Skip to main content
Important: We do not provide financial advice or custody funds. All transactions occur on third-party platforms.
CryptoBipto
Log in

Rollup Anatomy: Sequencer, Prover, Bridge

The three structural components of every rollup and the data flow that ties them together — from L2 transaction submission, through batching and L1 posting, to bridge state updates.

35 min · expert · part of Rollup Architecture & L2 Engineering

The Three-Component Mental Model

Strip away the marketing and every rollup — Arbitrum, Optimism, Base, zkSync Era, StarkNet, Polygon zkEVM, Linea, Scroll, Taiko — reduces to three components: a sequencer, a prover, and a bridge. The sequencer is the engine that accepts user transactions on L2, orders them, and executes them against L2 state. It is essentially the L2's block producer. The prover (in ZK rollups) or the challenge protocol (in optimistic rollups) is the trust-anchor: it convinces the L1 contract that the sequencer's claimed state is correct. The bridge is the L1 smart contract that holds the canonical L2 state root, processes deposits in, and authorizes withdrawals out. Understand those three boxes and the arrows between them, and you understand every rollup in production today. The differences between Arbitrum and zkSync — or between Base and Linea — are mostly differences in how each box is implemented, not in the topology itself. This lesson walks through each component in turn, then traces an end-to-end transaction through all three. By the end you should be able to look at any L2 explorer (Arbiscan, Optimistic Etherscan, Basescan, zkSync Explorer) and identify which transaction is a sequencer batch post, which is a state root assertion, which is a deposit, and which is a withdrawal claim.

Also in this lesson

  • 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.

Continue this lesson — 5 more sections in the CryptoBipto app.

Open lesson

Educational only — not financial advice.