Series: 30 Days DevOps Interview Preparation
Author: Tathagat Gaikwad

25 Docker & AWS Interview Questions with Detailed Answers

🌟 Introduction

If you’re preparing for a DevOps Engineer interview, two areas you will always be tested on are:

Docker – because containerization is at the core of modern DevOps.
AWS – because it is the most widely adopted cloud provider.

In this blog, we’ll go through 25 commonly asked Docker and AWS questions, explained with both theory and practical insights.

🐳 Docker Interview Questions

Q1: What is Docker?
👉 Docker is a containerization platform that packages applications and dependencies together into a lightweight, portable container. It ensures the app runs consistently across environments.

Q2: What is the difference between Docker containers and Virtual Machines?

VMs → Require a hypervisor, each VM runs a full OS (heavyweight).
Docker Containers → Share the host OS kernel, lightweight, faster startup.

Q3: Difference between Docker Image and Container?

Image → Blueprint for containers (read-only).
Container → Running instance of an image (read-write).

Q4: How do you persist data in Docker?
👉 By using Volumes or Bind Mounts. Volumes are managed by Docker and are the recommended approach for persistence.

Q5: What are common instructions in a Dockerfile?

FROM → Base image
COPY/ADD → Copy files into image
RUN → Execute commands while building
EXPOSE → Ports to be exposed
CMD → Default command

Q6: What is Docker Compose?
👉 A tool to define and run multi-container applications using a YAML file (docker-compose.yml). Example: web + database running together.

Q7: How do you reduce Docker image size?

Use smaller base images (like alpine)
Use multi-stage builds
Clean up caches and unused packages

Q8: Difference between COPY and ADD in Dockerfile?

COPY → Copies files/folders only.
ADD → Can copy files, extract .tar, and fetch remote URLs.

Q9: How do you monitor Docker containers?

Commands: docker ps, docker stats
Tools: Prometheus + cAdvisor, ELK stack, or Docker Desktop dashboard

Q10: What is the difference between ENTRYPOINT and CMD?

ENTRYPOINT → Defines the executable (main process).
CMD → Provides default arguments to ENTRYPOINT.

☁️ AWS Interview Questions

Q11: What is IAM in AWS?
👉 IAM (Identity and Access Management) is a service to manage users, groups, roles, and permissions securely.

Q12: Difference between Security Groups and NACLs?

Security Groups → Instance-level firewall, stateful (return traffic is allowed).
NACLs → Subnet-level firewall, stateless (rules must be defined for both inbound & outbound).

Q13: How do you achieve high availability with EC2?
👉 Use Auto Scaling Groups + Load Balancers (ELB/ALB) across multiple Availability Zones (AZs).

Q14: What is the difference between S3 Standard and S3 Glacier?

S3 Standard → Frequent access, low latency.
S3 Glacier → Archival storage, very low cost, but retrieval takes minutes to hours.

Q15: Explain Spot, Reserved, and On-Demand EC2 instances.

On-Demand → Flexible, pay per hour/second.
Reserved → 1–3 year commitment, cheaper.
Spot → Cheapest, but AWS can reclaim anytime.

Q16: What is AWS CloudFormation?
👉 Infrastructure as Code (IaC) tool to provision and manage AWS resources using YAML/JSON templates.

Q17: Difference between Elastic Beanstalk and ECS?

Elastic Beanstalk → PaaS for simple app deployments (abstracts infra).
ECS → Container orchestration (you manage containers).

Q18: What is AWS EKS?
👉 Elastic Kubernetes Service – Managed Kubernetes cluster service on AWS.

Q19: How do you secure sensitive data in AWS?
👉 Use Secrets Manager or SSM Parameter Store instead of hardcoding credentials.

Q20: Difference between ELB and ALB?

ELB (Classic Load Balancer) → Layer 4 & 7, basic routing.
ALB (Application Load Balancer) → Layer 7, path-based & host-based routing, better for microservices.

Q21: How do you connect on-premises servers to AWS?
👉 Use VPN, AWS Direct Connect, or hybrid networking.

Q22: Difference between RDS and DynamoDB?

RDS → Relational (SQL-based).
DynamoDB → NoSQL (key-value, document-based).

Q23: What is AWS Lambda used for?
👉 Serverless compute – runs code without managing servers, triggered by events (S3 upload, API Gateway, CloudWatch events).

Q24: How do you manage logs in AWS?
👉 Use CloudWatch Logs, store logs in S3, and analyze with Athena.

Q25: What is an Auto Scaling Policy?
👉 Rules that automatically scale EC2 instances up/down based on metrics like CPU, memory, or custom CloudWatch alarms.

🎯 Interview Preparation Tips

✅ Don’t just memorize answers → Practice hands-on with Docker containers and AWS Free Tier.
✅ Expect follow-up scenario-based questions (e.g., “How would you set up HA for a web app?”).
✅ Use the STAR Method (Situation, Task, Action, Result) to explain real-world experiences.
✅ Stay updated → Docker & AWS services evolve quickly, so always check official docs.

🏁 Conclusion

We covered 25 must-know Docker & AWS questions with detailed answers that will help you in your next DevOps interview.

💡 Which of these questions do you think interviewers ask most often? Let’s discuss in the comments!

#DevOps #Docker #AWS #InterviewPreparation #30DaysOfDevOps #Cloud

🚀 Day 21 of 30 – DevOps Interview Preparation Challenge