Sequencer

In the context of Layer 2 networks — particularly Optimistic Rollups and ZK-Rollups — a sequencer is a special node responsible for receiving user transactions, ordering them, and batching them together before submitting them to the Layer 1 blockchain. The sequencer is what gives Layer 2 networks their speed: rather than waiting for the slow and expensive Layer 1 to process each transaction individually, the sequencer handles fast preliminary ordering and provides users with near-instant transaction confirmations.

When you transact on a Layer 2 like Arbitrum or Optimism, your transaction is sent to the sequencer first. The sequencer immediately acknowledges it, gives you a soft confirmation (your balance updates right away), and then queues your transaction along with many others to be batched and eventually posted to Ethereum. The major concern with sequencers today is centralization — most production Layer 2 networks currently operate with a single sequencer controlled by the team that built the network. This creates a potential point of failure: a single entity could theoretically block certain transactions or go offline and halt the network. Decentralizing the sequencer is a major engineering challenge that most Layer 2 teams are actively working to solve.

Example: Think of the sequencer like a deli counter attendant who takes your order and hands you a numbered ticket with a confirmed wait time. You know your order is in the queue and approximately when it will be ready — even though the kitchen (the Layer 1) has not processed it yet. The attendant provides immediate acknowledgment and manages the ordering of all requests, while the kitchen handles the actual final preparation.