An active-active cluster consists of two availability zones and one witness node.
The witness node does not belong to any availability zone and can be either a physical machine or a virtual machine. It only participates in leader election and runs critical services (such as ZooKeeper, MongoDB, NTPD, and Master Monitor), but does not store data.
Each availability zone contains at least 2 nodes, all of which are master nodes (i.e., nodes running ZooKeeper). Network connections exist between the two availability zones, as well as between the witness node and the two availability zones, as shown below:
Based on business requirements, one availability zone needs to be designated as the primary availability zone, while the other automatically becomes the secondary availability zone.
