Overview of RDS for Oracle replicas
An Oracle replica database is a physical copy of your primary database. An Oracle replica in read-only mode is called a read replica. An Oracle replica in mounted mode is called a mounted replica. Oracle Database doesn't permit writes in a replica, but you can promote a replica to make it writable. The promoted read replica has the replicated data to the point when the request was made to promote it.
The following video provides a helpful overview of RDS for Oracle disaster recovery.
For more information, see the blog post Managed disaster recovery with
Amazon RDS for Oracle cross-Region automated backups - Part 1
Topics
Read-only and mounted replicas
When creating or modifying an Oracle replica, you can place it in either of the following modes:
- Read-only
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This is the default. Active Data Guard transmits and applies changes from the source database to all read replica databases.
You can create up to five read replicas from one source DB instance. For general information about read replicas that applies to all DB engines, see Working with DB instance read replicas. For information about Oracle Data Guard, see Oracle Data Guard concepts and administration
in the Oracle documentation. - Mounted
-
In this case, replication uses Oracle Data Guard, but the replica database doesn't accept user connections. The primary use for mounted replicas is cross-Region disaster recovery.
A mounted replica can't serve a read-only workload. The mounted replica deletes archived redo log files after it applies them, regardless of the archived log retention policy.
You can create a combination of mounted and read-only DB replicas for the same source DB instance. You can change a read-only replica to mounted mode, or change a mounted replica to read-only mode. In either case, the Oracle database preserves the archived log retention setting.
Read replicas of CDBs
RDS for Oracle supports Data Guard read replicas for Oracle Database 19c and 21c CDBs in the single-tenant configuration only. You can create, manage, and promote read replicas in a CDB just as you can in a non-CDB. Mounted replicas are also supported. You get the following benefits:
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Managed disaster recovery, high availability, and read-only access to your replicas
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The ability to create read replicas in a different Amazon Web Services Region.
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Integration with the existing RDS read replica APIs: CreateDBInstanceReadReplica, PromoteReadReplica, and SwitchoverReadReplica
To use this feature, you need an Active Data Guard license and an Oracle Database Enterprise Edition license for both the replica and primary DB instances. There are no additional costs related to using CDB architecture. You pay only for your DB instances.
For more information about the single-tenant and multi-tenant configurations of the CDB architecture, see Overview of RDS for Oracle CDBs.
Archived redo log retention
If a primary DB instance has no cross-Region read replicas, Amazon RDS for Oracle keeps a minimum of two hours of archived redo logs on the
source DB instance. This is true regardless of the setting for archivelog retention hours in
rdsadmin.rdsadmin_util.set_configuration.
RDS purges logs from the source DB instance after two hours or after the archive log retention hours setting has passed, whichever is longer. RDS purges logs from the read replica after the archive log retention hours setting has passed only if they have been successfully applied to the database.
In some cases, a primary DB instance might have one or more cross-Region read replicas. If so, Amazon RDS for
Oracle keeps the transaction logs on the source DB instance until they have been transmitted and applied to
all cross-Region read replicas. For information about rdsadmin.rdsadmin_util.set_configuration,
see Retaining archived redo logs.
Outages during Oracle replication
When you create a read replica, Amazon RDS takes a DB snapshot of your source DB instance and begins replication. The source DB instance experiences a very brief I/O suspension when the DB snapshot operation begins. The I/O suspension typically lasts about one second. You can avoid the I/O suspension if the source DB instance is a Multi-AZ deployment, because in that case the snapshot is taken from the secondary DB instance.
The DB snapshot becomes the Oracle replica. Amazon RDS sets the necessary parameters and permissions for the source database and replica without service interruption. Similarly, if you delete a replica, no outage occurs.