What is a sequencer?
A sequencer is the rollup component responsible for ordering transactions and posting them to L1. Every major rollup (Arbitrum, Optimism, Base, zkSync, StarkNet) currently operates with a centralised sequencer — typically run by the rollup’s development team. The sequencer represents the single most concentrated point of control in the rollup stack.
What sequencers do
Four core functions. (1) Transaction ordering — accept user transactions and determine execution order. (2) Pre-confirmation — provide near-instant confirmation to users (centralised trust). (3) State computation — execute transactions to produce updated state. (4) L1 posting — submit batched data to L1 with state assertion (optimistic) or proof (ZK).
The centralisation problem
Centralised sequencers introduce several risks. (1) Censorship — sequencer can refuse to include specific transactions. (2) MEV extraction — sequencer captures all front-running and back-running value. (3) Liveness — sequencer downtime halts the rollup. (4) Single point of failure — sequencer compromise affects entire L2. These problems collectively undermine the “credibly neutral” guarantees of L2 systems.
Decentralisation paths
Three approaches being explored. (1) Shared sequencers — Espresso, Astria, Radius operate sequencer-as-a-service for multiple rollups, decentralising the sequencer role across many operators. (2) Rotating sequencers — protocol-defined rotation among permissioned operators. (3) Based rollups — sequencing inherited from Ethereum L1 itself, eliminating the rollup-level sequencer.
Force inclusion mechanisms
All major rollups include “force inclusion” — users can submit transactions directly to L1 if sequencer censors them. The trade-off: force inclusion is slower (typically 24+ hours) and more expensive. Force inclusion exists as a censorship-resistance backstop rather than primary UX path.
Türkiye context
For Türk Web3 developers and users, the centralised sequencer is the most important risk to understand when assessing L2 dependencies. Türk consumer products built on rollups should plan for occasional sequencer downtime. Türk regulators evaluating L2 systems may scrutinise sequencer arrangements as effective centralised intermediaries with CASP-style licensing implications.
Related: Optimistic Rollup, ZK Rollup, MEV, Data Availability.