AWS Lambda Interview Questions
Core Concepts
Q: How does Lambda work? What is cold start?
Lambda is serverless compute — you provide code, AWS runs it on demand. No servers to manage.
Execution model:
1.Event triggers Lambda (API Gateway, S3, SQS, CloudWatch Events, etc.)
2.AWS finds/creates execution environment (container)
3.Runs your handler function
4.Returns response or writes to output
5.Environment stays "warm" briefly (reuse for next invocation) or is destroyed
Cold start: First invocation (or after idle period) requires:
•Spin up execution environment (~100ms)
•Download deployment package
•Initialise runtime (Python/Node.js)
•Run initialisation code outside handler
Cold start latency: ~100ms-1s (Java worst, Python/Node.js better, SnapStart for Java).
Mitigation:
•Provisioned Concurrency: pre-warms environments (eliminates cold starts, extra cost)
•Keep deployment package small
•Move heavy initialisation outside handler (connection pooling, SDK clients)
•Use Python/Node.js for latency-sensitive functions
Q: Lambda limits and concurrency.
|---|---|
| Max memory | 10,240 MB |
|---|
| Max timeout | 15 minutes |
| Deployment package | 50MB zipped, 250MB unzipped |
| Container image | Up to 10GB |
| Concurrent executions | 1,000 (soft limit, can increase) |
| Ephemeral storage (/tmp) | 512MB default, up to 10GB |
Concurrency types:
•Reserved concurrency: Caps maximum concurrent executions for a function (throttles beyond limit)
•Provisioned concurrency: Pre-initialised environments (warm, no cold start, costs money)
Q: Lambda invocation types — synchronous vs asynchronous.
Synchronous (request-response): Caller waits for result.
Triggers: API Gateway, ALB, Lambda (SDK call), Cognito
Asynchronous: Lambda queues event, returns immediately (202). Retries 2x on failure.
Triggers: S3, SNS, CloudWatch Events
Event Source Mapping (stream/queue): Lambda polls source.
Triggers: SQS, Kinesis, DynamoDB Streams, Kafka
Error handling:
•Sync: Error returned to caller
•Async: Retries, then DLQ (Dead Letter Queue)
•Event source mapping: Retries until expiry, BisectOnError
Q: How do you connect Lambda to a VPC?
python
# Lambda VPC config - add in function configuration
VpcConfig:
SubnetIds: [subnet-xxx, subnet-yyy] # Private subnets
SecurityGroupIds: [sg-xxx]
# Lambda needs AWSLambdaVPCAccessExecutionRole
# For internet access from VPC: needs NAT Gateway
# For AWS services: use VPC Endpoints (avoid NAT)
Why VPC?: Access RDS, ElastiCache, or other private resources.
VPC caveat: Adds ~100ms cold start. Use VPC Endpoints for AWS services to avoid NAT Gateway costs.
Q: Lambda layers and container images — when to use each?
Layers: Shared code/dependencies packaged as a ZIP.
•Max 5 layers per function
•Separate deployment from updates (update layer once, all functions benefit)
•Good for: shared libraries (pandas, requests), custom runtimes, security tools
Container Images (up to 10GB):
•Full control of environment
•Larger ML models, complex dependencies
•Familiar Docker workflow
•Better for ML inference, large dependency trees
Revision Notes
LAMBDA: serverless, event-driven, max 15min timeout, 10GB memory
COLD START: ~100ms-1s. Mitigate: Provisioned Concurrency, small packages, warm code outside handler.
CONCURRENCY:
Reserved: caps maximum (throttles beyond)
Provisioned: pre-warmed (no cold start, costs money)
Default limit: 1,000 concurrent (can raise)
INVOCATION TYPES:
Sync: API GW, ALB → wait for result
Async: S3, SNS → queued, retries 2x
Event Source Mapping: SQS, Kinesis → Lambda polls
LAYERS: shared code (max 5). Container: large deps/ML models.
VPC: adds cold start time. Use VPC Endpoints for AWS services.
