Rollup Anatomy: Sequencer, Prover, Bridge

What you'll learn

• The Three-Component Mental Model
• The Sequencer: L2 Block Production
• The Prover (or Challenger): The Trust Anchor
• The Bridge Contract: State and Asset Custodian
• End-to-End: One Transaction Through All Three
• Rollups-as-a-Service and the Superchain

Key terms

Sequencer

The L2 entity that accepts user transactions, orders them, executes them against L2 state, and posts batches to L1. Today centralized on every major rollup; decentralization roadmaps (Espresso, Astria, native rollup paths) are in flight.

Prover

In ZK rollups, the system that generates a zero-knowledge proof attesting a batch of L2 transactions transitions the prior state root to the claimed new state root. The L1 verifier contract checks the proof in a single transaction.

Bridge contract

The L1 smart contract that stores the canonical L2 state root, locks deposits, and authorizes withdrawals. The single most security-critical component of any rollup — a bridge bug or compromised upgrade key endangers all locked assets.

Soft confirmation

A receipt issued by the sequencer immediately after transaction submission (typically 250-500 ms). Reliable in practice but secured only by the sequencer's policy commitment, not by L1 finality.

L1 confirmation

The state in which an L2 batch has been posted to and finalized on Ethereum L1. Once L1-confirmed, the batch cannot be censored or reverted by the sequencer; any honest verifier can replay it from L1 data.

BoLD (Bounded Liquidity Delay)

Arbitrum's permissionless fraud-proof protocol, enabled in 2024 on Arbitrum One. Allows any bonded party (not just a whitelisted validator set) to challenge a claimed state root.

Cannon

Optimism's MIPS-based fault-proof virtual machine. Allows any user to run a verifier and submit fraud proofs against an incorrect state root. Permissionless on mainnet since 2024.

Superchain

A federation of OP Stack rollups (Optimism, Base, Mode, Zora, Frax, Worldchain, Lisk, Soneium, Ink, and others) that share a sequencer protocol, fault-proof system, governance, and cross-chain messaging.

RaaS (Rollup-as-a-Service)

Companies (Conduit, Caldera, Alchemy Rollups) that deploy and operate production L2 chains for client teams, abstracting away sequencer hosting and infrastructure.

Arbitrum Orbit

Arbitrum's framework for spawning custom L2/L3 chains on the Arbitrum stack. Powers chains like Xai (gaming), ApeChain, and Sanko.