RDS Oracle disaster recovery failover and reader Oracle cross-Region

 Amazon Relational Database Service (Amazon RDS) for Oracle have several managed high availability (HA) and disaster recovery (DR) capabilities to choose from based on your business requirements and use cases: 

• With Amazon RDS Multi-AZ, you get enhanced availability and durability for database (DB) instances within a specific AWS Region. This is often an effective DR solution for most use cases.

• You can use RDS for Oracle read replicas when running mission-critical databases with a business requirement for your DR configuration to span across different Regions. At the same time, you want to use your DR investments to handle some of your production read workloads in another Region closer to the users.

• RDS for Oracle mounted DB replicas are ideal if you just need DR but don’t have the requirement to route your read workloads to the replica.

Read-only and mounted replicas

Read-only-->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.

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

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.

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 

Read Replicas: Read replicas are copies of your primary database instance that allow you to offload read queries from your primary instance, thereby reducing its load. can be used for read-heavy workloads like reporting,

analytics, or read operations from applications.

Multi-AZ Deployments: Multi-AZ (Availability Zone) deployments provide high availability by automatically replicating your database across multiple Availability Zones within a single AWS Region. Amazon RDS automatically fails over to the Standby replica in case of a failure of the primary database instance, ensuring minimal downtime and data loss.

Standby Instances: Functionality: Provide a point-in-time backup of your primary database for:

• • Planned maintenance: Take offline maintenance on the primary database while applications can still access the standby with read-only access.

  • Disaster recovery: Quickly failover to the Standby in case of primary database failure, minimizing downtime and data loss.

• Limitations: Standby instances are typically configured in manual failover mode and require manual intervention to promote to the primary role

Failover: Functionality: An automated process that transitions the standby instance to become the primary database upon failure of the original primary.

• Two modes:

  • Automatic failover: Enabled by default in Multi-AZ deployments, automatically promotes the standby in the same Availability Zone (AZ) if the primary fails.

  • Manual failover: Requires manual intervention to promote the standby in a different AZ, offering more control but slower recovery time.

• Considerations: Configure automatic failover carefully to avoid unnecessary failovers triggered by transient issues.

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Read Replicas Across Regions: Amazon RDS also supports creating read replicas in different AWS Regions. This feature can be useful for disaster recovery,scaling read operations closer to end-users, or for other purposes.

Choosing the Right Option:

The best option depends on your specific needs and priorities:

• Use read replicas for offloading read workloads, development, and testing.

• Use standby instances for planned maintenance, disaster recovery, and backup/archiving.

• Consider automatic failover for high availability and fast recovery from primary database failures.

• Choose manual failover for more control over the failover process.

HA/DR  Feature Comparison

HA/DR (RTO/RPO) Feature Comparison

Understanding RTO and RPO

Different features of Amazon RDS support different RTOs and RPOs at different cost points: 

Feature              RTO    RPO      Cost           Scope    

Automated backups   Good    Better   Low      Single Region

Manual snapshots   Better   Good    Medium    Cross-Region

Read replicas      Best     Best     High     Cross-Region

NOTE  : If the replica is in read-only mode, make sure that you have an Active Data Guard license. If you place the replica in mounted mode, you don't need an Active Data Guard license. Only the Oracle DB engine supports mounted replicas.

 For major version upgrades of cross-Region replicas, Amazon RDS automatically does the following:

• Generates an option group for the target version.

• Copies all options and option settings from the original option group to the new option group.

• Associates the upgraded cross-Region replica with the new option group.