Working with RDS for Db2 replica backups

You can create and restore backups of an RDS for Db2 replica just like a primary database. However, there are important differences in how replica backups work, particularly regarding restore timing and backup retention settings.

RDS for Db2 supports both automatic backups and manual snapshots for replicas. RDS for Db2 doesn't support point-in-time restore. For information about RDS backups, see Backing up, restoring, and exporting data.

Key differences for replica backups

Replica backups differ from primary database backups in several important ways:

• Automatic backups aren't enabled by default for replicas.

• Restore operations use database time rather than backup creation time.

• Replica lag can affect the actual data restored. For information about monitoring replica lag, see Monitoring Db2 replication lag.

Enabling automatic backups for RDS for Db2 replicas

Unlike primary databases, RDS for Db2 replicas don't have automated backups enabled by default. You must manually configure the backup retention period to enable automatic backups. Enable automated backups by setting the backup retention period to a positive nonzero value.

Console

To enable automatic backups immediately

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

2. In the navigation pane, choose Databases, and then choose the DB instance that you want to modify.

3. Choose Modify.

4. For Backup retention period, choose a positive nonzero value, for example three days.

5. Choose Continue.

6. Choose Apply immediately.

7. Choose Modify DB instance to save your changes and enable automated backups.

AWS CLI

To enable automated backups, use the AWS CLI modify-db-instance command.

Include the following parameters:

• --db-instance-identifier

• --backup-retention-period

• --apply-immediately or --no-apply-immediately

The following example enables automated backups by setting the backup retention period to three days. The changes are applied immediately.

For Linux, macOS, or Unix:

aws rds modify-db-instance \
    --db-instance-identifier my_db_instance  \
    --backup-retention-period 3 \
    --apply-immediately

For Windows:

aws rds modify-db-instance ^
    --db-instance-identifier my_db_instance  ^
    --backup-retention-period 3 ^
    --apply-immediately

RDS API

To enable automated backups, use the RDS API ModifyDBInstance operation with the following required parameters:

• DBInstanceIdentifier

• BackupRetentionPeriod

Restoring an RDS for Db2 replica backup

You can restore an RDS for Db2 replica backup the same way that you can restore a backup of the primary database. For more information, see Restoring to a DB instance.

The most important consideration when restoring replica backups is understanding the difference between database time and backup creation time, especially when replica lag is present.

You can monitor replication lag and ensure that your backups contain the expected data. For information about the ReplicaLag metric, see Amazon CloudWatch metrics for Amazon RDS.

Understanding timing differences

When you restore a replica backup, you must determine the point in time to which you are restoring. The database time refers to the latest applied transaction time of the data in the backup. When you restore a replica backup, you restore to the database time, not the time when the backup completed. The difference is significant because a replica can lag behind the primary database by minutes or hours. Thus, the database time of a replica backup might be much earlier than the snapshot creation time.

To find the difference between database time and creation time, run the AWS CLI describe-db-snapshots command or call the RDS API DescribeDBSnapshots operation. Compare the SnapshotDatabaseTime value and the OriginalSnapshotCreateTime value. The SnapshotDatabaseTime value is the earliest database time among all the databases of the replica backup. The OriginalSnapshotCreateTime value is the latest applied transaction on the primary database. Note that replication lags could be different for multiple databases, and the database time could be in between these two times.

The following AWS CLI example shows the difference between the two times:

For Linux, macOS, or Unix:

aws rds describe-db-snapshots \
    --db-instance-identifier my_db2_replica \
    --db-snapshot-identifier my_replica_snapshot

For Windows:

aws rds describe-db-snapshots ^
    --db-instance-identifier my_db2_replica ^
    --db-snapshot-identifier my_replica_snapshot

This command produces output similar to the following example.

{
    "DBSnapshots": [
        {
            "DBSnapshotIdentifier": "my_replica_snapshot",
            "DBInstanceIdentifier": "my_db2_replica", 
            "SnapshotDatabaseTime": "2022-07-26T17:49:44Z",
            ...
            "OriginalSnapshotCreateTime": "2021-07-26T19:49:44Z"
        }
    ]
}