Interview › Databases & Caching
How do you scale an RDS database vertically and what is the downtime impact?
Databases & Caching · Basic level
Answer
Vertical scaling changes the RDS DB instance class to add CPU, memory, or network capacity. It can require downtime or a failover depending on engine, class change, Multi-AZ setup, and whether the change is applied immediately or in a maintenance window.
Technical explanation
Scale-up can buy time but does not fix bad SQL, locks, missing indexes, or connection storms.
Multi-AZ can reduce impact but still requires app reconnect and retry behavior.
Scale-down should be tested because less memory can reduce cache hit ratio and change query behavior.
Hands-on example
Scale-up example:
$ aws rds modify-db-instance --db-instance-identifier orders-prod --db-instance-class db.r7g.2xlarge --apply-immediately
Watch RDS events, p95 latency, DB load, CPU, free memory, slow queries, and application connection errors during the change.
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