You can combine Enterprise Replication and HDR to create a high-availability replication system in which a critical read-write database server in an Enterprise Replication system maintains a backup server with HDR.
An HDR system consists of two database servers: the primary database server, which receives updates, and the secondary, read-only copy of the primary database server. The secondary server is a mirror image of the primary system and is in perpetual recovery mode, applying logical-log records from the primary server to its dbspaces and sbspaces.
The HDR secondary server does not participate in Enterprise Replication; it receives updates through HDR. When the HDR primary server receives an update through Enterprise Replication, the transaction is not committed until the log records containing that transaction are sent to the secondary server. If the primary server becomes unavailable, you replace it with the secondary server by switching the secondary server into standard mode and redirecting connections to it.
High-availability replication systems are most useful for replication systems in which the failure of a critical server prevents other servers from participating in replication. Figure 13 illustrates the combination of a primary-target replication system with a high-availability replication system.
If the primary server fails, the secondary server is set to standard mode, the target database connections are redirected to it, and Enterprise Replication continues, as illustrated in Figure 14.
In an update-anywhere replication system, you can use HDR with any server for which you need high availability, as illustrated in Figure 15.
Using HDR with Enterprise Replication is particularly effective when you use a hierarchical or a forest of trees topology.
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