API Gateway Integration with Lambda

Connecting API Gateway to Lambda is the most common serverless pattern on AWS. This lesson covers integration types, request/response mapping, proxy vs custom integrations, error handling, and building a complete REST API backed by Lambda functions.

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Integration Types

API Gateway supports several integration types that determine how requests flow to your backend:

Integration Type	Description	When to Use
Lambda Proxy (AWS_PROXY)	Passes the entire request to Lambda as-is	Default choice — simplest approach
Lambda Custom (AWS)	Uses mapping templates to transform request/response	When you need to reshape data between API GW and Lambda
HTTP Proxy (HTTP_PROXY)	Forwards request to an HTTP endpoint	Proxying to existing HTTP services
HTTP Custom (HTTP)	Like HTTP proxy but with mapping templates	Transforming requests to legacy APIs
Mock (MOCK)	Returns a response without calling any backend	Health checks, CORS preflight, stubs

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Lambda Proxy Integration (AWS_PROXY)

Lambda proxy integration is the simplest and most common pattern. API Gateway forwards the entire HTTP request to Lambda in a standardised format.