CommitIndex

CommitIndex is a term used in distributed systems to denote the index of the highest log entry that is known to be committed across a group of servers. It is most commonly associated with replicated log protocols such as Raft, where each server maintains a log of commands and a state machine that applies those commands in order.

In a typical Raft-style setup, the leader maintains several state variables, including the commitIndex. A log entry becomes committed when it is stored on a majority of servers and certain term-related safety rules are satisfied. When the leader detects that an entry at index N, which has the current term, has been replicated on a majority of servers, it can advance commitIndex to N. Once commitIndex is advanced, all servers are expected to apply all log entries up to that index to their state machines in order.

Followers synchronize with the leader by receiving AppendEntries messages that carry new log entries and the leader’s commitIndex. Followers then advance their own commitIndex to the minimum of the leader’s commitIndex and their own last log index, and they apply all committed entries in sequence. This coordinated advancement ensures consistent application of commands across the cluster.

CommitIndex is typically distinct from the lastApplied index. lastApplied tracks the highest log entry already applied to the local state machine, while commitIndex tracks the highest entry known to be committed and safe to apply. The combination of these indices supports orderly, fault-tolerant progression of state across all participating servers.