Sequencer

A sequencer is the component of a Layer 2 rollup that receives transactions from users, orders them, executes them against the L2 state, and batches the results for eventual posting to Layer 1. In most current L2 deployments (Arbitrum, Optimism, Base, zkSync Era), the sequencer is operated by a single centralised entity — the L2's development company.

The sequencer provides fast 'soft' finality: once the sequencer includes your transaction in a batch (typically within seconds), you get a receipt and can proceed. Hard finality — guaranteed by the L1 — comes minutes to hours later when the batch is posted and proved on Ethereum. The sequencer's speed advantage is a major reason L2 transactions feel nearly instantaneous.

Centralised sequencers introduce risks: a sequencer can theoretically censor transactions (refuse to include them), reorder transactions to extract MEV, or go offline causing network downtime. Arbitrum, Optimism, and others have plans for decentralised sequencing — distributing the role among multiple validators — but as of 2025, most major L2s still rely on single sequencers operated by their teams. Forced-inclusion mechanisms allow users to submit transactions directly to L1 if the sequencer censors them.