3.2.4 Controlling the server switchover order
You can control the order in which the servers in a server group are to be terminated during a hot standby operation, which will also be the order in which the servers are started on the target host. To control these orders, you group multiple servers together and then define parent-child relationships among those servers that correspond to the desired order.
Server switchover order control is applied only after a server failure or host failure has occurred and when a grouped-system switchover is to be performed during a planned hot standby operation. Server switchover order cannot be controlled during normal server start and termination (the servers are started and terminated regardless of a specified start or termination order).
Note that if a server in the monitor mode is using the program management function, the switchover order among the UAPs and programs managed by the server cannot be controlled.
- Organization of this subsection
(1) Correspondence between server start order and parent-child relationships
This subsection explains how to define parent-child relationships for purpose of determining a server start order.
A server required in order to start another server is called a parent server. A server that is started after the parent server has started is called a child server.
Servers are started in order starting with the parent server. When the parent server has been started, its child servers are started. The termination order is the reverse of the start order. A parent server's termination processing begins when all its child servers have been terminated.
The figure below shows the parent-child relationships and start order for a server group. For example, if you want to start server1, server2, server3, and server4 in this order, their parent-child relationships are as shown in the figure.
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![[Figure]](GRAPHICS/F0000048.GIF)
In this example, server1 is the parent server of server2, and server2 is the parent server of both server3 and server4.
Take the following into account with respect to server parent-child relationships:
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A child server can have only one parent server. You must not define more than one parent server for each child server. On the other hand, a parent server can have multiple child servers, so you can define multiple child servers for a single parent server.
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You can assign to a server group servers for which no order relationship is defined.
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If the server group contains a resource server, the resource server is the highest parent server.
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While you can control the start and termination orders for the servers in a group, you cannot define order control among groups when there are multiple groups.
For details about the processing flow for controlling server switchover order, see 4.1.5 Processing flow for order control of server switchover.
(2) Required environment settings
You must define the server grouping settings in the group operand in the server environment definition beforehand.
When you define parent-child relationships for multiple servers in a group, be sure to take into account the desired server start order. You specify the parent operand in the server environment definition to define a parent-child relationship. There is no requirement to specify a parent operand for all servers.
You use the command for displaying the server order control status (monodrshw command) to check the defined start order (by checking the parent-child relationships within the group).