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Heartbeat Thread is an important component of a Raft Member. As the chracter of a Raft Member is alwayls one of LEADER, FOLLOWER, or ELECTOR(CANDIDATE), it utilizes Heartbeat Thread in three ways: as a LEADER, Heartbeat Thread sends heartbeats to followers; as a FOLLOWER, Heartbeat Thread periodicallly checks whether the leader has timed out; as a CANDIDATE, it starts elections and send election requests to other nodes until one of the nodes becomes a valid leader.

• HeartbeatThread: an abstarction of heartbeat-related transactions, and each RaftMember will have its own HeartbeatThread. When the associated member is a LEADER, the thread sends heartbeat to each follower periodically; if the member is a FOLLOWER, it checks the timestamp when the last heartbeat was received, and decide whether the leader has timed out; otherwise the member is an elector, the thread will start an indefinite election loop, asks for votes from other members in this Raft group, and break from the loop once itself or another node has become a leader.
• MetaHeartbeatThread: an extension of HeartbeatThread for MetaMember. It requries the the follower to generate a new identifier, if its identifier conflicts with a known node during the cluster-building phase. It also sends followers the partition table if it is required.
• DataHeartbeatThread: an extension of HeartbeatThread for DataMember. It includes the header node of its group so the receiver knows which group this heartbeat if from, and it includes the log progress of its associated MetaMember to ensure the leader of a data group also has up-to-date partition table.

RaftLogManager

RaftLogManager provides interfaces to append, commit, and get Raft logs, controls how Raft logs are organized in memory and persisted on disks, when to apply committed logs to the underlying IoTDB, and how to generate a snapshot before a certain log index. As each Raft group keeps its own log, each RaftMember has its own RaftLogManager.

• RaftLogManager: a basic RaftLogManager, which manages lastest logs in memory and colder logs on disks, providing interfaces such as append, commit, and get. It applies logs after they are committed, but how they are applied depends on the passed LogApplier. It provides no snapshot implementation, so sub-classes extending it should focus on their own definition of snapshots.
• MetaSingleSnapshotLogManager: an extension of RaftLogManager for MetaMembers, which provides MetaSimpleSnapshot containing storage groups, TTLs, users, roles, and partition tables.
• FilePartitionedSnapshotLogManager: an extension of RaftLogManager for DataMembers, which provides PartitionedSnapshot<FileSnapshot>, consisting of a map whose keys are slot numbers, and values are FileSnapshots. FileSnapshot contains timeseries schemas and hardlinked TsFiles (in the form of RemoteTsFileResource) of a slot.