Sequencer
The component that orders and batches a rollup's transactions — often centralized today, and a key risk to understand.
A sequencer is the entity in a rollup that receives users’ transactions, orders them, and batches them for posting to the base chain. It’s what gives rollups their fast, cheap user experience — the sequencer confirms transactions quickly before they settle on Layer 1.
Here’s the catch most rollup users don’t realize: nearly all major rollups currently run a single, centralized sequencer operated by the rollup team. That operator could theoretically censor or reorder transactions (extracting MEV), and a sequencer outage halts the chain. Rollups include escape hatches (forcing transactions directly through L1) and are working toward decentralized sequencing, but for now the sequencer is a real centralization point behind the “secured by Ethereum” branding.