Beginner-Level AWS Lambda Interview Questions.
1. What is AWS Lambda?
AWS Lambda is a serverless compute service that lets you run code without managing servers.
2. What does “serverless” mean in AWS Lambda?
Serverless means AWS manages servers, scaling, and infrastructure while you focus only on code.
3. What is a Lambda function?
A Lambda function is the code and configuration that runs in response to an event.
4. Which programming languages are supported by AWS Lambda?
Python, Node.js, Java, C#, Go, Ruby, and custom runtimes.
5. What is an event in AWS Lambda?
An event is a trigger that invokes a Lambda function.
6. Name common AWS Lambda triggers.
API Gateway, S3, DynamoDB, SQS, SNS, EventBridge.
7. What is the maximum execution time of a Lambda function?
15 minutes.
8. What is the minimum and maximum memory allocation?
128 MB (min) to 10,240 MB (max).
9. What happens if a Lambda function times out?
The function stops execution and returns a timeout error.
10. What is AWS Lambda pricing based on?
Number of requests and execution duration (GB-seconds).
11. What is an execution role in AWS Lambda?
An IAM role that grants permissions for Lambda to access AWS services.
12. Can AWS Lambda access other AWS services?
Yes, using IAM roles and policies.
13. What is the default timeout for Lambda?
3 seconds.
14. What is concurrency in AWS Lambda?
The number of Lambda functions running at the same time.
15. Does AWS Lambda scale automatically?
Yes, AWS Lambda automatically scales based on incoming requests.
16. What is CloudWatch used for in Lambda?
Logging, monitoring, and setting alarms.
17. Where are Lambda logs stored?
Amazon CloudWatch Logs.
18. What is a Lambda layer?
A way to share libraries and dependencies across multiple Lambda functions.
19. Can Lambda functions run inside a VPC?
Yes, Lambda can be configured to access VPC resources.
20. What is the Lambda function handler?
The entry point method that AWS Lambda calls when the function runs.
21. What is a deployment package?
A ZIP file containing function code and dependencies.
22. What is AWS Lambda cold start?
The delay when a Lambda function runs for the first time or after inactivity.
23. Is AWS Lambda stateless?
Yes, Lambda functions are stateless by default.
24. Can AWS Lambda be used for REST APIs?
Yes, using Amazon API Gateway.
25. What happens when a Lambda function fails?
AWS may retry execution or send the event to a DLQ (if configured).
26. What is the maximum package size for Lambda?
50 MB (zipped) and 250 MB (unzipped).
27. Can Lambda functions be scheduled?
Yes, using EventBridge (CloudWatch Events).
28. What is environment variable usage in Lambda?
To store configuration values securely.
29. Can AWS Lambda connect to a database?
Yes, via VPC or public endpoints.
30. Give a real-world use case of AWS Lambda.
Image processing, REST APIs, log processing, automation tasks.
Intermediate-Level AWS Lambda Interview Questions.
1. What happens internally when an AWS Lambda function is invoked?
AWS Lambda creates or reuses an execution environment, loads the code, and runs the handler in response to the event.
2. What is a cold start and why does it happen?
A cold start occurs when Lambda creates a new execution environment, causing initial latency.
3. How can you reduce cold start latency?
- Increase memory
- Use Provisioned Concurrency
- Minimize dependencies
- Choose lighter runtimes
4. What is a warm start in AWS Lambda?
A warm start happens when Lambda reuses an existing execution environment.
5. What is Lambda concurrency?
Concurrency is the number of Lambda instances executing simultaneously.
6. What is reserved concurrency?
Reserved concurrency guarantees a specific number of concurrent executions for a function.
7. What happens when Lambda concurrency limits are exceeded?
Requests are throttled and return a 429 error.
8. How does AWS Lambda scale automatically?
Lambda scales by creating more execution environments based on incoming events.
9. What is the difference between synchronous and asynchronous invocation?
- Synchronous: Caller waits for response
- Asynchronous: Event is queued and processed later
10. What AWS services invoke Lambda asynchronously?
S3, SNS, EventBridge, and CloudWatch Events.
11. How does error handling work for asynchronous invocations?
Lambda retries automatically and can send failed events to a DLQ or destination.
12. What are Lambda Destinations?
They route invocation results to another AWS service like SNS, SQS, or Lambda.
13. How do you monitor Lambda performance?
Using CloudWatch metrics, logs, and AWS X-Ray.
14. What metrics are important for Lambda monitoring?
Duration, Invocations, Errors, Throttles, and ConcurrentExecutions.
15. How do Lambda layers help in real-world applications?
They allow reuse of shared libraries and reduce deployment size.
16. How does Lambda integrate with API Gateway?
API Gateway triggers Lambda to handle HTTP requests and return responses.
17. What is the maximum payload size for Lambda invocation?
- 6 MB (synchronous)
- 256 KB (asynchronous)
18. How does Lambda work inside a VPC?
Lambda attaches ENIs to access VPC resources, which can increase cold start time.
19. What are common causes of Lambda timeouts?
Slow external calls, low memory allocation, infinite loops.
20. How do you handle secrets in Lambda?
Using AWS Secrets Manager or Parameter Store.
21. What is AWS X-Ray used for?
Tracing requests and identifying performance bottlenecks.
22. What is function versioning in Lambda?
Each code update creates a new immutable version.
23. What are Lambda aliases?
Aliases point to specific Lambda versions (e.g., dev, test, prod).
24. How do you deploy Lambda functions?
Using AWS Console, CLI, SAM, Serverless Framework, or Terraform.
25. How do you handle retries in Lambda?
Using built-in retries, DLQs, or custom retry logic.
26. What is the difference between DLQ and Lambda Destinations?
DLQ captures failed events; destinations route both success and failure results.
27. How does memory allocation affect Lambda performance?
More memory also provides more CPU power, improving execution speed.
28. Can Lambda functions call other Lambda functions?
Yes, directly or through services like Step Functions.
29. How do you manage configuration across environments?
Using environment variables and aliases.
30. What are common real-world use cases at this level?
REST APIs, event processing, ETL jobs, microservices.
Advanced-Level AWS Lambda Interview Questions.
1. How does AWS Lambda manage scalability at massive scale?
Lambda automatically creates multiple execution environments to handle concurrent requests, scaling horizontally without user intervention.
2. Explain Provisioned Concurrency and its use cases.
Provisioned Concurrency keeps execution environments pre-initialized, eliminating cold starts for latency-sensitive workloads.
3. What is Lambda throttling and how do you prevent it?
Throttling occurs when concurrency limits are exceeded. Prevent it using reserved concurrency, request smoothing, or buffering with SQS.
4. How do you design a highly available serverless application using Lambda?
Use multi-AZ services, decouple components with SQS/SNS, enable retries, and implement idempotency.
5. How does memory allocation affect CPU and network performance?
Higher memory allocation provides more CPU and network throughput, reducing execution time.
6. How do you handle long-running processes beyond 15 minutes?
Use Step Functions, break tasks into smaller Lambdas, or move to ECS/Fargate.
7. How does Lambda behave when connected to a VPC?
Lambda creates ENIs to access VPC resources, which can increase cold start latency.
8. How do you optimize Lambda cold start for Java-based functions?
Use smaller JARs, GraalVM native images, Provisioned Concurrency, and reduce initialization logic.
9. What security best practices should be followed for Lambda?
Least-privilege IAM roles, encrypted environment variables, Secrets Manager, and VPC security groups.
10. How do you implement zero-downtime deployments for Lambda?
Use versions and aliases with traffic shifting via CodeDeploy.
11. What is idempotency and why is it important in Lambda?
Idempotency ensures repeated executions do not produce duplicate side effects, crucial for retries.
12. How do you debug Lambda performance issues?
Analyze CloudWatch logs, metrics, AWS X-Ray traces, and optimize cold starts and dependencies.
13. Explain the difference between on-demand and provisioned concurrency.
On-demand scales automatically but may cause cold starts; provisioned concurrency avoids cold starts but costs more.
14. How do you secure Lambda API endpoints?
Use API Gateway authentication (IAM, Cognito, JWT authorizers), WAF, and throttling.
15. How does Lambda integrate with Step Functions for complex workflows?
Step Functions orchestrate Lambdas with retries, branching, and state management.
16. How do you control Lambda costs at scale?
Right-size memory, avoid unnecessary retries, batch events, and monitor usage.
17. How do you implement canary deployments with Lambda?
Use weighted aliases to shift traffic gradually between versions.
18. What happens when Lambda execution fails repeatedly?
AWS retries automatically and may route events to DLQs or destinations.
19. How do you manage secrets securely in Lambda?
Store secrets in AWS Secrets Manager or Parameter Store and fetch them securely at runtime.
20. When should you choose Lambda over containers or EC2?
For event-driven, short-lived, and unpredictable workloads.
21. How do you design Lambda for high throughput?
Use async processing, SQS buffering, concurrency controls, and optimized memory.
22. How does Lambda handle retries differently for sync vs async invocations?
Sync invocations return errors immediately; async invocations are retried automatically.
23. What are Lambda Destinations and advanced use cases?
They route invocation results to services like SNS or EventBridge for downstream processing.
24. How do you implement centralized logging for Lambda?
Use CloudWatch Logs, subscription filters, or third-party logging tools.
25. How does Lambda handle dependency management at scale?
Using Lambda layers and CI/CD pipelines.
26. How do you protect Lambda from traffic spikes?
Enable throttling, use SQS buffering, and configure API Gateway limits.
27. How does AWS Lambda ensure fault tolerance?
By isolating execution environments and running across multiple Availability Zones.
28. What challenges do you face in serverless system design?
Cold starts, observability, state management, and vendor lock-in.
29. How do you migrate a monolithic app to Lambda?
Decompose into microservices, introduce events, and gradually migrate workloads.
30. Describe a real-world production issue you faced with Lambda and how you resolved it.
Interviewers expect discussion around performance, cost, scaling, or security challenges and solutions.
Conclusion.
AWS Lambda has become a core service in modern cloud and serverless architectures, and interviewers expect candidates to demonstrate knowledge that goes far beyond basic definitions. From understanding serverless fundamentals at the beginner level, to handling concurrency, integrations, and monitoring at the intermediate level, and finally to designing scalable, secure, and cost-efficient systems at the advanced level each stage tests a different depth of expertise.
Preparing AWS Lambda interview questions from Beginner to Advanced helps you:
- Build a strong conceptual foundation
- Gain confidence in real-world implementation scenarios
- Make informed architectural and cost optimization decisions
- Clearly explain trade-offs between Lambda, containers, and EC2
Whether you are a fresher starting your cloud journey or an experienced professional aiming for senior or architect roles, mastering AWS Lambda concepts is essential for success in today’s cloud interviews.
Consistent hands-on practice, understanding real production challenges, and keeping up with AWS best practices will set you apart and help you perform confidently in any AWS Lambda interview.
Final Tip: Don’t just memorize answers focus on why AWS Lambda works the way it does and when to use it in real-world applications.
