matchIndex

matchIndex is a term used in distributed consensus protocols, most commonly Raft, to describe, for each follower, the index of the highest log entry known to be stored on that follower. It is a piece of leader-maintained state that tracks replication progress and is distinct from the follower’s own log boundary. For a given follower, matchIndex is non-decreasing as the follower catches up.

In Raft, each follower is tracked by two values: nextIndex and matchIndex. The matchIndex for a follower represents the highest log entry index that the leader believes has been replicated on that follower. This value is updated when the follower acknowledges successful reception and storage of log entries via AppendEntries RPC. If replication fails due to a conflict or missing entries, the leader may adjust nextIndex and retry, but matchIndex is only advanced upon a successful update.

A primary use of matchIndex is determining when new entries can be considered committed. The leader can advance its commitIndex to a larger value N when a majority of servers report a matchIndex of at least N and the log entry at N belongs to the current term. This majority-based rule ensures that committed entries are durable and replicated on or beyond a majority of servers before readers observe them.

Relation to persistence: matchIndex is typically volatile state kept by the leader and not persisted across leadership changes. After a new leader is elected, the system relies on follower logs and the new leader’s nextIndex/matchIndex values to re-establish replication progress.