## RDS Multi-AZ DB Instance Deployment  - RDS automatically creates a primary DB Instance and synchronously replicates the data to a standby instance in a different AZ. - RDS performs an automatic failover to the standby, so that database operations can be resumed as soon as the failover is complete. - RDS Multi-AZ deployment 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. - Multi-AZ is a High Availability feature and NOT a scaling solution for read-only scenarios; a standby replica can’t be used to serve read traffic. To service read-only traffic, use a Read Replica. - Multi-AZ deployments for Oracle, PostgreSQL, MySQL, and MariaDB DB instances use Amazon technology, while SQL Server DB instances use SQL Server Mirroring. ![[RDS_Multi-AZ_Instance_Deployment.png|650]] - In a Multi-AZ deployment, - RDS automatically provisions and maintains a **synchronous standby replica in a different Availability Zone**. - Copies of data are stored in different AZs for greater levels of data durability. - Primary DB instance is **synchronously replicated** across Availability Zones to a standby replica to provide - data redundancy, - eliminate I/O freezes during snapshots and backups - and minimize latency spikes during system backups. - DB instances may have increased write and commit latency compared to a Single AZ deployment, due to the synchronous data replication - Transaction success is returned only if the commit is successful both on the primary and the standby DB - There might be a change in latency if the deployment fails over to the standby replica, although AWS is engineered with low-latency network connectivity between Availability Zones. - When using the BYOL licensing model, a license for both the primary instance and the standby replica is required - For production workloads, it is recommended to use Multi-AZ deployment with Provisioned IOPS and DB instance classes (m1.large and larger), optimized for Provisioned IOPS for fast, consistent performance. - When Single-AZ deployment is modified to a Multi-AZ deployment (for engines other than SQL Server or Amazon Aurora) - RDS takes a snapshot of the primary DB instance from the deployment and restores the snapshot into another Availability Zone. - RDS then sets up synchronous replication between the primary DB instance and the new instance. - This avoids downtime during conversion from Single AZ to Multi-AZ. - An existing Single AZ instance can be converted into a Multi-AZ instance by modifying the DB instance without any downtime. ## RDS Multi-AZ Failover Process - In the event of a planned or unplanned outage of the DB instance, - RDS automatically switches to a standby replica in another AZ, if enabled for Multi-AZ. - The time taken for the failover to complete depends on the database activity and other conditions at the time the primary DB instance became unavailable. - Failover times are typically 60-120 secs. However, large transactions or a lengthy recovery process can increase failover time. - **Failover mechanism automatically changes the DNS record of the DB instance to point to the standby DB instance.** - Multi-AZ switch is seamless to the applications as there is no change in the endpoint URLs but just needs to re-establish any existing connections to the DB instance. - RDS handles failover automatically so that database operations can be resumed as quickly as possible without administrative intervention. - Primary DB instance switches over automatically to the standby replica if any of the following conditions occur: - Primary Availability Zone outage - Loss of network connectivity to primary - Primary DB instance fails - DB instance’s server type is changed - Operating system of the DB instance is undergoing software patching - Compute unit failure on the primary - Storage failure on the primary - A manual failover of the DB instance was initiated using **Reboot with failover** (also referred to as **Forced Failover**) - If the Multi-AZ DB instance has failed over, can be determined by - DB event subscriptions can be set up to notify you via email or SMS that a failover has been initiated. - DB events can be viewed via the Amazon RDS console or APIs. - The current state of the Multi-AZ deployment can be viewed via the RDS console and APIs. ![[RDS-Multi-AZ-Comparison.jpg|650]] ## Sources [RDS\_Multi-AZ\_DB\_Instance\_Deployment](https://docs.aws.amazon.com/AmazonRDS/latest/UserGuide/Concepts.MultiAZSingleStandby.html) [AWS RDS Multi-AZ DB Instance](https://jayendrapatil.com/aws-rds-multi-az-db-instance/)