What is a sequencer?
- Author
- CRYPTO PORT Editorial
- Published
- Updated
- Reading time
- 5 min
In short
A sequencer receives transactions sent to a layer 2, decides the order in which they execute, and packs them into blocks. On most rollups a single operator runs it, so if it halts, new transactions stop being processed. Because ordering power is also profit power, who holds that role matters.
Key points
- Decides transaction order and batches them into blocks
- Usually run by a single operator on today's rollups
- If it stops, new transactions cannot be accepted
- Ordering power is also the source of MEV
Definition
The node that accepts transactions submitted to a layer 2, fixes their execution order, produces blocks, and posts the resulting data to layer 1.
On layer 1, many validators or miners collectively decide which transaction goes first. On today's major rollups, that job instead falls to a single sequencer run by the operating team. Centralising it is what lets the chain acknowledge a transaction within roughly 0.2 to 2 seconds of receiving it.
That acknowledgement, though, is only the sequencer's promise. The ordered data still has to be posted to layer 1 and pass through proving or the dispute process before it carries layer 1's guarantees. This two-stage structure is why a bridge deposit can look instantly complete on the L2 and still not be withdrawable.
If the sequencer goes down, whether through a fault or a compliance decision, the normal path for sending or swapping stops working. Most rollups therefore provide a fallback: submit the transaction to a layer 1 contract directly and have it force-included after a delay. The exact procedure and waiting time differ by chain.
Watch out for
- · While the sequencer is down, ordinary transfers and swaps simply do not go through
- · A 'complete' badge in the interface means the sequencer accepted it, not that layer 1 has settled it
- · Force-inclusion procedures differ by chain and can take hours or a full day to take effect