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Fig.1 shows the class diagram of important classes in the cluster module. For simplicity, class members and class methods are omitted here as they are just too many. Helper classes and utility classes are also ignored, so you would find much more classes in the cluster module.

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Class Functionalities

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• MetaClusterServer: listens on to internal_meta_port, receives meta-groupMetaGroup-related Raft requests, and forwards them to MetaMember. It also starts the underlying IoTDB once it is up.
• MetaHeartbeatServer: listens to internal_meta_port, receives MetaGroup heartbeats, and forwards them to MetaMember.
• DataClusterServer: listens on to internal_data_port, receives data-groupDataGroup-related Raft requests, decides which DataMember will process it and forwards to it.
• DataHeartbeatServer: listens to internal_data_port + 1, only receives DataGroup heartbeats, decides which DataMember will process it and forwards to it.
• ClientClusterServer: listens on to external_client_port, receives database operations from clients, and forwards them to MetaMember.

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