Zookeeper

Zookeeper makes sure all the sub-servers (e.g. DataNode in HDFS) report to a single master server (e.g. NameNode in HDFS).

It provides the consistency to the distributed system.

ZAB (Zookeeper Atomic Broadcast)

ZAB is the core algo of Zookeeper.

According ZAB, there is a leader among all the servers.

When there is a write operation, the leader server will propose this operation to all the follower servers. If there are multiple write request arrive at the same time, the leader must select only one of them and propose.

If the follower server agrees with this write opertion, it will send an ACK signal to the leader server.

If all the follower servers agree, the leader server will ask all the followers to commit this write operation.