Leader & Follower

A single m3aggregator node for every shardset is elected to be a leader. Both leader and follower nodes are receiving the writes and performing the aggregation. The main difference between the leader and the follower is that the leader node is responsible for flushing (persisting) the data it has aggregated (see Flushing for more details). The follower is standing by ready to take over flushing in case the current leader fails.

Election state of the node can be checked by using /status endpoint:

curl http://localhost:6001/status
  "status": {
    "flushStatus": {
      "electionState": "follower",
      "canLead": true

canLead field is set to true on the follower node when that node is ready to take leader role without data loss. For that, the node must have accumulated all the data since the last flush done by the current leader (for the shardset that it owns).

canLead is always true on leader nodes.