High Availability (Multi-AZ)
Amazon RDS provides high availability and failover support for DB instances using Multi-AZ deployments. Multi-AZ deployments for Oracle, PostgreSQL, MySQL, and MariaDB DB instances use Amazon technology, while SQL Server DB instances use SQL Server Mirroring.
Amazon Aurora stores copies of the data in a DB cluster across multiple Availability Zones in a single region, regardless of whether the instances in the DB cluster span multiple Availability Zones. For more information on Amazon Aurora, see Aurora on Amazon RDS.
In a Multi-AZ deployment, Amazon RDS automatically provisions and maintains a synchronous standby replica in a different Availability Zone. The primary DB instance is synchronously replicated across Availability Zones to a standby replica to provide data redundancy, eliminate I/O freezes, and minimize latency spikes during system backups. Running a DB instance with high availability can enhance availability during planned system maintenance, and help protect your databases against DB instance failure and Availability Zone disruption. For more information on Availability Zones, see Regions and Availability Zones.
The high-availability feature is not a scaling solution for read-only scenarios; you cannot use a standby replica to serve read traffic. To service read-only traffic, you should use a Read Replica. For more information, see Working with PostgreSQL, MySQL, and MariaDB Read Replicas.
When using the BYOL licensing model, you must have a license for both the primary instance and the standby replica.
Using the RDS console, you can create a Multi-AZ deployment by simply specifying Multi-AZ when creating a DB instance. You can also use the console to convert existing DB instances to Multi-AZ deployments by modifying the DB instance and specifying the Multi-AZ option. The RDS console shows the Availability Zone of the standby replica, called the secondary AZ.
You can specify a Multi-AZ deployment using the CLI as well. Use the AWS CLI describe-db-instances command, or the Amazon RDS API DescribeDBInstances action to show the Availability Zone of the standby replica (called the secondary AZ).
The RDS console shows the Availability Zone of the standby replica (called the secondary AZ), or you can use the AWS CLI describe-db-instances command, or the Amazon RDS API DescribeDBInstances action to find the secondary AZ. When using the BYOL licensing model, you must have a license for both the primary instance and the standby replica.
DB instances using Multi-AZ deployments may have increased write and commit latency compared to a Single-AZ deployment, due to the synchronous data replication that occurs. You may have a change in latency if your deployment fails over to the standby replica, although AWS is engineered with low-latency network connectivity between Availability Zones. For production workloads, we recommend you use Provisioned IOPS and DB instance classes (m1.large and larger) that are optimized for Provisioned IOPS for fast, consistent performance.
If you have a Single-AZ deployment, and you modify it to be a Multi-AZ deployment (for engines other than SQL Server or Amazon Aurora), then Amazon RDS takes a snapshot of the primary DB instance from your deployment and restores the snapshot into another Availability Zone. Amazon RDS then sets up synchronous replication between your primary DB instance and the new instance. This action avoids downtime when you convert from Single-AZ to Multi-AZ, but you can experience a significant performance impact when first converting to Multi-AZ. This impact is more noticeable for large and write-intensive DB instances.
Failover Process for Amazon RDS
In the event of a planned or unplanned outage of your DB instance, Amazon RDS automatically switches to a standby replica in another Availability Zone if you have enabled Multi-AZ. The time it takes 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 seconds. However, large transactions or a lengthy recovery process can increase failover time. When the failover is complete, it can take additional time for the RDS console UI to reflect the new Availability Zone.
The failover mechanism automatically changes the DNS record of the DB instance to point to the standby DB instance. As a result, you will need to re-establish any existing connections to your DB instance. Due to how the Java DNS caching mechanism works, you may need to reconfigure your JVM environment. For more information on how to manage a Java application that caches DNS values in the case of a failover, see the AWS SDK for Java.
Amazon RDS handles failovers automatically so you can resume database operations as quickly as possible without administrative intervention. The primary DB instance switches over automatically to the standby replica if any of the following conditions occur:
An Availability Zone outage
The primary DB instance fails
The DB instance's server type is changed
The operating system of the DB instance is undergoing software patching
A manual failover of the DB instance was initiated using Reboot with failover
There are several ways to determine if your Multi-AZ DB instance has failed over:
DB event subscriptions can be setup to notify you via email or SMS that a failover has been initiated. For more information about events, see Using Amazon RDS Event Notification
You can view your DB events via the Amazon RDS console or APIs.
You can view the current state of your Multi-AZ deployment via the Amazon RDS console and APIs.
For information on how you can respond to failovers, reduce recovery time, and other best practices for Amazon RDS, see Best Practices for Amazon RDS.