AWS / Lambda Interview Questions 2026

Your team is debating between SQS Standard and SQS FIFO queues. List the actual tradeoffs with numbers.

Explain how CloudFront caching works. A developer updates an S3 file but users still see the old version for 24 hours. Fix this correctly.

Your EC2 instances are running at 90% CPU. Auto Scaling adds instances but they're not receiving traffic fast enough — users still see errors for 3 minutes during scale-out. How do you fix this?

Design a serverless architecture for an API that: handles 10K req/sec at peak, has sub-100ms response, costs near-zero at zero traffic.

Your Lambda function needs credentials to access an RDS database. A developer hardcoded the password in environment variables. What's the correct approach?

Explain the difference between NAT Gateway, Internet Gateway, and VPC Endpoints. When does each one make sense?

You need to process S3 files as soon as they're uploaded. Files come in bursts of 10,000 per hour. How do you architect this?

Your S3 bucket is being charged for 500TB of storage but your application only stores 100TB. Where is the rest coming from and how do you find and fix it?

You're using DynamoDB for a social media app. Users can like posts. You need to count total likes per post in real-time. A naive counter update causes hot partition issues at viral posts. How do you solve this?

Your API Gateway + Lambda architecture handles 10,000 requests/second normally. During a flash sale, it receives 100,000 requests/second. Lambda concurrency limit is 1000. What happens and how do you handle this?

Design a data architecture on AWS for a startup that needs to: store raw events, run real-time analytics, run daily batch reports, and serve an API with sub-10ms response time.

Your RDS MySQL instance CPU hits 100% every day between 2-4 PM. Your application traffic is steady. What are the most likely causes and how do you investigate?

You deploy a new version of your service on ECS Fargate. The deployment causes 2 minutes of 503 errors despite a rolling update configuration. What are the likely causes?

Your AWS Lambda function processes S3 events. Under high load, you notice some events are processed twice, causing duplicate records in DynamoDB. Lambda is configured with at-least-once delivery. How do you fix this?

Netflix popularized "chaos engineering" — explain what problem it actually solves that traditional testing doesn't, and how you would introduce it safely into a system that has never done it before.

Your RDS instance hits 100% CPU during peak hours. Queries are optimized. What are your scaling options?

What is AWS ECS vs EKS? When would you choose ECS over Kubernetes?

Explain S3 storage classes and how to set up lifecycle policies to optimize costs.

What is CloudFront? How does it reduce origin load and what cache behaviors can you configure?

Explain how AWS VPC peering differs from AWS Transit Gateway. When would you choose each?

Your S3 bucket is publicly accessible and contains sensitive data. You turned on Block Public Access but users say the data is still accessible. Why?

What is Lambda cold start and why does it happen? Name two concrete techniques to reduce its impact on a latency-sensitive API.

What is the difference between AWS IAM Role and IAM User? Best practice when your EC2 instance needs S3 access?

Explain the difference between AWS Application Load Balancer and Network Load Balancer. When would you choose NLB?

Your AWS Lambda functions have a shared utility layer. You update the layer and some functions get the new version, some don't. Why?

Explain the difference between AWS SQS Standard and FIFO queues. When would you choose each?

A client's AWS bill has grown 40% in 6 months with no corresponding growth in usage. What are the first three things you would check?

Your CloudFormation stack update fails and rolls back, leaving your application in a partially deployed state. How do you prevent this and handle it when it happens?

Explain how AWS IAM roles vs users vs policies work. Why should applications NEVER use IAM users with access keys?

You have an S3 bucket that's costing $5K/month just for PUT/GET requests. Traffic hasn't changed. What could cause a spike in request costs?

Your application uses ElastiCache Redis. During a Redis failover (primary → replica promotion), your app throws connection errors for 30 seconds. How do you reduce this to under 5 seconds?

Design a data lake architecture on AWS that ingests both real-time streams and batch uploads, makes data queryable within 5 minutes, and costs less than $10K/month for 100TB.

Your Lambda function needs to access a VPC resource (RDS in private subnet) but also needs to call external APIs. What's the networking concern and how do you resolve it?

Explain the difference between SQS, SNS, and EventBridge. A new developer says 'they're all message queues.' Correct this.

Your DynamoDB table has a hot partition because 80% of reads go to the same partition key (a popular user). How do you solve this?