Election States
When building on Aeron Cluster, it helps to understand the state machine that the elections run through. This is especially useful when troubleshooting an Aeron Cluster issue as the election state for every node is available in near real-time via AeronStat.
Cluster Election State¶
| Value | Description |
|---|---|
| 0 | INIT. The node is preparing for a new leadership term |
| 1 | CANVASS. The node is checking connectivity between nodes, which is used to ensure a successful election is possible. This also prevents leadership bouncing between nodes with partial connectivity - see Coracle: Evaluating Consensus at the Internet Edge |
| 2 | NOMINATE. Node is nominating a leader |
| 3 | CANDIDATE_BALLOT. Node is checking if other nodes will accept it as leader |
| 4 | FOLLOWER_BALLOT. Follower has voted for a leader |
| 5 | LEADER_LOG_REPLICATION. [Leader only] Leader is waiting for followers to replicate any missing log entries |
| 6 | LEADER_REPLAY [Leader only] Leader is replaying the local log |
| 7 | LEADER_INIT [Leader only] Leader is initialising internal state for new leadership term |
| 8 | LEADER_READY [Leader only] Leader is ready, and is awaiting followers to mark themselves FOLLOWER_READY |
| 9 | FOLLOWER_LOG_REPLICATION [Follower only] Follower is replicating missing log entries from the leader |
| 10 | FOLLOWER_REPLAY [Follower only] Follower is replaying the local log in preparing for following a new leader |
| 11 | FOLLOWER_CATCHUP_INIT [Follower only] The node is preparing for receiving a replay from the leader to catch up in current term |
| 12 | FOLLOWER_CATCHUP_AWAIT [Follower only] The node is awaiting a join from the leader for catch up |
| 13 | FOLLOWER_CATCHUP [Follower only] Follower is replaying the log in order to catch up to leader's live position and be able to join the live log |
| 14 | FOLLOWER_LOG_INIT [Follower only] Follower is initializing internal state to join the live log on the leader |
| 15 | FOLLOWER_LOG_AWAIT [Follower only] Follower's clustered service is processing the log, and the consensus module is awaiting this in order to join the live log of the leader |
| 16 | FOLLOWER_READY [Follower only] Follower is ready, and published the append position to the leader to let the leader know the follower is ready for the new term |
| 17 | CLOSED The election is closed |
See also:
State diagram¶
Info
This is diagram is correct as of Aeron 1.37.0.
Note the 3 node roles are highlighted in the 3 sections (Follower, Candidate and Leader)