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AWS Lambda Interview Questions (Top 100 Questions with Answers)

Master AWS Lambda Interview Questions with production-oriented questions covering serverless architecture, Lambda execution model, event sources, cold starts, concurrency, scaling, monitoring, security, deployment strategies, performance optimization, and real-world production scenarios.

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Introduction

AWS Lambda is Amazon's fully managed serverless compute service.

It allows developers to execute code without provisioning or managing servers.

Lambda is widely used for

  • REST APIs
  • Event-Driven Applications
  • File Processing
  • Data Pipelines
  • Scheduled Jobs
  • Serverless Microservices
  • Automation
  • Backend Processing

Today, almost every AWS interview includes Lambda questions because serverless architecture is widely adopted across industries.

This guide contains the Top 100 AWS Lambda Interview Questions frequently asked in

  • AWS Solution Architect
  • Cloud Engineer
  • DevOps Engineer
  • Java Developer
  • Platform Engineer
  • Site Reliability Engineer

AWS Lambda Learning Roadmap

Lambda Basics
      │
      ▼
Execution Model
      │
      ▼
Triggers
      │
      ▼
Concurrency
      │
      ▼
Scaling
      │
      ▼
Security
      │
      ▼
Monitoring
      │
      ▼
Performance
      │
      ▼
Production

Lambda Fundamentals

1. What is AWS Lambda?

AWS Lambda is a serverless compute service that runs code in response to events without managing servers.

2. Why use Lambda?

  • No Server Management
  • Auto Scaling
  • Pay Per Use
  • High Availability
  • Event Driven

3. What is Serverless Computing?

A cloud execution model where infrastructure is managed by the cloud provider.

4. Which languages does Lambda support?

  • Java
  • Python
  • Node.js
  • Go
  • C#
  • Ruby
  • Custom Runtime

5. Does Lambda require provisioning servers?

No.

Lambda Execution Model

6. What happens when Lambda is invoked?

Event
   │
   ▼
Lambda Service
   │
   ▼
Execution Environment
   │
   ▼
Run Function
   │
   ▼
Return Response

7. What is an Invocation?

Execution of a Lambda function.

8. Synchronous Invocation?

Caller waits for response.

Example

API Gateway.

9. Asynchronous Invocation?

Caller does not wait.

Example

S3 Event.

10. Poll-Based Invocation?

Lambda polls services like

  • SQS
  • DynamoDB Streams
  • Kinesis

Lambda Lifecycle

11. Lambda Execution Environment?

Container where function executes.

12. Initialization Phase?

Runtime starts.

Libraries loaded.

13. Invocation Phase?

Function executes.

14. Shutdown Phase?

Execution environment eventually removed after inactivity.

15. Can execution environments be reused?

Yes.

Warm starts reuse existing environments.

Cold Starts

16. What is Cold Start?

First execution requiring creation of a new execution environment.

17. Why Cold Starts happen?

  • First invocation
  • Scaling out
  • Idle timeout

18. Cold Start impact?

Higher latency.

19. Reduce Cold Starts?

  • Provisioned Concurrency
  • Smaller packages
  • Lightweight initialization

20. Which language usually experiences larger cold starts?

Java due to JVM startup.

Lambda Scaling

21. Does Lambda Auto Scale?

Yes.

Automatically.

22. Scaling unit?

Concurrent executions.

23. What is Concurrency?

Number of functions executing simultaneously.

24. Reserved Concurrency?

Guarantees concurrency for a function and limits its maximum concurrency.

25. Provisioned Concurrency?

Keeps execution environments initialized.

Lambda Limits

26. Maximum Timeout?

15 minutes.

27. Temporary Storage?

Configurable up to 10 GB.

28. Deployment Package Size?

Depends on ZIP or container image deployment limits.

29. Memory Range?

128 MB to 10,240 MB.

30. CPU Allocation?

CPU increases with configured memory.

Lambda Triggers

31. What can trigger Lambda?

  • API Gateway
  • S3
  • SNS
  • SQS
  • EventBridge
  • DynamoDB Streams
  • Kinesis
  • CloudWatch Events

32. API Gateway Trigger?

Invokes Lambda for REST or HTTP APIs.

33. S3 Trigger?

Object upload/delete events.

34. EventBridge Trigger?

Scheduled or event-driven execution.

35. DynamoDB Streams?

React to table changes.

36. Kinesis Trigger?

Real-time stream processing.

37. SNS Trigger?

Pub/Sub messaging.

38. SQS Trigger?

Processes messages from queues.

39. CloudWatch Trigger?

Scheduled cron jobs.

40. Step Functions?

Orchestrates multiple Lambda functions.

Lambda Security

41. IAM Execution Role?

Defines permissions for Lambda.

42. Resource Policy?

Controls who can invoke Lambda.

43. VPC Lambda?

Allows secure access to private resources.

44. Environment Variables?

Store configuration.

Sensitive values should be encrypted.

45. Secrets Management?

AWS Secrets Manager.

Monitoring

46. Logs?

CloudWatch Logs.

47. Metrics?

CloudWatch Metrics.

48. X-Ray?

Distributed tracing.

49. CloudTrail?

Tracks API activity.

50. Common Metrics?

  • Invocations
  • Errors
  • Duration
  • Throttles
  • Concurrent Executions

Performance

51. How to improve performance?

  • Increase Memory
  • Reduce Dependencies
  • Optimize Initialization

52. Why does increasing memory improve performance?

More CPU is allocated.

53. Deployment Package Optimization?

Remove unnecessary libraries.

54. Reuse database connections?

Yes.

Outside handler.

55. Cache SDK clients?

Yes.

Reuse across invocations.

Lambda Networking

56. Lambda inside VPC?

Supported.

57. Why deploy in VPC?

Access

  • RDS
  • ElastiCache
  • Internal Services

58. Internet access from VPC Lambda?

Requires NAT Gateway for private subnets.

59. Lambda without VPC?

Has Internet access by default.

60. VPC Endpoints?

Reduce Internet dependency.

Production Scenarios

61. API response slow.

Check

  • Cold Starts
  • Memory
  • Dependencies

62. Lambda timing out.

Increase timeout or optimize code.

63. Access denied.

Check IAM Role.

64. RDS connection exhaustion.

Use RDS Proxy.

65. High invocation cost.

Optimize execution duration.

66. Millions of S3 uploads.

Use S3 Event Notifications.

67. Retry failed processing.

Configure Dead Letter Queue or Lambda Destinations.

68. Processing SQS messages.

Use Batch Processing.

69. High traffic API.

Use Provisioned Concurrency.

70. Scheduled report generation.

Use EventBridge Scheduler.

Architect Questions

71. Lambda vs EC2?

Lambda is serverless.

EC2 requires server management.

72. Lambda vs ECS?

Lambda for event-driven workloads.

ECS for long-running containers.

73. Lambda vs EKS?

EKS manages Kubernetes workloads.

74. Lambda vs Fargate?

Fargate runs containers.

Lambda runs functions.

75. Lambda vs Step Functions?

Step Functions coordinate workflows.

Lambda executes business logic.

Event-Driven Architecture

76. Event-Driven Flow

S3 Upload
     │
     ▼
Event Notification
     │
     ▼
Lambda
     │
     ▼
Resize Image
     │
     ▼
Store in S3

77. Benefits?

  • Decoupled
  • Scalable
  • Cost Effective

78. Idempotency?

Functions should safely handle duplicate events.

79. Retry Behavior?

Depends on event source.

80. Dead Letter Queue?

Captures failed events.

Senior Interview Questions

81. Common Lambda mistakes?

  • Large deployment packages
  • Opening DB connection per request
  • Ignoring retries
  • No monitoring

82. Best practices?

  • Least Privilege IAM
  • Small Functions
  • Idempotency
  • Monitoring

83. Cost Optimization?

  • Optimize Duration
  • Reduce Memory
  • Remove Idle Invocations

84. Java Lambda optimization?

  • Provisioned Concurrency
  • SnapStart (for supported Java runtimes)
  • Lightweight frameworks

85. Lambda SnapStart?

Reduces Java cold starts by restoring pre-initialized execution environments.

86. Deployment methods?

  • ZIP Package
  • Container Image

87. Blue-Green Deployment?

Supported using Lambda aliases and weighted traffic shifting.

88. Canary Deployment?

Gradually shifts traffic to new versions.

89. What should be monitored?

  • Errors
  • Duration
  • Throttles
  • Concurrency
  • Memory Usage

90. Production Readiness Checklist?

  • IAM Role
  • Monitoring
  • Retry Strategy
  • DLQ
  • Logging
  • Alerts

91. Lambda Layers?

Reusable libraries shared across functions.

92. Function URLs?

Built-in HTTPS endpoint for Lambda.

93. Lambda Extensions?

Integrate monitoring, logging, and security agents.

94. Reserved vs Provisioned Concurrency?

Reserved controls concurrency limits.

Provisioned reduces cold starts.

95. RDS Proxy?

Manages database connections efficiently.

96. Event Source Mapping?

Connects Lambda with poll-based services.

97. How to secure Lambda?

  • IAM Roles
  • KMS
  • Secrets Manager
  • VPC
  • Least Privilege

98. What do interviewers expect?

  • Serverless fundamentals
  • Event-driven architecture
  • Production experience
  • Cost optimization

99. Common troubleshooting steps?

  • Check CloudWatch Logs
  • Review Metrics
  • Verify IAM
  • Analyze X-Ray traces
  • Review Event Source

100. How should you prepare?

  • Build REST APIs
  • Process S3 Events
  • Connect Lambda with DynamoDB
  • Integrate SQS/SNS
  • Practice Step Functions
  • Deploy Java Lambda applications

Production Lambda Architecture

                 Client
                    │
                    ▼
              API Gateway
                    │
                    ▼
              AWS Lambda
          ┌─────────┼─────────┐
          ▼         ▼         ▼
      DynamoDB    Amazon RDS   S3
          │           │         │
          └───────────┼─────────┘
                      ▼
            CloudWatch Logs
                      │
                      ▼
               AWS X-Ray

Event-Driven Serverless Architecture

S3 Upload
     │
     ▼
 Lambda Function
     │
     ▼
 SNS Notification
     │
     ▼
 Email / SMS

OR

SQS Queue
     │
     ▼
 Lambda Consumer
     │
     ▼
 DynamoDB

Quick Revision

Topic Key Point
Lambda Serverless Compute
Invocation Function Execution
Cold Start First Execution Delay
Warm Start Reused Environment
Reserved Concurrency Concurrency Limit
Provisioned Concurrency Reduced Cold Starts
IAM Role Permissions
CloudWatch Monitoring
EventBridge Scheduler
RDS Proxy Connection Management

Interview Tips

During AWS Lambda interviews:

  • Explain the Lambda execution lifecycle (Initialization → Invocation → Reuse).
  • Understand Cold Starts, Provisioned Concurrency, and SnapStart for Java.
  • Know the difference between synchronous, asynchronous, and poll-based invocations.
  • Explain integrations with API Gateway, S3, SQS, SNS, DynamoDB Streams, and EventBridge.
  • Discuss production practices such as IAM Roles, RDS Proxy, DLQs, CloudWatch, X-Ray, and idempotency.
  • Relate answers to serverless microservices and event-driven architectures.

Summary

AWS Lambda is the core serverless compute service in AWS and is widely used for event-driven applications, APIs, automation, and microservices. Strong Lambda interview performance requires understanding execution lifecycle, cold starts, concurrency, event sources, security, monitoring, performance optimization, and production best practices.

Mastering these 100 AWS Lambda interview questions prepares you for AWS Cloud Engineer, DevOps Engineer, Java Developer, Platform Engineer, Site Reliability Engineer, and Solution Architect interviews.