Tags
In addition to selecting the initial number of nodes (see How Many Servers to Use for a Cluster), SHIPOPS uses a limited set of tags for more precise resource planning: "control", "storage", and "worker".
Resource Placement Tags
Nodes with the "control" tag host administration and monitoring applications. Nodes with the "storage" tag host applications that use physical disk space, such as databases. Some "control" applications also use physical disk space and therefore are placed only on nodes with both tags.
Nodes with the "worker" tag host user applications. If a user application uses a physical disk, the node on which it will be placed must have the "storage" tag.
Until more precise resource planning is required, you do not need to think about tags: by default, every cluster node includes all tags.
System Status and Safety
It is important to account for the special status of nodes included in the cluster during its creation, when fault-tolerant or resource-saving mode was selected. In addition to the tags listed above, such nodes have the "system" status. Unlike regular tags, this status cannot be removed. System applications that manage the cluster are installed on these nodes.
System nodes cannot be intentionally removed or disabled, except for an emergency stop of one of the nodes in a fault-tolerant cluster.
The number of system nodes is always odd, which means that one of the nodes added during creation may not receive system status (see Assigning Node Status During Cluster Creation).
Nodes added to the cluster later are not system nodes. They can be disabled and enabled with a special SHIPOPS command. Below, they are referred to as "non-system" nodes.