Sequencers

The sequencer is the node that orders incoming L2 transactions, executes them, and produces batches for L1. On most rollups today — Arbitrum, Base, Optimism — a single operator runs the sequencer. That delivers fast, predictable confirmation but introduces a centralization tradeoff.

A centralized sequencer can reorder or delay transactions — a form of MEV on L2. It cannot steal funds because state transitions must still be valid when posted to L1, but it can degrade user experience or extract value through ordering.

Research is active on shared sequencers, based rollups where L1 validators order L2 blocks, and decentralized sequencer networks. Today's pragmatic reality: sequencers are centralized, but the escape hatches to L1 keep the security model honest.