Definition
A sequencer is a core logical role in many rollup and other Layer 2 architectures that is responsible for determining the canonical order of transactions. It aggregates user transactions into batches, assigns them a sequence, and forwards this ordered data to the underlying base chain, where it can ultimately be finalized. By centralizing or coordinating transaction ordering, the sequencer defines the state transition history for the Layer 2 system. In many designs, the sequencer is distinct from a validator on the base chain, even though both interact with the same underlying blockchain.
In rollup-based designs, the sequencer typically interfaces with a data availability layer to ensure that the ordered transaction data is published in a way that others can reconstruct the Layer 2 state. The sequencer’s ordering power can create opportunities for MEV, since it can choose how to arrange transactions within a batch. Different systems may implement a single sequencer, a committee of sequencers, or a decentralized protocol for sequencing, each with its own trust and liveness assumptions. Overall, the sequencer defines the temporal structure of Layer 2 activity while relying on the base chain for settlement and security guarantees.
Context and Usage
In practice, the term sequencer is most commonly associated with optimistic and zero-knowledge rollups that operate as Layer 2 extensions of a base blockchain. In these contexts, the sequencer is the entity that first accepts transactions, determines their order, and produces the rollup blocks or batches that represent Layer 2 state transitions. The base chain’s validators then process the sequencer’s submissions according to the rollup protocol, but they do not independently reorder the Layer 2 transactions.
Discussions about sequencers often focus on centralization, censorship resistance, and MEV extraction, because control over ordering can influence which transactions are included and in what sequence. Research on decentralized sequencing aims to distribute this ordering power among multiple parties, sometimes integrating with specialized data availability solutions. As Layer 2 ecosystems mature, the design and governance of sequencers have become central topics in evaluating the security and trust model of rollup-based systems.