Module openraft::docs::data::effective_membership

Effective membership

The membership config log in Openraft becomes effective upon being viewed, resulting in the latest log being the effective membership.

As a result, the effective membership in Openraft is unstable until it is committed, as uncommitted logs may be overwritten by a new leader. Therefore, Openraft will revert membership config to the previous membership config log, the second-to-last entry, when it conflicts with the leaders’ logs.

Raft prohibits the proposal of a new membership config until the previous one has been committed. Therefore, the Raft Engine only needs to maintain two membership configurations at most: the last one that has been committed and the effective one.

The second to last membership log has to be committed.

An alternative solution is to make the membership config log effective only after it has been committed. However, this does not simplify the problem, as followers still need to keep track of the last two membership config logs.

In Openraft, the membership state contains these two configs:

pub struct MembershipState<NID: NodeId> {
    pub committed: Arc<EffectiveMembership<NID>>,
    pub effective: Arc<EffectiveMembership<NID>>,
}

If conflicting logs that contain a membership configuration log need to be deleted on a Follower/Learner, it is necessary to revert at most one membership configuration to the previous one. This involves discarding the effective configuration and making the last committed configuration effective.