Siemens Simatic PCS 7 Function Manual page 98

High availability solutions in PCS 7
4.6 Solutions for OS clients
Preferred server
A "preferred server" is an OS server in the redundant OS server pair that is preferred by the
OS client for a connection. A preferred server can be defined separately for each OS client in
order to ensure permanent operability. The distribution of the OS clients between the OS
servers distributes the loads and increases the performance of the system as a whole.
Operating principle
If the active OS server fails, the process values on all of the connected OS clients are no longer
updated and there is no operator control on these OS clients during the switchover. Other OS
clients that are connected in parallel to the redundant OS partner server are not affected by
this. The plant operator can therefore change to these OS clients if needed.
Generally, the following applies: The OS clients always connect to the specified preferred
server if it is available. If it is not available, the OS clients automatically connect to its redundant
OS partner server. If you do not specify a preferred server for an OS client, it connects to the
OS server that has the master identification.
When the failed OS server comes online again, the OS client automatically reconnects to its
preferred server. The master identification of the OS server does not change even when the
failed OS server comes back online.
Additional information
● Section "How to configure an OS client for permanent operability (Page 165)"
● Online help for
98
WinCC
High Availability Process Control Systems (V9.0)
Function Manual, 05/2017, A5E39221836-AA