A master node refers to a node that runs the ZooKeeper and Meta services. ZooKeeper provides a distributed consensus protocol that enables coordination among multiple tasks in a distributed system. Meta provides metadata management services and is primarily responsible for managing storage objects, data replicas, access control, and data consistency. In addition, data storage services such as Chunk also run on the master node, allowing them to provide data storage services for the cluster.
The master node assumes one ZooKeeper role and one Meta role, which can be classified into the following four types:
ZooKeeper Leader / ZooKeeper Follower
All master nodes in an ACOS cluster run the ZooKeeper service. Through a voting mechanism among nodes, one node is elected as the ZooKeeper Leader. The ZooKeeper Leader acts as the decision-making node of the ACOS cluster and is responsible for handling data write, modification, and deletion requests, as well as controlling data synchronization among nodes. Nodes that are not elected as the ZooKeeper Leader act as ZooKeeper Followers and are responsible for synchronizing ZooKeeper metadata.
At any given time, there is exactly one ZooKeeper Leader node in an ACOS cluster. If the ZooKeeper Leader node fails, the remaining ZooKeeper Follower nodes will elect a new ZooKeeper Leader through another round of voting.
Meta Leader / Meta Follower
Master nodes running the Meta service are classified as Meta Leader or Meta Follower.
Meta provides services in a clustered manner and relies on ZooKeeper service to designate one master node as the Meta Leader. At any given time, only one Meta Leader in the ACOS cluster provides Meta services externally, while Meta Follower nodes are responsible only for metadata synchronization. If the Meta Leader node fails, the Meta Follower nodes will elect a new Meta Leader.
