SOA-C03 practice question: Multi-AZ and fault tolerance

Reliability and Business Continuity A free question from our original SOA-C03 practice exam. Try it, then open the answer to see why each option is right or wrong.

A solar installer's scheduling application connects to an Amazon RDS for MySQL Multi-AZ DB instance. During a planned failover test, the database failed over in under two minutes, but the application kept timing out for 40 minutes until its servers were restarted. Investigation shows the application resolves the database endpoint name once at startup and then reuses the IP address. What should a CloudOps engineer recommend so that the application recovers automatically from future failovers?

Choose one answer.

A. Convert the DB instance to a Single-AZ deployment with automated backups, so that the endpoint IP address never changes.
B. Create a read replica in another Availability Zone and point the application at the replica's IP address during failover tests.
C. Add an Elastic IP address to the DB instance so that the same address moves to the standby during each failover.
D. Make the application honor DNS TTLs and re-resolve the DB endpoint name when it reconnects.

Show the answer and explanation

Answer: D

A. Wrong: A Single-AZ instance has no standby to fail over to, so an Availability Zone problem would cause a long outage. It removes fault tolerance instead of fixing the client. Learn more: Multi-AZ DB instance deployments (opens in a new tab)
B. Wrong: A replica would need manual promotion, and hardcoding IP addresses repeats the original problem. Multi-AZ already provides the automatic standby. Learn more: Working with DB instance read replicas (opens in a new tab)
C. Wrong: Elastic IP addresses attach to EC2 network interfaces, not to RDS DB instances. RDS handles failover by updating the endpoint's DNS record. Learn more: Failover process for Amazon RDS (opens in a new tab)
D. Right: Correct. During failover RDS points the endpoint's DNS record at the new primary. Clients that re-resolve the name reconnect within minutes, while clients that cache the old IP address keep failing. Learn more: Failover process for Amazon RDS (opens in a new tab)

Explanation

RDS Multi-AZ failover works by changing the DNS record behind the DB endpoint to the standby's address. Applications must connect by endpoint name and must not cache DNS results longer than the record's TTL, or they keep reaching the failed address. Long client-side DNS caches are a common reason failovers look longer than they are.

SOA-C03 practice question: Multi-AZ and fault tolerance

Explanation

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