Amazon RDS Multi-AZ deployments provide enhanced availability and durability for Database (DB) Instances, making them a natural fit for production database workloads. When you provision a Multi-AZ DB Instance, Amazon RDS automatically creates a primary DB Instance and synchronously replicates the data to a standby instance in a different Availability Zone (AZ). Each AZ runs on its own physically distinct, independent infrastructure, and is engineered to be highly reliable. In case of an infrastructure failure, Amazon RDS performs an automatic failover to the standby (or to a read replica in the case of Amazon Aurora), so that you can resume database operations as soon as the failover is complete. Since the endpoint for your DB Instance remains the same after a failover, your application can resume database operation without the need for manual administrative intervention.

Amazon Aurora employs an SSD-backed virtualized storage layer purpose-built for database workloads. Amazon Aurora automatically replicates your storage six ways, across three Availability Zones. Amazon Aurora storage is fault-tolerant, transparently handling the loss of up to two copies of data without affecting database write availability and up to three copies without affecting read availability. Amazon Aurora storage is also self-healing. Data blocks and disks are continuously scanned for errors and replaced automatically. For more information about high availability with Amazon Aurora, see the online documentation.

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Converting an Amazon RDS Instance to Multi-AZ

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Multi-AZ deployments for the MySQL, MariaDB, Oracle, and PostgreSQL engines utilize synchronous physical replication to keep data on the standby up-to-date with the primary. Multi-AZ deployments for the SQL Server engine use synchronous logical replication to achieve the same result, employing SQL Server-native Mirroring technology. Both approaches safeguard your data in the event of a DB Instance failure or loss of an Availability Zone.

If a storage volume on your primary instance fails in a Multi-AZ deployment, Amazon RDS automatically initiates a failover to the up-to-date standby (or to a replica in the case of Amazon Aurora). Compare this to a Single-AZ deployment: in case of a Single-AZ database failure, a user-initiated point-in-time-restore operation will be required. This operation can take several hours to complete, and any data updates that occurred after the latest restorable time (typically within the last five minutes) will not be available.

You also benefit from enhanced database availability when running Multi-AZ deployments. If an Availability Zone failure or DB Instance failure occurs, your availability impact is limited to the time automatic failover takes to complete: typically under one minute for Amazon Aurora (as little as 30 seconds when using the MariaDB Connector/J) and one to two minutes for other database engines (see the RDS FAQ for details).

The availability benefits of Multi-AZ deployments also extend to planned maintenance and backups. In the case of system upgrades like OS patching or DB Instance scaling, these operations are applied first on the standby, prior to the automatic failover. As a result, your availability impact is, again, only the time required for automatic failover to complete.

Unlike Single-AZ deployments, I/O activity is not suspended on your primary during backup for Multi-AZ deployments for the MySQL, MariaDB, Oracle, and PostgreSQL engines, because the backup is taken from the standby. However, note that you may still experience elevated latencies for a few minutes during backups for Multi-AZ deployments. 

On instance failure in Amazon Aurora deployments, Amazon RDS uses RDS Multi-AZ technology to automate failover to one of up to 15 Amazon Aurora Replicas you have created in any of three Availability Zones. If no Amazon Aurora Replicas have been provisioned, in the case of a failure, Amazon RDS will attempt to create a new Amazon Aurora DB instance for you automatically.

DB Instance failover is fully automatic and requires no administrative intervention. Amazon RDS monitors the health of your primary and standbys, and initiates a failover automatically in response to a variety of failure conditions.

Amazon RDS detects and automatically recovers from the most common failure scenarios for Multi-AZ deployments so that you can resume database operations as quickly as possible without administrative intervention. Amazon RDS automatically performs a failover in the event of any of the following:

  • Loss of availability in primary Availability Zone
  • Loss of network connectivity to primary
  • Compute unit failure on primary
  • Storage failure on primary

Note: When operations such as DB Instance scaling or system upgrades like OS patching are initiated for Multi-AZ deployments, for enhanced availability, they are applied first on the standby prior to an automatic failover (see the Aurora documentation for details on update behavior). As a result, your availability impact is limited only to the time required for automatic failover to complete. Note that Amazon RDS Multi-AZ deployments do not failover automatically in response to database operations such as long running queries, deadlocks or database corruption errors.

Please refer to Amazon RDS pricing page for more details.

Using the AWS Management Console, you can easily create new Multi-AZ deployments or modify existing Single-AZ instances to become Multi-AZ deployments. To create a new Multi-AZ deployment using the AWS Management Console, simply click the "Yes" option for "Multi-AZ Deployment" when launching a DB Instance. To convert an existing Single-AZ DB Instance to a Multi-AZ deployment, use the "Modify" option corresponding to your DB Instance in the AWS Management Console.

Amazon RDS Multi-AZ deployments complement Read Replicas for Amazon RDS for MySQL, MariaDB, and PostgreSQL. While both features maintain a second copy of your data, there are differences between the two:

Multi-AZ Deployments Read Replicas
Synchronous replication – highly durable Asynchronous replication – highly scalable
Only database engine on primary instance is active All read replicas are accessible and can be used for read scaling
Automated backups are taken from standby No backups configured by default
Always span two Availability Zones within a single Region Can be within an Availability Zone, Cross-AZ, or Cross-Region
Database engine version upgrades happen on primary Database engine version upgrade is independent from source instance
Automatic failover to standby when a problem is detected Can be manually promoted to a standalone database instance

You can combine Multi-AZ deployments and read replicas to enjoy the benefits of each. For example, you can configure a source database as Multi-AZ for high availability and create a read replica (in Single-AZ) for read scalability.

With RDS for MySQL and MariaDB, you can also set the read replica as Multi-AZ, allowing you to use the read replica as a DR target. When you promote the read replica to be a standalone database, it will already be Multi-AZ enabled. Note that RDS for PostgreSQL does not yet support this feature.