What is sequencer?

The component of a rollup that receives transactions, orders them, and publishes the result, giving users near-instant confirmations before the batch settles on the base chain.

Most production rollups run a single sequencer operated by the development team, which is fast and cheap and also a central point of failure and censorship. If it stops, the chain stops accepting new transactions, which is why credible rollups also provide a way to submit transactions directly to the base layer. Decentralizing the sequencer is an open problem across the sector. See rollup and layer 2.

The word reaches ordinary users mainly through outage notices. When a layer 2 network is described as down, it is almost always the sequencer that stopped, not the base chain and not the funds.

How it works

A transaction sent to a rollup goes to the sequencer rather than to a public mempool. The sequencer picks an order, executes against the rollup's state, and returns a soft confirmation within a second or two. That confirmation is a promise from one operator, not a settled fact.

Separately, the sequencer batches transaction data and posts it to the base chain. Since EIP-4844 that data usually travels as a blob rather than as calldata, which is what made rollup fees fall sharply (source: EIP-4844). Only once the batch is on the base chain, and once the rollup's proof or challenge period has run, is the state final in the sense that matters for withdrawals.

Because the sequencer alone decides ordering, it captures the value that ordering is worth. Some rollups run first-come-first-served ordering to limit this, some auction the right, and some publish a policy and ask to be held to it. See mev.

The escape hatch is forced inclusion. A rollup with a credible one lets you submit a transaction to a base-chain contract that the rollup must eventually process, so a sequencer that censors you delays you rather than traps you. Whether that hatch exists, and how long it takes, is documented per network in each rollup's own documentation.

Example

Illustrative timings of the sort a rollup publishes. You swap on a layer 2 and see a confirmation in about two seconds. The sequencer posts your batch to Ethereum roughly a minute later, and the batch is included in an Ethereum block. If the network is an optimistic rollup with a seven-day challenge window, a withdrawal back to Ethereum initiated at that moment completes seven days later. The two-second confirmation and the seven-day withdrawal are both true, and they answer different questions.

Why it matters when you buy

If you buy on an exchange and withdraw to a layer 2 to save on fees, the sequencer is the party between you and your funds until the batch settles. Compare what each chain charges and how it settles on the chain pages, and check on the exchange pages whether your venue supports direct deposits and withdrawals on that network rather than only on the base chain.

Related terms

• rollup — the design a sequencer serves
• layer 2 — the broader category
• optimistic rollup — the challenge-window variant
• zk rollup — the validity-proof variant
• mev — the value in choosing the order
• finality — when the result stops being provisional

Questions

What happens to my funds if a sequencer goes offline?

They stay on the rollup and remain provable against data already posted to the base chain. Networks with a forced-inclusion path let you exit through the base chain directly, though it is slower than the normal route.

Is a single sequencer a security risk?

It is a liveness and censorship risk rather than a theft risk on a rollup that posts its data to the base chain. The operator can stop or reorder your transaction. It cannot forge a state the proof system would reject.

Why are layer 2 fees so much lower?

Costs are shared across everyone in a batch, and since EIP-4844 the batch travels as blob data priced separately from ordinary base-chain gas. See blob transaction and gas.