Sequencer
A sequencer is the component of many Layer 2 networks that collects users' transactions, puts them in order, executes them quickly and prepares them to be posted to the base chain in batches.
What a sequencer does
When you send a transaction on a rollup, it usually goes to the sequencer instead of to a public mempool. The sequencer decides the order of transactions, executes them and gives you a fast confirmation, often within a second or two. It then compresses many transactions into a batch and posts the data to the base chain, such as Ethereum, where it becomes part of the permanent record.
Soft and hard confirmation
The quick confirmation from the sequencer is a promise, sometimes called a soft confirmation. The transaction is only truly settled once its batch is posted to the base chain and, depending on the rollup type, the challenge period has passed or a validity proof has been verified. In normal operation the two match; the difference matters if the sequencer misbehaves or fails.
Why centralization is an issue
Most rollups today run a single sequencer operated by the team behind the project. That makes it fast and simple, but it creates a single point of failure. If the sequencer goes down, the rollup can stop processing transactions until it is back. A sequencer can also delay or censor specific transactions, and since it controls ordering, it is in a position to extract MEV, value from reordering trades.
Many rollups provide an escape route: users can submit transactions directly to the base chain to force inclusion after a delay. Projects are also working on shared or decentralized sequencers, but this is still developing.
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Frequently asked questions
Can a sequencer steal my funds?
In a functioning rollup, no: it cannot forge signatures or post invalid state that survives fraud or validity proofs. It can, however, delay, reorder or refuse transactions.
What happens if the sequencer goes offline?
New transactions on the rollup may pause. Funds stay safe, and many rollups let users force transactions through the base chain, though this is slower and more expensive.
Is a sequencer the same as a validator?
No. A validator helps secure a blockchain's consensus; a sequencer orders transactions for a rollup that relies on another chain for final security.
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