A sequencer is a component that determines transaction order in systems such as Layer 2 rollups. It receives transactions and arranges them for execution in rollup blocks. This ordering matters because a swap, liquidation, or other contract interaction can produce different results depending on which transaction comes first.
Part of a larger pipeline
A sequencer can offer a quick indication that a transaction has entered the proposed sequence. That indication is not automatically final settlement on the underlying blockchain.
Execution, batch publication, and state commitments are related tasks that may be handled by separate components. In the OP Stack, for example, a batcher compresses and publishes transaction data to Ethereum. Distinguishing these roles avoids assuming the sequencer alone proves every state change or provides data availability.
Availability and ordering power
Some designs use a single active sequencer; others distribute sequencing responsibilities. Concentrated control can simplify coordination but introduces outage, censorship, and transaction-ordering risks, including opportunities to extract value from ordering.
Certain rollups provide an alternative route through the base chain when the sequencer is unavailable or excludes transactions. The conditions, costs, and delays are protocol-specific. Such a route should not be assumed to provide immediate inclusion or instant withdrawals.