AWS Lambda is one of the most frequently asked topics in AWS, Cloud Architecture, DevOps, Data Engineering, AI/ML, and Solution Architect interviews.

1. What is AWS Lambda?

Answer:

AWS Lambda is a serverless compute service that allows you to run code without provisioning or managing servers.

You upload your code, and Lambda automatically:

Executes code
Scales automatically
Handles infrastructure
Performs patching and maintenance
Charges only for actual execution time

Benefits:

No server management
Auto-scaling
Pay-per-use
High availability
Event-driven architecture

Example:

A file uploaded to S3 triggers a Lambda function that resizes images automatically.

2. What is Serverless Computing?

Answer:

Serverless computing means developers focus only on application code while AWS manages:

Servers
OS patches
Capacity planning
Scaling
Availability

Serverless does NOT mean no servers exist.

AWS manages them behind the scenes.

Examples:

Lambda
API Gateway
DynamoDB
Step Functions

3. How Does AWS Lambda Work?

Answer:

Workflow:

Event occurs
Trigger invokes Lambda
Lambda execution environment starts
Code executes
Response returned

Example:

S3 Upload → Lambda → Process File → Store Results in DynamoDB

4. What Can Trigger a Lambda Function?

Answer:

Lambda supports many event sources:

AWS Services

S3
DynamoDB Streams
Kinesis
SNS
SQS
EventBridge
CloudWatch
API Gateway
Cognito
Step Functions

External Sources

REST APIs
Webhooks
Mobile Apps

Example

User uploads file → S3 Event → Lambda Trigger

5. What Runtime Environments Does Lambda Support?

Answer:

Supported runtimes include:

Python
Java
Node.js
.NET
Ruby
Go
Custom Runtime

Common Interview Answer:

Python and Node.js are most popular due to fast startup times.

6. What is a Lambda Execution Environment?

Answer:

Execution environment is the runtime container where Lambda code runs.

Contains:

Runtime
Function code
Memory
CPU allocation
Temporary storage

Lifecycle:

Init Phase
Invoke Phase
Shutdown Phase

7. What is a Cold Start?

Answer:

Cold start occurs when Lambda creates a new execution environment.

Steps:

Container creation
Runtime loading
Code initialization
Dependency loading

This increases latency.

Causes:

First invocation
Scaling events
Long idle periods

8. How Do You Reduce Cold Starts?

Answer:

Methods:

Provisioned Concurrency

Pre-warms Lambda environments.

Optimize Package Size

Reduce dependencies.

Use Lightweight Runtime

Python and Node.js generally start faster.

Reuse Connections

Initialize outside handler.

Example:

import boto3

s3 = boto3.client('s3')

def lambda_handler(event, context):
    return "Success"

9. What is Provisioned Concurrency?

Answer:

Provisioned Concurrency keeps Lambda environments warm.

Benefits:

Predictable latency
Reduced cold starts
Better user experience

Use cases:

Banking applications
APIs
Real-time systems

10. What is Reserved Concurrency?

Answer:

Reserved concurrency guarantees a specific number of concurrent executions.

Example:

Lambda A reserved = 100

Other Lambdas cannot consume these 100 executions.

Benefits:

Resource isolation
Traffic control

11. Difference Between Reserved and Provisioned Concurrency?

Feature	Reserved	Provisioned
Limits execution	Yes	No
Guarantees capacity	Yes	Yes
Eliminates cold start	No	Yes
Additional cost	No	Yes

Interview favorite question.

12. What is Lambda Timeout?

Answer:

Maximum execution duration.

Range:

1 second
Up to 15 minutes

Default:

3 seconds

Best practice:

Set realistic timeout based on workload.

13. What is Lambda Memory Allocation?

Answer:

Memory can be configured from:

128 MB to 10,240 MB

Increasing memory also increases:

CPU
Network throughput

Sometimes higher memory reduces overall cost due to faster execution.

14. How is Lambda Pricing Calculated?

Answer:

Based on:

Requests

Number of invocations

Duration

Execution time

Memory

Configured memory

Formula:

Cost = Requests + GB-Seconds

Example:

512 MB running for 2 seconds

= 1 GB-second

15. What is Lambda Layers?

Answer:

Layers allow sharing code across functions.

Examples:

Common libraries
SDKs
Security modules
Utility functions

Benefits:

Smaller deployment packages
Reusability
Easier maintenance

16. What is Lambda Versioning?

Answer:

Versions create immutable snapshots of code and configuration.

Example:

Version 1
Version 2
Version 3

Each version remains unchanged.

Useful for rollback.

17. What are Lambda Aliases?

Answer:

Aliases provide friendly names for versions.

Examples:

DEV
TEST
PROD

Example:

PROD → Version 10

Allows deployment without changing application code.

18. What is Blue-Green Deployment in Lambda?

Answer:

Deploy new version alongside old version.

Traffic routing:

90% → Old Version
10% → New Version

Gradually increase traffic.

Implemented using:

Lambda aliases
CodeDeploy

19. What is Canary Deployment?

Answer:

A small percentage of traffic is sent to a new version.

Example:

95% Old Version
5% New Version

Monitor before full rollout.

20. What is Lambda Destinations?

Answer:

After asynchronous execution:

Success → Destination
Failure → Destination

Supported:

SNS
SQS
EventBridge
Lambda

Useful for auditing and workflows.

21. Difference Between Synchronous and Asynchronous Invocation?

Synchronous

Caller waits for response.

Examples:

API Gateway
ALB

Asynchronous

Caller doesn’t wait.

Examples:

SNS
EventBridge
S3

22. What Happens if Lambda Fails?

Answer:

Depends on invocation type.

Async

Automatic retries.

SQS

Message becomes visible again.

Streams

Batch retried.

DLQ

Failed events stored.

23. What is Dead Letter Queue (DLQ)?

Answer:

Stores failed events after retries.

Supported:

Benefits:

Prevent data loss
Debug failures

24. What is Event Source Mapping?

Answer:

Connects Lambda with:

SQS
Kinesis
DynamoDB Streams

Lambda automatically polls source and processes records.

25. How Does Lambda Scale?

Answer:

Lambda automatically scales horizontally.

Example:

1 request → 1 execution

1000 requests → 1000 concurrent executions

AWS handles scaling automatically.

26. What is Lambda Concurrency?

Answer:

Concurrency = Number of simultaneous executions.

Formula:

Concurrency = Requests per second × Average duration

Example:

100 req/sec × 2 sec

= 200 concurrent executions

27. What is Lambda SnapStart?

Answer:

Feature primarily for Java workloads.

Benefits:

Reduces startup time
Faster cold starts
Better performance

AWS snapshots initialized environment.

28. How Do You Secure Lambda?

Answer:

IAM Roles

Grant minimum permissions.

VPC Security

Private network access.

Encryption

KMS encryption.

Secrets Manager

Store credentials securely.

Environment Variables Encryption

Protect sensitive data.

29. What is Lambda Execution Role?

Answer:

IAM role assumed by Lambda during execution.

Example permissions:

{
  "Effect":"Allow",
  "Action":"s3:GetObject",
  "Resource":"*"
}

Follow least privilege principle.

30. Can Lambda Run Inside a VPC?

Answer:

Yes.

Use cases:

Access RDS
Access internal services
Private workloads

Need:

Subnets
Security Groups

31. Challenges of Running Lambda in VPC?

Answer:

Historically:

Higher cold start latency

Current improvements significantly reduced this issue.

Still consider:

NAT Gateway costs
Network architecture complexity

32. How Do You Monitor Lambda?

Answer:

CloudWatch Metrics

Invocations
Errors
Duration
Throttles
Concurrent executions

CloudWatch Logs

Application logs

X-Ray

Distributed tracing

33. What is AWS X-Ray?

Answer:

Tracing service for debugging distributed applications.

Tracks:

Request flow
Latency
Service dependencies

Useful for microservices.

34. What are Lambda Best Practices?

Answer:

Use least-privilege IAM
Reuse SDK clients
Keep deployment package small
Use Provisioned Concurrency for APIs
Store secrets in Secrets Manager
Monitor with CloudWatch
Handle retries properly
Use DLQs
Implement idempotency
Use Layers

35. What is Idempotency?

Answer:

Executing same request multiple times produces same result.

Example:

Payment processing.

Bad:

Duplicate charges.

Good:

Transaction ID check before processing.

Critical interview topic.

36. Explain a Real Enterprise Lambda Architecture

Answer:

AI Document Processing Platform

Flow:

User uploads PDF to S3
S3 triggers Lambda
Lambda extracts text
Lambda sends text to Amazon Bedrock
Bedrock generates summary
Lambda stores result in DynamoDB
SNS sends notification
CloudWatch monitors execution

Services involved:

AWS Lambda
Amazon S3
Amazon Bedrock
DynamoDB
SNS
IAM
CloudWatch

This is an excellent Solution Architect interview example.