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.

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.

Cost

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

Questions are collected from Internet and the answers are marked as per my knowledge and understanding (which might differ with yours).

AWS services are updated everyday and both the answers and questions might be outdated soon, so research accordingly.

AWS exam questions are not updated to keep up the pace with AWS updates, so even if the underlying feature has changed the question might not be updated

Open to further feedback, discussion and correction.