RDS DB instances replicas can be created in two ways Multi-AZ & Read Replica, which provide high availability, durability, and scalability to RDS. ![[RDS-Multi-AZ-vs-Read-Replica.jpg|650]] ## Purpose - [[AWS RDS Multi-AZ DB Instance|Multi-AZ DB Instance deployments]] provide high availability, durability, and automatic failover support. - [[AWS RDS Read Replicas|Read replicas]] enable increased scalability and database availability in the case of an AZ failure. Read Replicas allow elastic scaling beyond the capacity constraints of a single DB instance for read-heavy database workloads ![[RDS-Read-Replicas-vs-Multi-AZ.png|650]] ## Region & Availability Zones - RDS Multi-AZ deployment automatically provisions and manages a standby instance in a different AZ (independent infrastructure in a physically separate location) within the same AWS region. - RDS Read Replicas can be provisioned within the same AZ, Cross-AZ or even as a Cross-Region replica. ## Replication Mode - RDS Multi-AZ deployment manages a **synchronous** standby instance in a different AZ - RDS Read Replicas has the data replicated **asynchronously** from the Primary instance to the read replicas ## Standby Instance can Accept Reads - Multi-AZ DB instance deployment is a high-availability solution and the standby instance does not support requests. - Read Replica deployment provides readable instances to increase application read-throughput. ## Automatic Failover & Failover Time - Multi-AZ DB instance deployment performs an automatic failover to the standby instance without administrative intervention, and the failover time can be up to 120 seconds based on the crash recovery. - Planned database maintenance - Software patching - Rebooting the Primary instance with failover - Primary DB instance connectivity or host failure, or an - Availability Zone failure - RDS maintains the same endpoint for the DB Instance after a failover, so the application can resume database operation without the need for manual administrative intervention. - Read Replica deployment does not provide automatic failover. Read Replica instance needs to be manually promoted to a Standalone instance. ## Upgrades - For a Multi-AZ deployment, Database engine version upgrades happen on the Primary instance. - For Read Replicas, the Database engine version upgrade is independent of the Primary instance. ## Automated Backups - Multi-AZ deployment has the Automated Backups taken from the Standby instance - Read Replicas do not have any backups configured, by default. ## Sources [AWS RDS Replication - Multi-AZ vs Read Replica](https://jayendrapatil.com/aws-rds-replication-multi-az-read-replica/)