or ZK-STARK) that attests to the correctness of the state transition. This computationally intensive process proves the batch execution was valid without revealing the underlying data, enabling fast finality on L1.

The client interacts directly with smart contracts on the base layer (L1). Key interactions include:

• Submitting batches and state roots to the rollup's core contract.
• Processing deposits from L1 into the rollup.
• Facilitating withdrawal requests by including exit proofs in batches.

The client participates in a P2P network with other rollup nodes (e.g., other sequencers, full nodes, validators). This network is used to:

• Receive raw transactions from users and other nodes.
• Propagate newly created batches.
• Sync the latest rollup state and history.

In most current rollups, the sequencer is a single, centralized entity. The client software connects to this specific sequencer, creating a critical vulnerability. If the sequencer goes offline or acts maliciously (e.g., censoring transactions), the entire network's liveness is compromised. This centralization is a primary security concern addressed by decentralized sequencer sets.

A centralized sequencer client can theoretically exclude transactions based on content or origin. True censorship resistance requires a permissionless, decentralized sequencer network where multiple independent parties run clients to propose blocks. Mechanisms like MEV auction houses or proof-of-stake validator sets are proposed solutions to mitigate this risk.

The client's role in posting transaction data to the base layer (e.g., Ethereum) is crucial. If data is withheld (data availability failure), users cannot reconstruct state or submit fraud proofs. Clients must ensure data is published to a data availability layer (like Ethereum calldata or a dedicated DA chain) to enable trustless verification and escape hatches.

Similar to Ethereum's execution and consensus clients, a healthy rollup ecosystem requires multiple, independently developed sequencer clients (e.g., different implementations in Rust, Go, or Nim). This client diversity reduces the risk of a single software bug halting the network and prevents a single team from having excessive control over the protocol's evolution.

• Permissioned Sequencer: A whitelisted entity runs the client. This offers speed and simplicity but sacrifices decentralization.
• Permissionless Sequencer: Anyone can run a client and participate in sequencing via a cryptographic protocol (e.g., PoS). This enhances security but adds complexity. The choice defines the rollup's security trust model.

A critical security feature for users is the ability to bypass a malicious or offline sequencer. Clients must support force inclusion protocols, allowing users to submit transactions directly to the base layer contract. This enables users to exit the rollup with their funds even if the primary sequencer client is censoring them, preserving self-custody guarantees.

The central node or service responsible for ordering transactions within a rollup or Layer 2 network. It receives raw transactions, arranges them into a sequence, batches them, and generates cryptographic proofs. This is the primary function a sequencer client software implements.

A network participant, often the same entity as the sequencer, that submits the state root and compressed transaction data to the base layer (e.g., Ethereum). The proposer's role is critical for finalizing the rollup's state and enabling trustless withdrawals.

A fault-tolerant sequencing model where the right to order transactions is distributed among multiple parties, using mechanisms like Proof-of-Stake (PoS) or leader election. This contrasts with a single, centralized sequencer and aims to improve liveness and censorship resistance.

A safety mechanism that allows users to bypass a censoring or inactive sequencer by submitting transactions directly to a contract on the base layer after a timeout. This is a critical liveness guarantee in rollup designs, enforced by the protocol.