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Class Functionalities

Program Entry Point

Cluster Main ClusterMain is the main class in the cluster module, which performs some developer-level customization and start a MetaClusterServer, which will further initialize the Raft logics and the underlying IoTDB.

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• RaftMember: an abstraction of common Raft procedures, like log generation, log replication, heartbeat, election, and catch-up of falling behind nodes.
• MetaMember: an extension of RaftMember, representing a member in the MetaGroup, where cluster configuration, authentication info, and storage groups are managed. It holds the partition table, processes operations that may modify the partition table, and also acts as a coordinator to route database opertions to responsible DataMember with the help of its partition table. One node has only one MetaMember.
• DataMember: an extension of RaftMember, representing a member in one DataGroup, where timeseries schema schemas and timeseries data of one cluster-level partition are managed. It also handles data queries and thus manages query resources. One node may have multiple DataMembers, depending on how many data partition it manages, which is further decided by the number of replications and partitioning strategy.

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Partition Table

Partition Table maintains the mapping from a partition key (storage group name and time partition id in IoTDB) to a DataMember(or a node). As timeseries schemas and data are replicated only in a subset of the cluster (i.e., a DataGroup), opertions involving them can only be applied to specific nodes. When a node receives an operation, and it does not manage the corresponding data or schemas of the operation, the node must forward it to the right node, and this is where a Partition Table applies.

• SlotPartitionTable: a consistent-slot-hashing-based Partition Table, which divides the whole data into a fixed number of slots (10000 slots by default), distributes the slots evenly to all DataGroups, hashes the partition key to one of the slots, and the data holder will be the holder of the slot. Upon the DataGroups are updated, the slots are re-distributed and the changes are recorded for data migration.

Heartbeat Thread

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.