Working with Oracle replicas for RDS Custom for Oracle

You can create Oracle replicas for RDS Custom for Oracle DB instances. Both container databases (CDBs) and non-CDBs are supported. Creating an Oracle replica is similar to the process in Amazon RDS, but with important differences. For general information about creating and managing Oracle replicas, see Working with DB instance read replicas and Working with read replicas for Amazon RDS for Oracle.

Overview of RDS Custom for Oracle replication

The architecture of RDS Custom for Oracle replication is analogous to RDS for Oracle replication. A primary DB instance replicates asynchronously to one or more Oracle replicas.

As with RDS for Oracle, you can create up to five managed Oracle replicas of your RDS Custom for Oracle primary DB instance. You can also create your own manually configured (external) Oracle replicas. External replicas don't count toward your instance limit. They also lie outside the RDS Custom support perimeter. For more information about the support perimeter, see RDS Custom support perimeter and unsupported configurations.

Oracle replica names are based on the database unique name. The format is DB_UNIQUE_NAME_X, with letters appended sequentially. For example, if your database unique name is ORCL, the first two replicas are named ORCL_A and ORCL_B. The first six letters, A–F, are reserved for RDS Custom. RDS Custom copies database parameters from your primary DB instance to the replicas. For more information, see DB_UNIQUE_NAME in the Oracle documentation.

By default, RDS Custom Oracle replicas use the same backup retention period as your primary DB instance. You can modify the backup retention period to 1–35 days. RDS Custom supports backing up, restoring, and point-in-time recovery (PITR). For more information about backing up and restoring RDS Custom DB instances, see Backing up and restoring an Amazon RDS Custom for Oracle DB instance.

You can promote managed Oracle replicas in RDS Custom for Oracle using the console, promote-read-replica AWS CLI command, or PromoteReadReplica API. If you delete your primary DB instance, and all replicas are healthy, RDS Custom for Oracle promotes your managed replicas to standalone instances automatically. If a replica has paused automation or is outside the support perimeter, you must fix the replica before RDS Custom can promote it automatically. You can only promote external Oracle replicas manually.

Requirements and limitations for RDS Custom for Oracle replication

When you create RDS Custom for Oracle replicas, not all RDS Oracle replica options are supported.

Topics

• General requirements and limitations
• Networking requirements and limitations
• External replica requirements and limitations
• Replica promotion requirements and limitations

General requirements and limitations

RDS Custom for Oracle replicas have the following limitations:

• You can create RDS Custom for Oracle replicas in mounted mode only. However, you can manually change the mode of mounted replicas to read-only, and from read-only to mounted. For more information, see the documentation for the create-db-instance-read-replica AWS CLI command.

• Cross-Region Oracle replicas aren't supported.

• You can't change the value of the Oracle Data Guard CommunicationTimeout parameter. This parameter is set to 15 seconds for RDS Custom for Oracle DB instances.

• Make sure not to modify the RDS_DATAGUARD user. This user is reserved for RDS Custom for Oracle automation. Modifying this user can result in undesired outcomes, such as an inability to create Oracle replicas for your RDS Custom for Oracle DB instance.

• Make sure not to change the replication user password. It is required to administer the Oracle Data Guard configuration on the host. If you change the password, RDS Custom for Oracle might put your Oracle replica outside the support perimeter. For more information, see RDS Custom support perimeter and unsupported configurations.

  The password is stored in AWS Secrets Manager, tagged with the DB resource ID. Each Oracle replica has its own secret in Secrets Manager. The format for the secret is the following.

  do-not-delete-rds-custom-db-DB_resource_id-6-digit_UUID-dg

• Make sure not to change the DB_UNIQUE_NAME for the primary DB instance. Changing the name causes any restore operation to become stuck.

• While creating a Oracle replica, RDS Custom temporarily pauses the cleanup of redo log files. In this way, RDS Custom ensures that it can apply these logs to the new Oracle replica after it becomes available.

• If your RDS Custom DB instance is a CDB, we recommend that you don't specify the clause STANDBYS=NONE in a CREATE PLUGGABLE DATABASE command. The goal is to ensure that your standby CDB contains all PDBs if a failover occurs.

Networking requirements and limitations

Make sure that your network configuration supports RDS Custom for Oracle replicas. Consider the following requirements and limitations:

• Make sure to enable port 1140 for both inbound and outbound communication within your virtual private cloud (VPC) for the primary DB instance and all of the replicas. This is required for Oracle Data Guard communication between the read replicas.

• RDS Custom for Oracle validates the network while creating a Oracle replica. If the primary DB instance and the new replica can't connect over the network, RDS Custom for Oracle doesn't create the replica and places it in the INCOMPATIBLE_NETWORK state.

• For external Oracle replicas, such as those you create on Amazon EC2 or on-premises, use another port and listener for Oracle Data Guard replication. Trying to use port 1140 could cause conflicts with RDS Custom automation.

• The /rdsdbdata/config/tnsnames.ora file contains network service names mapped to listener protocol addresses. Note the following requirements and recommendations:

  • Entries in tnsnames.ora prefixed with rds_custom_ are reserved for RDS Custom when handling Oracle replica operations.

    When creating manual entries in tnsnames.ora, don't use this prefix.

  • In some cases, you might want to switch over or fail over manually, or use failover technologies such as Fast-Start Failover (FSFO). If so, make sure to manually synchronize tnsnames.ora entries from the primary DB instance to all of the standby instances. This recommendation applies to both Oracle replicas managed by RDS Custom and to external Oracle replicas.

    RDS Custom automation updates tnsnames.ora entries on only the primary DB instance. Make sure also to synchronize when you add or remove a Oracle replica.

    If you don't synchronize the tnsnames.ora files and switch over or fail over manually, Oracle Data Guard on the primary DB instance might not be able to communicate with the Oracle replicas.

External replica requirements and limitations

RDS Custom for Oracle external replicas, which include on-premises replicas, have the following limitations:

• RDS Custom for Oracle detects instance role changes upon manual failover, such as FSFO, for managed Oracle replicas but not for external Oracle replicas.

  The role change is noted in the event log. You can also see the new state by using the describe-db-instances AWS CLI command.

• RDS Custom for Oracle detects high replication lag for managed Oracle replicas but not for external Oracle replicas.

  High replication lag produces the Replication has stopped event. You can also see the replication status by using the describe-db-instances AWS CLI command, but there might be a delay for it to be updated.

• RDS Custom for Oracle doesn't promote external Oracle replicas automatically after you delete your primary DB instance. The automatic promotion feature is available only for managed Oracle replicas. For information about promoting Oracle replicas manually, see the white paper Enabling high availability with Data Guard on Amazon RDS Custom for Oracle.

Replica promotion requirements and limitations

Promoting RDS Custom for Oracle managed Oracle replicas is the same as promoting RDS managed replicas, with some differences. Note the following requirements and limitations for RDS Custom for Oracle:

• You can't promote a replica while RDS Custom for Oracle is backing it up.

• When you promote your Oracle replica, you can't change the backup retention period to 0.

• Make sure not to initiate a failover while RDS Custom for Oracle is promoting your replica. Otherwise, the promotion workflow could become stuck.

• Make sure not to switch over your primary DB instance while RDS Custom for Oracle is promoting your Oracle replica. Otherwise, the promotion workflow could become stuck.

• Make sure not to shut down your primary DB instance while RDS Custom for Oracle is promoting your Oracle replica. Otherwise, the promotion workflow could become stuck.

• Make sure not to attempt to restart replication with your newly promoted DB instance as a target. After RDS Custom for Oracle promotes your Oracle replica, it becomes a standalone DB instance and no longer has the replica role.

• If you issue delete-db-instance on the primary DB instance, RDS Custom for Oracle validates that each managed Oracle replica is healthy and available for promotion. A replica might be ineligible for promotion because automation is paused or it is outside the support perimeter. In such cases, RDS Custom for Oracle publishes an event explaining the issue so that you can repair your Oracle replica manually.

• If you maintain external Oracle replicas and delete your primary DB instance, you must promote your external Oracle replicas manually.

For more information, see Troubleshooting replica promotion for RDS Custom for Oracle.

Promoting an RDS Custom for Oracle replica to a standalone DB instance

Just as with RDS for Oracle, you can promote an RDS Custom for Oracle replica to a standalone DB instance. When you promote a Oracle replica, RDS Custom for Oracle reboots the DB instance before it becomes available. For more information about promoting Oracle replicas, see Promoting a read replica to be a standalone DB instance.

The following steps show the general process for promoting a Oracle replica to a DB instance:

1. Stop any transactions from being written to the primary DB instance.

2. Wait for RDS Custom for Oracle to apply all updates to your Oracle replica.

3. Promote your Oracle replica by choosing the Promote option on the Amazon RDS console, the AWS CLI command promote-read-replica, or the PromoteReadReplica Amazon RDS API operation.

Promoting a Oracle replica takes a few minutes to complete. During the process, RDS Custom for Oracle stops replication and reboots your replica. When the reboot completes, the Oracle replica is available as a standalone DB instance.

Console

To promote an RDS Custom for Oracle replica to a standalone DB instance

1. Sign in to the AWS Management Console and open the Amazon RDS console at https://console.aws.amazon.com/rds/.

2. In the Amazon RDS console, choose Databases.

   The Databases pane appears. Each Oracle replica shows Replica in the Role column.

3. Choose the RDS Custom for Oracle replica that you want to promote.

4. For Actions, choose Promote.

5. On the Promote Oracle replica page, enter the backup retention period and the backup window for the newly promoted DB instance. You can't set this value to 0.

6. When the settings are as you want them, choose Promote Oracle replica.

AWS CLI

To promote your RDS Custom for Oracle replica to a standalone DB instance, use the AWS CLI promote-read-replica command.

Example

For Linux, macOS, or Unix:

aws rds promote-read-replica \
--db-instance-identifier my-custom-read-replica \
--backup-retention-period 2 \
--preferred-backup-window 23:00-24:00

For Windows:

aws rds promote-read-replica ^
--db-instance-identifier my-custom-read-replica ^
--backup-retention-period 2 ^
--preferred-backup-window 23:00-24:00

RDS API

To promote your RDS Custom for Oracle replica to be a standalone DB instance, call the Amazon RDS API PromoteReadReplica operation with the required parameter DBInstanceIdentifier.