As ConfigMgr does not provide data in real time, short intermittent down times should not usually be considered a problem.
Note
ConfigMgr does not support any high availability (HA) cluster solution for the application node other than switching clients to a different ConfigMgr server. However, you might use SQL clustering or a feature that started to be supported with ConfigMgr 1602 by Microsoft--Always On availability groups for SQL--to implement HA on the database level.
Always On availability groups continuously synchronize transactions from the primary replica to each of the secondary replicas. This replication can be configured as synchronous or asynchronous to support local high availability or remote disaster recovery.
The preceding mechanism cannot be used for secondary site databases, and secondary site databases cannot be restored from the backup--this applies only to the central administration site and the primary site. The only way to recover the secondary site is to recreate it from its parent--the primary site.
Maintaining the central administration site and more than one primary site allows the redirecting of clients to the other server while the first one is inaccessible. The same is the case with management points and distribution points.
By configuring the sites to publish the data about the site servers, and services in Active Directory and DNS, it becomes available for clients to identify when new site system servers that can provide important services, such as management points, are available.