This section describes standby databases in general. The basic concept of a standby database is to set up a copy of the production database on a second hardware system, so greatly improving the overall reliability of the database service. The standby database is used if the production database fails.
With standby database solutions, you need to consider the effects on the SAP application hosts connected to the database, when the database fails and the database host changes.
If the two database hosts form a high availability cluster, it might be possible to use a standard switchover solution. For more information about related issues (such as how to configure the database, application hosts inside and outside the cluster and the database reconnect feature), refer to “See also” at the end of this section.
If the two database hosts do not form a cluster the effects on application hosts vary, depending on where they run:
· All application hosts run external to the two database hosts
After failure, all application hosts have to be restarted using a different profile to connect to the standby database. It is sufficient to set the profile parameter SAPDBHOSTto point to the new database host.
· SAP CI (central instance, including enqueue and message service) runs on the same host as the production database
If the database fails, both database and CI are restarted on the standby database host. At the very least, SAPDBHOST has to be different in the profile of the CI. You need to change several parameters in the profiles of all external application hosts and these have to be restarted.
If the standby database was set up using asynchronous transfer of transactions (that is, log file shipping), all users have to be made aware – after work resumes using the standby database – that transactions might be lost. Users also have to be told to check whether the last changes they made are still available. The following applies:
· If the last transaction of a user is committed to the standby database, all previous transactions for that user are definitely committed as well.
· If the last transaction for a user is missing, the user has to check back to find the last transaction that was successfully applied to the standby database. All work done after this transaction is lost and has to be repeated manually.
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