SOA-C02 Deployment, Provisioning, and Automation Practice Question
A SysOps administrator is automating the creation of Amazon RDS DB instances using AWS CloudFormation. The template includes a DB instance with a Multi-AZ deployment. During a stack update, the administrator changes the DB instance class from db.t3.small to db.t3.medium. What is the expected behavior during the update?
⚠ Common exam trap
A common mix-up: candidates assume any modification to a Multi-AZ instance causes a full replacement or simultaneous outage, but AWS specifically designed the Multi-AZ update process to minimize downtime by modifying the standby first and then failing over.
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
RDS will modify the standby instance first, then fail over to it, and finally modify the original primary, resulting in minimal downtime.
When updating a Multi-AZ RDS DB instance class via CloudFormation, AWS performs a 'rolling upgrade' to minimize downtime. RDS first modifies the standby instance to the new class, then initiates a failover to make the standby the new primary, and finally modifies the original primary (now the standby) to the new class. This results in only a brief outage during the failover, typically lasting 60–120 seconds.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
RDS will create a new DB instance with the new class and delete the old one.
Why it's wrong here
This answer confuses an in-place configuration change with a replacement operation. RDS's ModifyDBInstance call with the new DBInstanceClass parameter mutates the existing DB instance, preserving the database, endpoints, and storage rather than creating a new one. CloudFormation does not orchestrate a new instance creation for a class update because the resource is not marked as 'replaced'—if it were, you would need to restore from a snapshot, which is not how class modifications work.
- ✗
RDS will modify both instances simultaneously, causing a brief outage.
Why it's wrong here
A simultaneous modification of both Multi-AZ members is incorrect because RDS intentionally sequences the change to maintain availability. The actual order is: modify the standby, trigger a failover so that the standby becomes primary, then modify the old primary (now the standby). Updating both at the same time would leave both instances unavailable concurrently, negating the purpose of a Multi-AZ deployment and causing a full outage instead of the brief failover period that RDS normally imposes.
- ✗
The update will fail because CloudFormation cannot modify a Multi-AZ DB instance class.
Why it's wrong here
CloudFormation fully supports updating the DBInstanceClass property on an AWS::RDS::DBInstance resource, including when the instance is deployed in a Multi-AZ configuration. The update is performed by calling the RDS ModifyDBInstance API, which accepts the new class for Multi-AZ instances without any limitation that would cause a CloudFormation stack update to fail. Instead, the stack update succeeds once RDS completes the class change, possibly with a brief failover but with no requirement to recreate the stack.
- ✓
RDS will modify the standby instance first, then fail over to it, and finally modify the original primary, resulting in minimal downtime.
Why this is correct
This is exactly how RDS handles instance-class changes for Multi-AZ DB instances. Because the primary and standby run in different Availability Zones, RDS first applies the new class to the standby while the primary continues to serve traffic. It then performs a failover—typically causing a short DNS/connection disruption of a few seconds—to promote the upgraded instance to primary, and finally applies the new class to the original primary, which is now the standby. This rolling pattern keeps downtime to a minimum and is the documented behavior for class modifications on Multi-AZ deployments.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This SOA-C02 practice question is part of Courseiva's free Amazon Web Services certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the SOA-C02 exam.