Sequencer
What Is a Sequencer?
A sequencer decides which transaction comes first and packages activity for settlement. More specifically, it’s the component of a
layer-2 network, typically a rollup, that receives user transactions, decides their order, and groups them into batches before posting them to the underlying layer-1 blockchain.
How a Sequencer Works
On many
rollups, a user sends their transaction directly to the sequencer rather than to a public waiting area. The sequencer checks if the transaction is valid, assigns it a place in the order, and often executes it against the current layer-2 state right away. This lets it give the user a fast "soft confirmation" that the transaction has been accepted, well before it's finalized on layer 1.
The sequencer then combines many transactions into a single
block of compressed data and posts that batch to the layer-1 chain. Once the batch is recorded on layer 1, the order it set becomes part of the rollup’s permanent history. Bundling activity this way is a big part of how rollups keep fees low.
Trade-Offs to Understand
Because the sequencer controls ordering, it holds a lot of power. Many rollups today run a single, operator-controlled sequencer, which is efficient but raises concerns about centralization: if it goes offline, the network can stall, and whoever controls ordering could in theory extract
maximal extractable value (MEV) by reordering transactions. For this reason, decentralizing the sequencer, for example through shared or rotating sequencers, is an active area of development.
For everyday users, the sequencer is usually invisible: it's simply the reason transactions on a rollup feel fast and cheap. But it's worth knowing that this speed can depend on trusting a single operator to behave fairly, at least until more decentralized designs become widely used.