sequencer

A sequencer is the actor on a rollup that receives users' transactions, decides their order, executes them, and batches the results to be posted to the base layer. It is the rollup's traffic controller and the reason a Layer 2 feels fast: when you submit a transaction to Arbitrum or an OP-Stack chain, the sequencer gives you a near-instant 'soft confirmation' (a promise about the ordering) within a fraction of a second, long before the batch is settled on Ethereum. Almost every major rollup today runs a single sequencer operated by the rollup team.

That centralization is the chief criticism, and it has two faces. A single sequencer can censor — refusing to include certain users' transactions — and it can extract value by reordering transactions to its own advantage (a form of MEV), since it alone chooses the order. It is also a liveness risk: if the sole sequencer goes offline, the rollup stops producing new blocks until it recovers, as has happened during real outages. Importantly, a sequencer generally cannot steal funds or forge invalid state, because the rollup's fraud or validity proofs still constrain correctness; its power is over ordering and inclusion, not over the rules.

The standard safety valve is forced inclusion. A well-designed rollup lets a user bypass a misbehaving sequencer by submitting their transaction directly to the rollup's contract on Ethereum; after a delay, the protocol obliges the rollup to include it, guaranteeing censorship resistance and the ability to exit even if the sequencer refuses to cooperate. The roadmap beyond this is to decentralize sequencing itself — through shared sequencers, sequencer sets with their own consensus, or based rollups that hand ordering to L1 proposers — so that no single party holds this chokepoint.