Serverless Architecture Interview Questions 2026

Serverless is more than just Lambda functions. It's about designing resilient, event-driven architectures.

1. How do you handle cold starts in AWS Lambda?

A cold start happens when a Lambda function is invoked after inactivity, requiring AWS to provision a new container.

Solutions: Provisioned Concurrency, optimizing package size, choosing faster languages (Node.js/Go over Java), and keeping functions warm via EventBridge.

2. What is the difference between SQS and SNS?

SNS: Pub/Sub model. Pushes messages to multiple subscribed endpoints (fan-out).

SQS: Message queue. Consumers pull messages from the queue. Often used together: SNS fans out to multiple SQS queues.

3. What is an Event-Driven Architecture (EDA)?

An architecture pattern where decoupled services communicate by publishing and subscribing to events (state changes) rather than synchronous API calls. It increases scalability and fault tolerance.

4. How does AWS API Gateway integrate with Lambda?

API Gateway acts as an HTTP endpoint that triggers Lambda via proxy integration. It handles request routing, rate limiting, authentication, and payload transformation before invoking the Lambda.

5. What are Lambda Layers?

Lambda Layers allow you to package libraries, custom runtimes, or other dependencies separately from your function code. Multiple functions can share a single Layer, reducing deployment size and duplicated code.

6. How do you manage state in a Serverless application?

Since Lambda is stateless, state must be stored externally using services like DynamoDB (for fast NoSQL storage), S3 (for objects), or ElastiCache/Redis (for transient caching). Step Functions can also maintain workflow state.

7. What is AWS Step Functions?

Step Functions is a serverless orchestration service that lets you combine AWS Lambda functions and other AWS services to build complex, stateful workflows, retry logic, and error handling without writing boilerplate code.

8. What is the maximum execution time of a Lambda function?

15 minutes. If a task takes longer, it should be broken down, run in AWS Fargate/Batch, or orchestrated via Step Functions.

9. How do you secure a Serverless application?

Use IAM roles with least privilege per function. Use API Gateway authorizers (Cognito/Custom JWT). Secure data at rest (KMS encryption) and in transit. Validate all input payloads to prevent injection.

10. Explain the concept of Idempotency.

An idempotent operation produces the same result no matter how many times it is executed. In serverless, where at-least-once delivery (like SQS) is common, functions must be idempotent to handle duplicate events safely.

11. What is EventBridge?

Amazon EventBridge is a serverless event bus that makes it easy to connect applications using data from your own apps, SaaS apps, and AWS services. It uses routing rules to deliver events to targets.

12. How do you monitor and debug Serverless apps?

Use AWS CloudWatch for logs and metrics, AWS X-Ray for distributed tracing across services, and structured JSON logging within your function code.

13. What is the Dead Letter Queue (DLQ) in Serverless?

A DLQ is a queue where messages that fail to be processed after a certain number of retries are sent. This prevents poisoned messages from blocking the queue and allows for manual inspection.

14. What is a Serverless Framework or SAM?

They are Infrastructure as Code (IaC) tools optimized for serverless. AWS SAM (Serverless Application Model) and the Serverless Framework simplify defining API Gateways, Lambdas, and IAM roles in YAML.

15. How does Lambda scaling work?

AWS automatically provisions new execution environments to handle concurrent requests. However, there is an account-level concurrency limit (default 1000).

16. What is reserved concurrency?

Reserved concurrency guarantees that a certain number of concurrent executions are reserved for a specific function, and limits the function to never exceed that amount, preventing it from exhausting the account limit.

17. How do you test Serverless applications locally?

Unit testing the business logic, using tools like \sam local invoke\, or using mock services like LocalStack. However, testing in a real cloud dev environment is highly recommended.

18. What is DynamoDB Streams?

A feature that captures a time-ordered sequence of item-level modifications in a DynamoDB table. It can trigger a Lambda function to react to data changes (e.g., updating an elasticsearch index).

19. How do you handle database connections in Lambda?

Connecting to relational databases (RDS) from Lambda can exhaust connection pools. Solutions include using RDS Proxy, or connecting outside the handler function so connections are reused across warm invocations.

20. What is a "Poison Pill" message?

A message that causes a consumer to fail consistently. If not handled (e.g., via a DLQ), the consumer will continuously retry the message, blocking the processing of other valid messages in the queue.