AWS

How do you architect a highly available, zero-downtime database migration in AWS RDS?

Zero downtime means you never ask for a maintenance window. Bring the new database up next to the old one, keep it in sync while the old one carries all the traffic, check the data constantly, then switch connections over in a few seconds, with a way back you set up before you need it.

Your EC2 instances in a private subnet need to reach S3 and a third-party API, but must never be reachable from the internet. Design the networking.

Outbound-only access comes from a NAT Gateway in a public subnet, but S3 traffic should skip it entirely through a VPC endpoint, which is free and never leaves the AWS network. Nothing can reach in because there's no route for it to take, not because a rule is blocking it, and that distinction is usually the real follow-up question.

How do you prepare an EC2-based application to fail over to a second AWS region if the primary region has an outage?

Disaster recovery for EC2 is really three separate problems: getting a bootable image into the DR region ahead of time, having a way to launch capacity there fast, and having something outside both regions to redirect traffic. The RTO and RPO you're targeting decide how much of that you keep running warm versus building on demand.

You have a dozen VPCs that all need to reach each other and a shared-services VPC. Would you use VPC Peering or Transit Gateway, and why?

Peering connects two VPCs directly and doesn't scale past a handful of them, since connections grow quadratically and traffic can't transit through a peered VPC to reach a third. Transit Gateway is a central routing hub that turns the same problem into one connection per VPC, and it's the right call anywhere past three or four VPCs.

How would you securely connect an on-premises data center to a VPC, and when would you reach for Direct Connect instead of a Site-to-Site VPN?

A Site-to-Site VPN is fast to set up, encrypted by default, and runs over the public internet, which is also its ceiling: bandwidth and latency aren't guaranteed. Direct Connect is a dedicated physical link with predictable performance, but it takes weeks to provision and isn't encrypted on its own. Most serious hybrid setups end up running both.

How do you continuously monitor an AWS account for security and compliance drift, instead of only finding out during an annual audit?

Point-in-time audits catch a configuration that was wrong when someone happened to look. AWS Config evaluates resources continuously against rules and records every configuration change, CloudTrail records who made it, and Security Hub aggregates both into one place with a severity score so drift gets flagged the hour it happens, not the quarter someone checks.

What happens if an Auto Scaling instance never becomes healthy?

Auto Scaling terminates and replaces an instance that never passes health checks, which is fine once and expensive in a loop. How long you burn depends on the grace period, the check thresholds, and which health check type the group is actually using. The fix is working out why it's unhealthy: usually startup config, a wrong health endpoint, or a grace period shorter than boot time.