DevOps Interview Questions

An image is a read-only template; a container is a running instance of an image

A Docker image is a read-only template containing the application code, dependencies, and runtime. A container is a running instance of an image with its own writable filesystem layer. You can create multiple containers from the same image.

To reduce the final image size by separating build dependencies from the runtime image

Multi-stage builds use multiple FROM statements in a Dockerfile. Build tools and dependencies stay in earlier stages, and only the final artifacts are copied to the runtime stage. This dramatically reduces the final image size (e.g., a Java app: build with Maven/JDK, run with JRE only).

To define and run multi-container Docker applications using a YAML configuration file

Docker Compose uses a docker-compose.yml file to define multi-container applications. It allows you to configure services, networks, and volumes in one file, then start everything with docker compose up. Ideal for local development environments with multiple interdependent services.

The smallest deployable unit in Kubernetes, containing one or more containers that share network and storage

A Pod is the smallest deployable unit in Kubernetes. It encapsulates one or more containers that share the same network namespace (IP address and ports) and can share storage volumes. Pods are typically managed by higher-level controllers like Deployments.

A Deployment manages a set of identical Pod replicas, handling scaling, rolling updates, and self-healing

A Deployment is a higher-level controller that manages a ReplicaSet of Pods. It provides declarative updates, rolling deployments, rollbacks, and scaling. Unlike a bare Pod, if a Pod managed by a Deployment fails, the Deployment controller automatically creates a replacement.

ClusterIP: internal only; NodePort: exposes on each node's IP; LoadBalancer: provisions an external cloud load balancer

ClusterIP (default) exposes the service on an internal cluster IP, accessible only within the cluster. NodePort exposes the service on a static port on every node's IP. LoadBalancer extends NodePort by provisioning an external load balancer (e.g., AWS ELB) that routes traffic to the service.

Storing non-confidential configuration data as key-value pairs that can be consumed by Pods

ConfigMaps store non-confidential configuration data (environment variables, config files, command-line args) separately from container images. Pods can consume ConfigMaps as environment variables, volumes, or command-line arguments. For sensitive data, use Secrets instead.

To automate the process of building, testing, and deploying code changes

CI/CD (Continuous Integration/Continuous Delivery) pipelines automate the software delivery process. CI automatically builds and tests code on every commit. CD automatically deploys validated changes to staging or production environments, reducing manual errors and accelerating delivery.

Sets owner to read/write/execute, group and others to read/execute

chmod 755 sets permissions using octal notation: 7 (rwx) for the owner, 5 (r-x) for the group, and 5 (r-x) for others. This is a common permission set for executable scripts and web directories — the owner can modify, everyone else can read and execute.

Lists all running processes and filters for Java processes

ps aux lists all running processes with details (user, PID, CPU, memory). The pipe | sends the output to grep java, which filters for lines containing 'java'. This is a common way to find running Java applications and their process IDs.

Merge creates a merge commit preserving branch history; rebase replays commits on top of the target branch for a linear history

Git merge creates a merge commit that combines two branches, preserving the full branch history. Git rebase moves (replays) your commits on top of the target branch, creating a cleaner, linear history. Rebase rewrites commit history, so avoid rebasing shared/public branches.

To specify files and directories that Git should not track or include in commits

A .gitignore file tells Git which files and directories to ignore (not track). Common entries include build artifacts (target/, dist/), dependency directories (node_modules/), IDE files (.idea/, .vscode/), and environment files (.env). It keeps the repository clean.

An Infrastructure as Code tool that lets you define, provision, and manage cloud resources declaratively

Terraform is an Infrastructure as Code (IaC) tool by HashiCorp. It uses a declarative configuration language (HCL) to define infrastructure resources across multiple cloud providers. Terraform tracks resource state, plans changes, and applies them in the correct order.

plan shows what changes will be made without applying them; apply actually creates/modifies/destroys resources

terraform plan is a dry run that shows what resources will be created, modified, or destroyed without making changes. terraform apply executes the planned changes. Always run plan before apply to review changes. In CI/CD, you can save a plan file and apply it exactly.

Terraform state maps your configuration to real-world resources, enabling change detection and resource tracking

Terraform state (terraform.tfstate) is a JSON file that maps your Terraform configuration to real infrastructure resources. It tracks resource IDs, metadata, and dependencies. Without state, Terraform cannot determine what exists and what needs to change. Remote state backends (S3, Terraform Cloud) enable team collaboration.

A text file that defines a Jenkins pipeline as code, typically stored in version control

A Jenkinsfile defines a Jenkins pipeline as code using either Declarative or Scripted syntax. It is stored in the project's source control repository, enabling pipeline versioning, code review, and reproducibility. It defines stages like Build, Test, and Deploy.

To specify where the pipeline or a stage should run (which Jenkins node or Docker container)

The agent directive in a Jenkinsfile specifies where the pipeline or a specific stage runs. Options include any (any available agent), label (agents with a specific label), docker (run inside a Docker container), or none (no global agent, each stage must define its own).

A logically isolated virtual network in AWS where you can launch resources with your own IP address range

A VPC is a logically isolated section of the AWS Cloud where you define your own virtual network. You control IP address ranges, subnets, route tables, and network gateways. Resources like EC2 instances, RDS databases, and Lambda functions run within a VPC.

Horizontal scaling adds more machines (scale out); vertical scaling adds more resources to an existing machine (scale up)

Horizontal scaling (scale out) adds more machines/instances to distribute the load. Vertical scaling (scale up) adds more CPU, RAM, or disk to an existing machine. Horizontal scaling provides better fault tolerance and is preferred for cloud-native applications. Vertical scaling has physical limits.

S3 is object storage accessible via HTTP; EBS is block storage attached to EC2 instances like a virtual hard drive

S3 (Simple Storage Service) is object storage accessed via HTTP APIs — ideal for files, backups, static assets, and data lakes. EBS (Elastic Block Store) provides block-level storage volumes attached to EC2 instances — ideal for databases and application data that needs a filesystem. S3 is virtually unlimited; EBS has a provisioned size.