Integrating AWS Lambda with Amazon API Gateway for Scalable APIs

Instruction: Explain how you would integrate AWS Lambda with Amazon API Gateway to create a scalable, serverless API. Describe the steps involved in setting up the integration, how you would handle versioning of your API, and strategies for maintaining high availability and security.

Context: This question evaluates the candidate's experience with AWS services for building serverless applications, specifically focusing on their ability to leverage AWS Lambda in conjunction with Amazon API Gateway. Candidates should demonstrate an understanding of the setup process, including defining resources, methods, and stages in API Gateway, and how they link these to Lambda functions. Additionally, the question probes into the candidate's strategies for API versioning, security measures like authentication and authorization, and ensuring high availability.

Updated

Example answer

For a REST API in API Gateway, I would define the resource paths and methods, create a Lambda function for the application logic, and configure a Lambda proxy integration. The handler must accept the selected API event format and return the required response envelope, including an HTTP status and a string body. A custom integration instead needs explicit mapping rules. HTTP APIs have different features and event formats, so I would choose the API type before copying a configuration.

There are separate permission boundaries:

  • Caller to API: configure an appropriate IAM, Cognito user-pool or Lambda-authorizer scheme and enforce the caller's permissions for the requested resource. API keys identify usage-plan clients; they are not authentication or authorization credentials.
  • API Gateway to Lambda: grant invocation permission through a scoped Lambda resource-based policy, tied to the intended API/stage/method as appropriate.
  • Lambda to downstream services: give the function's execution role only the required data and service permissions. That role does not authenticate the API's caller.

For a breaking public contract change, expose a deliberate version such as /v2/orders while maintaining the supported /v1/orders contract. Deployment stages and Lambda versions/aliases serve deployment management; they do not by themselves define or guarantee client compatibility. Keep configuration in infrastructure as code, test the old and new contracts, and retain a tested rollback target.

Lambda manages execution across Availability Zones. I would make databases and other dependencies resilient too, and, if attaching Lambda to a VPC, select subnets in multiple Availability Zones. This does not make the whole API immune to regional failures or exhausted quotas.

Use TLS, validate inputs, avoid logging credentials, and set monitoring for latency, errors, throttles and downstream saturation. For a proposed rollout, test a valid request, missing/invalid authorization, access to another tenant's resource, a malformed request, a dependency failure and a burst beyond capacity. Check the client-visible error and recovery behavior. Usage-plan quotas are best-effort controls, not a guaranteed cost or security boundary.

Sources

Lambda and API Gateway integration and permissions; API Gateway usage-plan and API-key limitations; Lambda resilience and Availability Zones.

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