class

Raft::StateMachine

Abstract base class for user-provided state machines.

Implement this class to define the application logic that is replicated across the Raft cluster. All methods must be deterministic — given the same sequence of commands, every replica must produce identical state.

class KVStore < Raft::StateMachine
  def apply(command : Bytes) : Bytes
    # parse and execute command, return result
  end

  def snapshot : Bytes
    # serialize entire state
  end

  def restore(io : IO) : Nil
    # replace state from snapshot
  end
end

Instance methods

apply(command : Bytes) : Bytes

Applies a committed command to the state machine and returns a response.

Called by the Raft node after a log entry is committed (replicated to a majority). The returned bytes are sent back to the client that proposed the command via Node#propose.

Source

apply(commands : Array(Bytes)) : Array(Bytes)

Applies a batch of committed commands and returns their responses in order.

Override this for bulk optimizations (e.g., a single DB transaction per batch). The default implementation calls apply for each command individually.

Source

restore(io : IO) : Nil

Replaces the current state from a snapshot.

Called when this node receives a snapshot from the leader because it is too far behind to catch up via normal log replication.

Source

snapshot

Serializes the entire current state for snapshotting.

Called when the leader needs to send a snapshot to a lagging follower via the InstallSnapshot RPC.

Source