Sequencer
Also: Rollup sequencer
The party that decides the order of transactions on a rollup before they are published to the chain underneath — usually one party, usually the operator.
A rollup executes transactions off the main chain and posts the results. Something has to choose what order those transactions ran in, and on almost every production rollup that something is a single service run by the team that built it.
Why it is the business model
Ordering is worth money — through fees and through the value of choosing what goes first — and sequencer revenue is where rollup economics actually live. That is not a criticism, but it does explain why decentralising the role has proved slower in practice than in roadmaps.
What the centralisation does and does not risk
- It cannot steal funds. Validity or fraud proofs constrain what state transitions are accepted, whoever ordered them.
- It can censor, delay and reorder. Every rollup with a centralised sequencer therefore needs a forced-inclusion path to the base layer, and the usability of that path is what decides whether the guarantee is real.
- It is a single point of failure. A sequencer outage stops the chain, and this has happened to most major rollups at least once.
The question to ask of any rollup is not whether the sequencer is centralised — it almost certainly is — but what a user can do, unilaterally and today, when it stops behaving.