Interview Questions

This page covers the most common cloud/AWS interview questions — from beginner to advanced. Use these to prepare for your next interview.

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    Basic["Easy<br/>Cloud Concepts & Definitions"] --> Medium["Medium<br/>Services & Architecture"]
    Medium --> Advanced["Hard<br/>Design & Best Practices"]

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    style Medium fill:#7c3aed,color:#fff
    style Advanced fill:#ef4444,color:#fff

Q1: What is cloud computing?

Cloud computing is the on-demand delivery of IT resources over the internet with pay-as-you-go pricing. Instead of buying and maintaining physical data centers, you access computing power, storage, and databases from providers like AWS, Azure, or Google Cloud.

Q2: What is the difference between IaaS, PaaS, and SaaS?

IaaS (Infrastructure as a Service) provides virtual servers and storage — you manage the OS and apps. PaaS (Platform as a Service) provides a platform for deploying apps — you manage only the code. SaaS (Software as a Service) provides ready-to-use software — you just use it.

Q3: What is an AWS Region and Availability Zone?

A Region is a geographic area with multiple data centers (e.g., us-east-1). An Availability Zone (AZ) is one or more data centers within a region, isolated from other AZs for fault tolerance.

Q4: What is IAM and why is it important?

IAM (Identity and Access Management) controls who can access AWS resources and what they can do. It’s important because it enforces the principle of least privilege — giving only the necessary permissions.

Q5: What is the difference between a Security Group and a NACL?

Security Groups operate at the instance level and are stateful (return traffic is automatically allowed). NACLs operate at the subnet level and are stateless (return traffic must be explicitly allowed).

Q6: What is the difference between EC2 and Lambda?

Q7: Explain the difference between S3, EBS, and EFS.

S3 is object storage — files accessed via HTTP, unlimited scaling. EBS is block storage — like a virtual hard drive attached to one EC2 instance. EFS is file storage — like a network shared drive that multiple EC2 instances can mount simultaneously.

Q8: What is an Auto Scaling Group and how does it work?

An Auto Scaling Group maintains a desired number of EC2 instances across availability zones. Based on scaling policies (e.g., CPU > 70%), it launches or terminates instances to match demand while staying within configured min/max limits.

Q9: What is CloudFront and when would you use it?

CloudFront is a CDN (Content Delivery Network) that caches content at 300+ edge locations worldwide. Use it to reduce latency, offload traffic from your origin, and protect against DDoS attacks.

Q10: What is the difference between a public and private subnet?

A public subnet has a route to an Internet Gateway — instances can receive inbound traffic from the internet. A private subnet does not — instances can only access the internet through a NAT Gateway (outbound only).

Q11: How would you design a highly available web application on AWS?

1. Multi-AZ — Deploy EC2 instances across at least 2 Availability Zones
2. Load balancer — ALB distributes traffic across AZs
3. Auto Scaling — Automatically replace failed instances
4. Multi-AZ RDS — Database with standby replica in another AZ
5. CloudFront — CDN for static assets and DDoS protection
6. Route53 health checks + failover routing
7. S3 for static assets and backups

Q12: How would you reduce AWS costs?

1. Right-size EC2 instances (don’t over-provision)
2. Use Reserved Instances or Savings Plans for steady workloads
3. Use Spot Instances for fault-tolerant batch jobs
4. S3 lifecycle policies — move infrequent data to Glacier
5. Delete unattached EBS volumes, old snapshots, unused load balancers
6. Set AWS Budgets with alerts
7. Use Lambda instead of EC2 for sporadic workloads

Q13: What is the difference between Vertical and Horizontal scaling?

Vertical scaling = making the instance bigger (t2.micro → t2.large). Limited by max instance size. Horizontal scaling = adding more instances (2 EC2 → 10 EC2). Virtually unlimited and provides better fault tolerance.

Q14: How does S3 achieve 99.999999999% durability?

S3 automatically replicates objects across multiple Availability Zones within a region. It also uses checksums to detect and repair data corruption, and redundant storage across multiple devices. Even if two facilities fail, your data survives.

Q15: Explain the shared responsibility model in AWS.

AWS is responsible for security OF the cloud — hardware, software, networking, and facilities. You are responsible for security IN the cloud — customer data, IAM permissions, OS patching, network configuration, encryption, and firewall rules. The boundary shifts based on the service (e.g., Lambda shifts more responsibility to AWS than EC2).

• Know the cloud models (IaaS/PaaS/SaaS) and global infrastructure (Regions/AZs)
• Understand the core services: EC2, S3, Lambda, RDS, VPC, IAM
• Explain the difference between vertical vs horizontal scaling
• Know when to use Lambda vs EC2 and S3 vs EBS vs EFS
• Understand security groups vs NACLs and the shared responsibility model
• Practice designing a highly available architecture across multiple AZs