precommits

Precommits are a type of validator message used in some Byzantine fault-tolerant (BFT) consensus protocols, notably Tendermint. A precommit expresses a validator’s readiness to commit a proposed block to the blockchain, serving as a key step toward finality.

In these protocols, consensus proceeds through rounds with distinct voting stages. After a block proposal and an initial voting phase (often called prevote), validators may send a precommit for a specific block at a given height if certain conditions are met, such as having observed a valid proposal and being locked on that block. A block gains finality when it receives a supermajority of precommits, meaning more than two-thirds of voting power have expressed commitment for that block. Once this threshold is reached, the block is considered finalized and the chain can advance to the next height.

Precommits help prevent forks by requiring broad agreement before a block can be committed. They are part of a locking mechanism: if a validator is locked on a block, it will only switch to a different candidate block if certain safety conditions are satisfied, which reinforces consistency across honest nodes.

If no block attains the required precommit quorum within a round, the protocol can repeat rounds or switch locks, potentially with new proposals. While the precommit stage is central to finality in Tendermint-like systems, other BFT protocols use conceptually similar stages with different terminology, such as prepare and commit messages in PBFT.

See also: Byzantine fault tolerance, Tendermint, PBFT, finality, consensus protocol.