AWS RDS Multi-AZ DB Instance vs DB Cluster Deployment

RDS Multi-AZ DB Instance vs DB Cluster

RDS Multi-AZ DB Instance vs DB Cluster

  • RDS Multi-AZ deployments provide high availability and automatic failover support for DB instances
  • Multi-AZ helps improve the durability and availability of a critical system, enhancing availability during planned system maintenance, DB instance failure, and Availability Zone disruption.
  • A Multi-AZ DB instance deployment has one standby DB instance that provides failover support but doesn’t serve read traffic.
  • A Multi-AZ DB cluster deployment has two standby DB instances that provide failover support and can also serve read traffic.

RDS Multi-AZ DB Instance vs DB Cluster

Instances & Availability Zones

  • A Single AZ instance creates a single DB instance in any specified AZ.
  • A Multi-AZ DB Instance deployment creates a Primary and a Standby instance in two different AZs
  • A Multi-AZ DB Cluster deployment creates a Primary Writer and two Readable Standby instances in three different AZs

Replication Mode

  • Multi-AZ DB instance deployment synchronously replicates the data from the primary DB instance to a standby instance in a different AZ.
  • Multi-AZ DB cluster deployment semi-synchronously replicates data from the writer DB instance to both reader DB instances using the DB engine’s native replication capabilities.

Standby Instance can Accept Reads

  • Multi-AZ DB instance deployment is a high-availability solution and the standby instance does not support requests.
  • Multi-AZ DB cluster deployment provides readable standby instances to increase application read-throughput.

Commit Latency

  • Single AZ instance has the lowest commit latency.
  • Multi-AZ DB instance deployment has a high commit latency as compared to the Single AZ instance as the data needs to be synchronously replicated to the standby instance.
  • Multi-AZ DB cluster deployment provides up to two thirds faster commits for commits compared to Multi-AZ DB instance as it performs semi-synchronous replication.

Automatic Failover & Failover Time

  • Single AZ instances do not support automatic failover and failure would result in data loss. Use point-in-time recovery with backups to restore the database.
  • Multi-AZ DB instance deployment performs an automatic failover to the standby instance, and the failover time can be up to 120 seconds based on the crash recovery.
  • Multi-AZ DB cluster deployment performs an automatic failover to a reader DB instance in a different AZ, and the failover time can be up to 75 seconds depending on the replica lag.

Supported Engines

  • Single AZ and Multi-AZ DB instance deployments support all DB engines
  • Multi-AZ DB clusters are supported only for the MySQL and PostgreSQL DB engines.


  • Single AZ is the most cost-effective option.
  • Multi-AZ DB Instance deployment costs more than a Single AZ as it maintains a synchronous standby instance.
  • Multi-AZ DB Cluster would be an expensive option as it creates 3 instances, supports specific instance classes that do not include burstable classes, and does not support general-purpose SSD volumes.

Use Cases

  • Single AZ deployments are suitable for non-critical dev, test environments.
  • Multi-AZ deployments are suitable for critical, production-based environments requiring high availability, data redundancy, and scalability for read workloads.

AWS Certification Exam Practice Questions

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