Handling Multi-AZ Failover
A company's production RDS for PostgreSQL instance experienced a failover to the standby replica. After the failover, the application team reports that writes are failing with 'ReadOnlyException: Connection is read-only'. The DB instance status in the RDS console shows 'available'. Which action should the database administrator take to resolve the issue?
Quick Answer
The answer is to update the application connection string to use the new writer endpoint from the RDS console. This is correct because after a Multi-AZ failover, the former primary database instance is automatically promoted to a standby replica, which is read-only; the application is still pointing to the old endpoint, which now resolves to a read-only node, causing the ReadOnlyException. On the AWS Certified Database Specialty DBS-C01 exam, this scenario tests your understanding of Multi-AZ failover handling and the critical distinction between the writer and reader endpoints—a common trap is assuming a reboot or instance modification will fix the endpoint mapping. Remember that the RDS console updates the CNAME for the writer endpoint automatically, but the application must reconnect to that new endpoint; a simple memory tip is “failover flips the writer, so flip your connection string.”
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
✓
Update the application connection string to use the new writer endpoint from the RDS console.
After a Multi-AZ failover, the standby becomes the primary, and the old primary becomes a read-only standby. The application must update its connection string to use the new writer endpoint (which automatically points to the new primary) to perform writes. Option B is incorrect because creating a read replica and promoting it is unnecessary; Multi-AZ already provides a standby. Option C is incorrect because rebooting would force another failover but does not change the endpoint. Option D is incorrect because Multi-AZ is already enabled; modifying it again does not resolve the issue.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
Update the application connection string to use the new writer endpoint from the RDS console.
Why this is correct
The application must connect to the writer endpoint after failover.
- ✗
Create a read replica and promote it to a new primary.
Why it's wrong here
Creating a read replica is not the immediate fix; the existing primary is available.
- ✗
Reboot the DB instance to force a failover back to the original primary.
Why it's wrong here
Rebooting causes another failover, not resolution.
- ✗
Modify the DB instance to enable Multi-AZ again.
Why it's wrong here
Multi-AZ is already enabled; modification doesn't fix read-only status.
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Same concept, more angles
1 more way this is tested on DBS-C01
These questions test the same concept from different angles. Work through them to make sure you can recognise it however the exam phrases it.
Variation 1. A database specialist runs the CLI command shown. The output indicates the instance is Multi-AZ and available. The application, however, is experiencing intermittent connection timeouts to the database endpoint. What is the most likely cause?
hard- ✓ A.A failover occurred due to a Multi-AZ event.
- B.The MySQL engine version is not supported.
- C.The DB instance class is not available in the current region.
- D.The database storage is full.
Why A: A Multi-AZ failover can cause transient connection timeouts if the application does not have proper retry logic. The instance status shows 'available', so storage is not full (eliminating D). MySQL 8.0.28 is a supported version (eliminating B). The instance class db.r5.xlarge is current generation and available in the region (eliminating C). Therefore, the intermittent timeouts are most likely due to a failover event.
JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This DBS-C01 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 DBS-C01 exam.