AWS Interview Questions and Answers

This page is designed for real-world AWS interviews across Cloud Engineer, DevOps Engineer, and SRE roles. The questions progress from core cloud fundamentals to production-ready architecture, security, scalability, and troubleshooting scenarios, with answers that explain why each service is used, how it is implemented in practice, and the trade-offs involved in real AWS environments.

Basic AWS Interview Questions

1. What is AWS and why is it used?

   Amazon Web Services (AWS) is a cloud computing platform that provides on-demand infrastructure, managed services, and application components over the internet.

   AWS eliminates the need for upfront hardware investment and allows organizations to scale resources dynamically based on demand. It is widely used to improve availability, reduce operational overhead, enable global reach, and support modern application architectures such as microservices and serverless systems.

2. What is EC2 and when would you use it?

   Amazon EC2 provides resizable virtual servers that give users full control over the operating system, networking, and installed software.

   EC2 is used when applications require custom OS configurations, legacy software, specific networking rules, or predictable compute behavior. It is commonly used for backend services, custom APIs, and workloads that cannot easily be containerized or serverless.

3. What is the difference between Security Groups and Network ACLs?

   Security Groups act as stateful firewalls at the instance level, controlling inbound and outbound traffic.

   Network ACLs are stateless and operate at the subnet level, allowing both allow and deny rules. Security Groups are typically used for fine-grained access control, while NACLs provide an additional layer of network-wide security.

4. What is Amazon S3 and why is it so widely used?

   Amazon S3 is an object storage service designed for durability, scalability, and high availability.

   It is widely used for backups, static website hosting, log storage, data lakes, and media assets because it automatically scales, offers multiple storage classes, and integrates easily with other AWS services.

5. What is IAM and why is it critical in AWS?

   AWS IAM controls authentication and authorization for AWS resources.

   It is critical because misconfigured permissions are a common cause of security breaches. Best practices include using IAM roles, enforcing least privilege, and avoiding the use of root credentials for daily operations.

6. What is an AWS Region and Availability Zone?

   A Region is a geographical area that contains multiple Availability Zones (AZs).

   Availability Zones are isolated data centers designed to prevent single points of failure. Architecting across multiple AZs improves fault tolerance and availability for production workloads.

7. Why is high availability important in AWS?

   High availability ensures applications remain accessible during failures.

   AWS supports high availability through multi-AZ architectures, load balancers, and managed services that automatically handle infrastructure failures.

8. What is an Elastic Load Balancer (ELB)?

   ELB distributes incoming traffic across multiple targets such as EC2 instances.

   It improves fault tolerance, supports health checks, and ensures traffic is routed only to healthy instances, which is critical for scalable web applications.

9. What is Auto Scaling and why is it useful?

   Auto Scaling automatically adjusts the number of EC2 instances based on demand.

   It helps handle traffic spikes, reduce costs during low usage, and maintain application performance without manual intervention.

10. What is Amazon RDS?

    Amazon RDS is a managed relational database service that supports engines like MySQL, PostgreSQL, and SQL Server.

    It simplifies database management by handling backups, patching, replication, and failover, allowing teams to focus on application development.

11. What is the difference between EC2 and Lambda?

    EC2 provides full control over virtual machines, while Lambda is a serverless compute service.

    Lambda is ideal for event-driven, short-running tasks, whereas EC2 is better for long-running or stateful applications requiring OS-level access.

12. What is Amazon VPC?

    Amazon VPC allows you to create a logically isolated network within AWS.

    It gives full control over IP addressing, routing, and security, making it essential for designing secure and compliant cloud architectures.

13. What is the purpose of an Internet Gateway?

    An Internet Gateway enables communication between VPC resources and the internet.

    Without it, instances in public subnets cannot be accessed externally, even if they have public IP addresses.

14. What is CloudWatch and how is it used?

    Amazon CloudWatch provides monitoring and observability for AWS resources.

    It collects metrics, logs, and events, allowing teams to detect issues, set alerts, and analyze system performance.

15. What is AWS CloudTrail?

    CloudTrail records API calls made within an AWS account.

    It is essential for auditing, security investigations, and compliance by tracking who did what and when.

16. What is the shared responsibility model?

    AWS is responsible for the security of the cloud infrastructure.

    Customers are responsible for securing their applications, data, configurations, and access controls within AWS.

17. What are AWS Tags and why are they important?

    Tags are key-value metadata attached to AWS resources.

    They are crucial for cost allocation, automation, access control, and resource organization in large environments.

18. What is AWS Elastic IP?

    Elastic IP is a static public IP address that can be reassigned between instances.

    It is often used for failover scenarios where IP consistency is required.

19. What is the difference between public and private subnets?

    Public subnets have routes to the internet via an Internet Gateway.

    Private subnets do not have direct internet access and are used for internal services like databases.

20. Why should you avoid using the root account?

    The root account has unrestricted access to all AWS resources.

    It should only be used for initial setup, with daily operations handled through IAM users and roles with limited permissions.

Advanced & Scenario-Based AWS Interview Questions

1. How would you design a highly available web application in AWS?

   A highly available architecture in AWS is designed to eliminate single points of failure.

   It typically includes an Application Load Balancer distributing traffic across EC2 instances in multiple Availability Zones, backed by an Auto Scaling Group. A managed database like RDS with Multi-AZ ensures database failover, while static assets are served from S3 and CloudFront. This design allows the application to remain available even if an entire AZ goes down.

2. Your EC2 instance is running and healthy, but users cannot access the application. What do you check?

   This scenario usually indicates a networking or security misconfiguration.

   You should verify Security Group inbound rules, Network ACLs, route tables, and whether the instance is in a public subnet with an Internet Gateway attached. Additionally, confirm that the application is listening on the expected port and that the load balancer health checks are passing.

3. When would you choose AWS Lambda over EC2?

   Lambda is ideal for event-driven, short-lived workloads that do not require server management.

   It is commonly used for API backends, file processing, and automation tasks. However, Lambda is not suitable for long-running processes, heavy CPU workloads, or applications requiring persistent local state.

4. How do you securely store and access database credentials in AWS?

   Database credentials should never be hard-coded in application code or configuration files.

   AWS Secrets Manager or SSM Parameter Store (SecureString) should be used to store secrets. Applications should access secrets using IAM roles, allowing credentials to be rotated automatically without redeploying the application.

5. How does Auto Scaling work and what metrics can trigger scaling?

   Auto Scaling dynamically adjusts the number of EC2 instances based on demand.

   Common triggers include CPU utilization, memory usage (via CloudWatch agent), request count, or custom application metrics. Proper scaling policies ensure performance during traffic spikes while minimizing cost during low usage periods.

6. How would you design a secure VPC architecture for production?

   A secure VPC design separates public and private resources using multiple subnets.

   Public subnets host load balancers and bastion hosts, while private subnets contain application servers and databases. Security Groups, NACLs, IAM roles, and VPC endpoints are used to restrict access and reduce attack surface.

7. Your RDS database is slow under load. How do you troubleshoot?

   Database slowness can be caused by inefficient queries, insufficient resources, or storage bottlenecks.

   You should analyze CloudWatch metrics, enable Performance Insights, review slow query logs, and check connection counts. Scaling the instance, adding read replicas, or optimizing queries may be required.

8. How do you implement disaster recovery in AWS?

   Disaster recovery strategies depend on business requirements for RTO and RPO.

   Options include backup-and-restore, pilot light, warm standby, or multi-region active-active architectures. Services like S3 replication, RDS snapshots, and Route 53 health checks support automated recovery.

9. What is the AWS Shared Responsibility Model and why is it important?

   The shared responsibility model defines security ownership between AWS and the customer.

   AWS secures the underlying infrastructure, while customers are responsible for their data, applications, IAM, and network configurations. Misunderstanding this model is a common cause of security incidents.

10. How do you restrict internet access for private EC2 instances?

    Private instances should not have public IPs or direct internet routes.

    Outbound internet access can be provided via a NAT Gateway, while inbound access is restricted entirely. VPC endpoints can be used to access AWS services without exposing traffic to the public internet.

11. What happens if an Availability Zone goes down?

    If an application is deployed across multiple AZs, traffic is automatically routed to healthy instances.

    Auto Scaling replaces failed instances, and managed services like RDS Multi-AZ perform automatic failover. Single-AZ deployments, however, experience downtime.

12. How do you monitor and alert on AWS infrastructure?

    Monitoring is handled using CloudWatch metrics, logs, and alarms.

    Alarms trigger notifications or automated actions when thresholds are breached. For deeper visibility, logs can be centralized and analyzed using CloudWatch Logs or integrated observability tools.

13. Your S3 bucket was accidentally made public. How do you prevent this?

    Public access to S3 is a common security risk.

    Use S3 Block Public Access, bucket policies, IAM conditions, and regular audits using AWS Config to prevent accidental exposure of sensitive data.

14. How do you control costs in AWS?

    Cost control involves monitoring usage, right-sizing resources, and using pricing models like Reserved Instances or Savings Plans.

    AWS Cost Explorer, budgets, and tagging help track and optimize spending across teams and environments.

15. How do you manage access for applications running on EC2?

    Applications should use IAM roles attached to EC2 instances.

    This avoids hard-coded credentials and allows fine-grained access control to AWS services such as S3, DynamoDB, or Secrets Manager.

16. What is CloudFormation and why is it used?

    CloudFormation allows infrastructure to be defined as code.

    It enables repeatable, version-controlled infrastructure deployments and simplifies rollback and environment consistency.

17. Your load balancer health checks are failing. What do you investigate?

    Health check failures usually indicate application or network issues.

    Check application logs, health check path configuration, security groups, and ensure the application responds within the expected timeout.

18. How do you handle secrets rotation without downtime?

    Secrets Manager supports automatic rotation using Lambda.

    Applications retrieve secrets dynamically, allowing credentials to rotate without redeployment or service interruption.

19. How do you expose a private application securely to the internet?

    Use an Application Load Balancer in a public subnet.

    Backend services remain in private subnets, protected by Security Groups and accessed only through the load balancer.

20. Why is multi-account strategy recommended in AWS?

    Multiple accounts provide strong isolation between environments.

    They improve security, simplify billing, and reduce blast radius in case of misconfiguration or compromise.

These advanced questions are commonly asked for DevOps Engineer, Cloud Engineer, and SRE roles.