Module openraft::docs::data::vote

Vote

struct Vote<NID: NodeId> {
    leader_id: LeaderId<NID>,
    committed: bool,
}

In Openraft, the Vote defines the pseudo time which determines the leader in a distributed consensus. Essentially, each vote represents a distinct time point, similar to the concept of rnd or ballot-number in Paxos.

In a standard Raft, the corresponding concept is (term, voted_for: Option<NodeId>).

The validity checking for RPCs in Openraft, such as when handling vote or append-entries requests, is straightforward. Essentially, a node will grant a Vote only if it is greater than or equal to the last Vote it has seen. Refer to the PartialOrd implementation for Vote. The pseudocode about vote order checking is as follows:

fn handle_vote(vote: Vote) {
    if !(vote >= self.vote) {
        return Err(())
    }
    save_vote(vote);
    Ok(())
}

Partial order

Vote in Openraft is partially ordered value, i.e., it is legal that !(vote_a => vote_b) && !(vote_a <= vote_b). Because Vote.leader_id may be a partial order value:

Openraft provides two election modes.

• the default mode: every term may have more than one leader.
• and the standard Raft mode: every term has only one leader(enabled by single-term-leader),

The only difference between these two modes is the definition of LeaderId, and the PartialOrd implementation of it. See: leader-id.

Vote and Membership define the server state

In the default mode, the Vote defines the server state (leader, candidate, follower or learner). A server state has a unique corresponding vote, thus vote can be used to identify different server states, i.e, if the vote changes, the server state must have changed.

Note: follower and learner in Openraft is almost the same. The only difference is a learner does not try to elect itself.

Note: a follower will switch to a learner and vice versa without changing the vote, when a new membership log is replicated to a follower or learner.

E.g.:

• Node-2 with vote (term=1, node_id=2, committed=true):

  • is a leader if it is present in config, either a voter or non-voter.
  • is a learner if it is absent in config.

• Node-2 with vote (term=1, node_id=2, committed=false):

  • is a candidate if it is present in config, either a voter or non-voter.
  • is a learner if it is absent in config.

• Node-3 with vote (term=1, node_id=99, committed=false|true):

  • is a follower if it is a voter in config,
  • is a learner if it is a non-voter or absent in config.

For node-2:

vote \ membership	Voter	Non-voter	Absent
(term=1, node_id=2, committed=true)	leader	leader	learner
(term=1, node_id=2, committed=false)	candidate	candidate	learner
(term=1, node_id=99, committed=true)	follower	learner	learner
(term=1, node_id=99, committed=false)	follower	learner	learner