Here’s a set of interview questions with answers on API Gateway and AWS Lambda, tailored for a DevOps/Cloud Engineer with 4-5 years of experience.

1. What is Amazon API Gateway and why is it used?

Answer:

Amazon API Gateway is a fully managed service that makes it easy to create, publish, maintain, monitor, and secure APIs at any scale. It acts as a “front door” for applications to access data, business logic, or functionality from backend services, such as AWS Lambda functions, EC2 instances, or other APIs. API Gateway is commonly used for:

• Managing RESTful APIs and WebSocket APIs.
• Handling API requests and managing traffic.
• Securing APIs through authentication and authorization mechanisms.
• Monitoring and logging API usage with CloudWatch.
• Throttling and rate-limiting requests to prevent abuse.

2. Explain how AWS Lambda works with API Gateway.

Answer:

AWS Lambda is a serverless compute service that allows code execution in response to events. When integrated with API Gateway, Lambda functions can be triggered directly by HTTP requests. Here’s a typical flow:

• A user makes an HTTP request to an API Gateway endpoint.
• API Gateway receives the request and forwards it to a designated Lambda function.
• The Lambda function executes the code and returns a response back to API Gateway.
• API Gateway then forwards the response to the user. This setup enables building serverless, HTTP-based applications where the backend is managed by Lambda and frontend HTTP requests are managed by API Gateway.

3. What are the different types of endpoints available in API Gateway?

Answer:

API Gateway supports three endpoint types:

• Edge-optimized: This is the default endpoint type. Requests are routed through the AWS CloudFront network, reducing latency by using a global network of edge locations.
• Regional: For clients in the same region as the API, reducing the cost and latency by avoiding CloudFront routing.
• Private: Used to expose an API only within a specific VPC via VPC endpoints. This is ideal for internal applications or microservices that should not be accessible from the internet.

4. How does API Gateway handle authentication and authorization?

Answer:

API Gateway offers multiple ways to secure APIs:

• IAM Permissions: Uses AWS Identity and Access Management (IAM) to control who can invoke the API.
• Lambda Authorizers: Uses a Lambda function to authorize API requests. The Lambda function can validate tokens or other credentials, allowing more granular control.
• Cognito User Pools: Uses Amazon Cognito to manage and authenticate users. This is useful for applications with a user authentication requirement.
• API Keys: Can be used to identify API clients, often combined with throttling limits.

5. What are the benefits and limitations of using AWS Lambda with API Gateway?

Answer:

Benefits:

• Scalability: Lambda automatically scales in response to the number of incoming requests.
• Cost-effectiveness: You pay only for the compute time, reducing the cost of maintaining dedicated servers.
• Serverless: No need to manage or provision servers, simplifying deployment.
• Integration with other AWS services: AWS Lambda can easily interact with services like DynamoDB, S3, and more.

Limitations:

• Execution Time Limit: Lambda has a maximum execution time of 15 minutes, which may be a constraint for long-running tasks.
• Cold Start: Lambda functions may have a "cold start" delay, especially with infrequent usage.
• Memory and Timeout Constraints: Limited to 10GB of memory and 15-minute timeout, making it unsuitable for some high-compute or extended tasks.

6. What is a "cold start" in AWS Lambda, and how can you minimize its impact?

Answer:

A cold start occurs when a Lambda function is invoked after not being used for a period, resulting in extra time to initialize resources. This can introduce latency, especially noticeable in environments with sporadic traffic.

Ways to minimize cold start impact:

• Provisioned Concurrency: Ensures a certain number of Lambda instances are always warm, reducing latency.
• Optimize code and dependencies: Reducing package size and optimizing code can improve initialization time.
• Increase memory allocation: Lambda scales CPU with memory, so more memory can lead to faster execution.

7. How do you enable detailed logging for API Gateway and Lambda?

Answer:

For API Gateway:

• Go to the API Gateway Console.
• Under Stages, select a stage, then enable CloudWatch Logs under the Logs/Tracing tab.
• You can set the logging level (INFO, ERROR) for both request and response logs.

For AWS Lambda:

• Lambda logs are automatically sent to CloudWatch Logs.
• You can add additional logging in your Lambda code using the console.log (Node.js) or print (Python) statements.
• Enable X-Ray for distributed tracing to capture detailed metrics on function performance.

8. Explain how to implement throttling and rate-limiting in API Gateway.

Answer:

API Gateway allows you to set throttling and rate-limiting policies to control the number of requests clients can make to your API:

• Account-level throttling: Sets the default throttling limits across all APIs in the account.
• Stage-level throttling: Allows you to set request limits at the API stage (e.g., dev, prod).
• Method-level throttling: Provides even finer control, enabling limits on specific API methods.
• Usage Plans with API Keys: Allows defining rate and burst limits for individual users or clients identified by API keys.

Rate-limiting helps to prevent abuse and ensures fair usage across clients, while also protecting backend resources from being overwhelmed.

9. What is "Provisioned Concurrency" in AWS Lambda, and when would you use it?

Answer:

Provisioned Concurrency keeps a predefined number of Lambda instances initialized and ready to handle requests, reducing the latency caused by cold starts. It is particularly useful in scenarios where:

• Predictable Traffic Patterns: Applications experience predictable traffic spikes, such as e-commerce events, seasonal campaigns, etc.
• Low-latency Requirements: Applications where consistent, low latency is essential, such as user-facing APIs.

Provisioned Concurrency is configured at a cost, so it should be used where performance requirements justify it.

10. How would you secure an API Gateway endpoint?

Answer:

To secure an API Gateway endpoint, you can use one or more of the following approaches:

• IAM Policies: Allow or restrict access through IAM users, roles, or policies. Suitable for internal APIs.
• Lambda Authorizer: Use a Lambda function to validate requests. This allows custom authentication, like token validation or integrating with external identity providers.
• Amazon Cognito User Pools: Provides a managed solution for handling authentication and authorization.
• API Keys and Usage Plans: Limit access and usage for different consumers by using API keys combined with usage plans.
• TLS/SSL Encryption: Enforce HTTPS for secure data transmission over the network.

11. How can you manage versioning and deployment stages with API Gateway?

Answer:

API Gateway allows managing different versions and deployment stages:

• Deployment Stages: Each API can have multiple stages (e.g., dev, test, prod). Each stage represents a different deployment of your API, which can have distinct configurations like throttling, logging, and caching.
• Stage Variables: Variables that can be used in your Lambda functions or API configurations, enabling dynamic changes without redeployment.
• Versioning APIs: You can version your API endpoints using version identifiers in the URL (e.g., /v1/resource, /v2/resource) to manage breaking changes.

This setup enables developers to deploy, test, and release new API versions without affecting existing clients.

These questions cover fundamental concepts as well as specific use cases that demonstrate practical knowledge of managing API Gateway and AWS Lambda in a DevOps/cloud engineering environment.

Here are some scenario-based questions that can help assess hands-on knowledge in API Gateway and AWS Lambda for a DevOps/Cloud Engineer role.

1. Scenario: Logging and Monitoring

• Question: Your team has reported that an API exposed through API Gateway is experiencing inconsistent response times. Some requests are taking significantly longer than others. How would you investigate this issue using AWS tools, and what steps would you take to identify the root cause?
• What to look for:
  • Enabling detailed logging in API Gateway and reviewing CloudWatch logs.
  • Configuring X-Ray tracing to pinpoint bottlenecks in API calls or Lambda execution.
  • Checking for cold start delays in Lambda or looking into Lambda memory/timeout configurations.
  • Setting up CloudWatch metrics and alarms for monitoring API latency.

2. Scenario: API Throttling and Rate-Limiting

• Question: Your API has recently gained popularity, and now you’re experiencing a high volume of requests, which is putting strain on your backend resources. How would you set up throttling and rate-limiting to protect your backend services while ensuring fair usage?
• What to look for:
  • Configuring account-level, stage-level, or method-level throttling settings in API Gateway.
  • Creating usage plans and assigning API keys to different client groups to implement different rate limits.
  • Ability to explain the difference between burst and rate limits and when each is applied.
  • Adjusting Lambda concurrency settings to manage request spikes effectively.

3. Scenario: Cold Start Issue

• Question: You’ve deployed a Lambda function that’s accessed frequently by users through API Gateway. Some users have reported high response times during initial requests. How would you reduce the cold start time for this Lambda function?
• What to look for:
  • Explanation of how cold starts affect AWS Lambda, especially with VPC integration.
  • Implementing Provisioned Concurrency to keep instances warm.
  • Code optimization techniques to reduce initialization time, such as reducing package size or optimizing dependencies.
  • Discussing potential trade-offs of using Provisioned Concurrency vs. on-demand concurrency.

4. Scenario: Authentication and Authorization

• Question: You’re deploying a public API through API Gateway, but you need to restrict access to only authenticated users from your application. Explain how you would implement this setup.
• What to look for:
  • Integration of Amazon Cognito for authentication and setting up user pools.
  • Setting up Cognito Authorizers on API Gateway to validate user tokens.
  • For more custom requirements, discussing the use of Lambda Authorizers to verify access tokens or additional custom authentication logic.
  • Understanding of IAM permissions and roles in case the API is only for internal use.

5. Scenario: Cross-Origin Resource Sharing (CORS)

• Question: Your API Gateway is being used by a client-side web application, and users are getting CORS errors when trying to access resources. Describe the steps you would take to resolve this issue.
• What to look for:
  • Knowledge of enabling CORS in API Gateway settings.
  • Properly setting up HTTP headers to allow the necessary origins, methods, and headers.
  • Explaining how to configure the OPTIONS method to handle preflight requests.
  • Discussing testing strategies to verify CORS configuration, such as testing from different browsers or using tools like Postman.

6. Scenario: Versioning and Rollback

• Question: You’ve released a new version of an API with breaking changes, but some clients are still using the old version. How would you manage this situation while ensuring uninterrupted service?
• What to look for:
  • Creating separate versions in API Gateway (e.g., /v1/resource, /v2/resource).
  • Using deployment stages to deploy and test different versions independently (e.g., dev, test, prod stages).
  • Implementing canary deployment to gradually roll out the new version while monitoring for issues.
  • Discussion of rollback options in case of critical issues.

7. Scenario: Securing Internal APIs

• Question: You’ve created an internal API that should only be accessible within your VPC. Explain how you would secure this API so that it isn’t accessible over the internet.
• What to look for:
  • Understanding of Private Endpoints in API Gateway and using VPC endpoints.
  • Configuring necessary VPC security groups and route tables to allow access only from specified internal IP ranges or services.
  • Awareness of additional security measures such as using IAM policies or Lambda Authorizers for internal traffic control.

8. Scenario: Scaling Lambda with High Traffic API

• Question: You’re running a highly used API backed by Lambda, but during peak traffic, your Lambda functions are throttled, leading to failed requests. How would you handle this scaling issue?
• What to look for:
  • Explanation of Lambda’s concurrency limits and how to request a service quota increase.
  • Configuring reserved concurrency and provisioned concurrency for critical Lambda functions.
  • Implementing DLQs (Dead Letter Queues) to handle failed requests and ensure important tasks aren’t lost.
  • Setting up retries in API Gateway and Lambda error handling to make the system more resilient.

9. Scenario: Debugging API Gateway and Lambda Integration Issues

• Question: You’ve set up an API Gateway endpoint that triggers a Lambda function, but clients are getting 500 Internal Server Error responses. How would you debug this issue?
• What to look for:
  • Checking API Gateway logs in CloudWatch for error messages and response codes.
  • Verifying Lambda execution logs in CloudWatch to see if there are runtime errors.
  • Ensuring that the Lambda function’s IAM role has the correct permissions to execute and interact with required resources.
  • Using test events in Lambda and the API Gateway Test feature to isolate and troubleshoot the problem.

10. Scenario: Handling Large Data Payloads

• Question: You’re implementing a Lambda function that processes large payloads (over 6 MB), but API Gateway has a payload size limit of 10 MB. How would you design the system to handle this requirement?
• What to look for:
  • Understanding API Gateway’s payload limits and potential workarounds.
  • Proposing S3 as an intermediate storage, where the client uploads large payloads to S3 and passes the object link to Lambda via API Gateway.
  • Discussing S3 event triggers for Lambda as an alternative, where Lambda directly processes objects uploaded to S3.
  • Knowledge of how to secure S3 buckets and manage permissions for this setup.

11. Scenario: Transforming Requests and Responses

• Question: Your API needs to convert incoming JSON payloads to a format required by an external system, and then transform the response before sending it to the client. How would you handle this requirement in API Gateway and Lambda?
• What to look for:
  • Setting up Mapping Templates in API Gateway to transform requests and responses.
  • Using Lambda for more complex transformations that aren’t achievable in API Gateway alone.
  • Understanding of Content-Type headers and integration request/response configuration in API Gateway.
  • Knowledge of best practices for managing transformation logic in a scalable, maintainable way.

12. Scenario: High Availability and Disaster Recovery

• Question: You need to ensure high availability for a critical API hosted on API Gateway with Lambda integration. How would you set up the infrastructure to handle disaster recovery and minimize downtime?
• What to look for:
  • Implementing regional API Gateway endpoints in multiple AWS regions and configuring Route 53 for failover routing.
  • Using CloudFormation or Infrastructure as Code (IaC) for replicable, automated deployments.
  • Ensuring data resilience for any persistent data layer (e.g., DynamoDB Global Tables) and having a cross-region replication strategy.
  • Setting up alarms and monitoring in CloudWatch to detect failures and trigger automated recovery actions.

These scenario-based questions can help assess practical skills and how well the candidate can apply their knowledge to real-world situations with API Gateway and AWS Lambda.