Consider the following issues when an HDR failure occurs:
If a real failure occurs (not just transitory network slowness or failure), you probably want clients that are using the failed database server to redirect to the other database server in the pair. For instruction on how to redirect clients, see Redirection and Connectivity for Data-Replication Clients.
Which administrative actions to take after an HDR failure depends on whether the primary database server or the secondary database server failed. For a discussion of this topic, see Actions to Take If the Secondary Database Server Fails and Actions to Take If the Primary Database Server Fails.
If you redirect clients, consider what sort of load the additional clients place on the remaining database server. You might need to increase the space devoted to the logical log or back up the logical-log files more frequently.
If the secondary database server fails, the primary database server remains online.
To redirect clients that use the secondary database server to the primary database server, use any of the methods explained in Redirection and Connectivity for Data-Replication Clients. If you redirect these clients, the primary database server might require an additional temporary dbspace for temporary tables and sorting.
You do not need to change the type of the primary database server to standard.
To restart data replication after a failure of the secondary database server, follow the steps in Restarting If the Secondary Database Server Fails.
If the primary database server fails, the secondary database server can operate in the following ways:
Automatic switchover means that the secondary database server automatically becomes a standard database server after it detects an HDR failure. It first rolls back any open transactions and then comes into online mode as a standard database server. Automatic switchover occurs only if the parameter DRAUTO in the ONCONFIG file of the secondary database server is set to 1 (RETAIN_TYPE) or 2 (REVERSE_TYPE).
Because the secondary database server becomes a standard database server, you must be sure that either:
The automatic switchover changes only the type of the database server. It does not redirect client applications to the secondary database server. For information on redirecting clients, see Redirection and Connectivity for Data-Replication Clients.
Automatic switchover has the following advantages over manual switchover:
The main disadvantage to automatic switchover is that it requires a very stable network to function appropriately. For more information see Using Automatic Switchover Without a Reliable Network.
See The Secondary Database Server Is Changed to a Standard Database Server Automatically for the steps required to restart data replication after an automatic switchover.
When you succeed in bringing the original primary database server back online, the HDR connection is automatically established.
Although automatic switchover might appear to be the best solution, it is not appropriate for all environments.
Consider what might happen if the primary database server does not actually fail but appears to the secondary database server to fail. For example, if the secondary database server does not receive responses when it signals (pings) the primary database server because of a slow or unstable network, the secondary server assumes that the primary database server failed and switches automatically to the standard type. If the primary database server also does not receive responses when it signals the secondary database server, it assumes that the secondary database server failed and turns off data replication but remains in online mode. Now the primary database server and the secondary database server (switched to the standard type) are both in online mode.
If clients can update the data on both database servers independently, the database servers in the pair reach a state in which each database server has the logical-log records that are needed by the other. In this situation, you must start again and perform initial data replication with a level-0 dbspace backup of one entire database server, as described in Starting HDR for the First Time. Therefore, if your network is not entirely stable, you might not want to use automatic switchover.
Manual switchover means that the administrator of the secondary database server changes the type of the secondary database server to standard. The secondary database server rolls back any open transactions and then comes into online mode as a standard database server, so that it can accept updates from client applications. For an explanation of how to perform the switchover, see Changing the Database Server Type.
For a list of the steps involved in restarting data replication after a manual switchover, see The Secondary Database Server Is Changed to a Standard Database Server.
If the secondary database server is not changed to type standard, follow the steps in The Secondary Database Server Was Not Changed to a Standard Database Server.
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