The API Gateway & Lambda Tricky Integration Configs

The API Gateway & Lambda Tricky Integration Configs


3 min read

Creating demos with LocalStack often leads me into the depths of configurations and scenarios I wouldn't typically encounter. It really never gets boring, and it's how I learn. This hands-on exploration is crucial because it ensures that the solutions I demo are robust, match the AWS ecosystem, and go beyond your typical "Hello world!". I generally appreciate configurations that are straightforward, but this is not a story of an easy one. However, every wrong turn is always a learning opportunity.

The Challenge of Accurate Implementation

In a recent project, I aimed to illustrate a setup involving AWS API Gateway and Lambda functions, using Terraform for infrastructure provisioning. The goal was clear: an API Gateway integrating with two Lambda functions, one handling HTTP GET requests and the other managing POST requests. However, the simplicity of the concept did not reflect the complexity of its implementation.

Let's have a look at the snippet that started it all:

resource "aws_api_gateway_integration" "get_product_integration" {
  rest_api_id             =
  resource_id             =
  http_method             = aws_api_gateway_method.get_product.http_method
  type                    = "AWS_PROXY"
  integration_http_method = "GET"
  uri                     = aws_lambda_function.get_product.invoke_arn

The real challenge arose while configuring the integration_http_method argument, under the aws_api_gateway_integration resource. AWS documentation, dense and meticulous like a legal contract, requires careful parsing to ensure you're extracting the correct information. If you're running low on patience, it's easy to misinterpret the purpose of integration_http_method as indicating the method the Lambda function will accept.

A Twist in the Configuration

Using LocalStack to run the infrastructure makes it very obvious whether a Lambda function is invoked or not. Since every function starts its own container, it was clear that one was missing. I encountered the same behavior on AWS, so something strange was happening. This sent me on a quest through Google, StackOverflow, and AWS's documentation. Needless to say, I had to make many adjustments to the Terraform config file before finding the (not-so-obvious) answer.

Lessons lernt: configure Cloudwatch logs, but don't forget that the LocalStack logs will show you a one-liner hinting at a deeper misconfiguration.

localstack-main  | raise ApiGatewayIntegrationError("Internal server error", status_code=500)
localstack-main  | Internal server error
localstack-main  | 2024-04-25T20:28:58.226  INFO --- [   asgi_gw_2] localstack.request.http: GET /dev/productApi => 500

It was the ApiGatewayIntegrationError that started to make things clear. The breakthrough came soon. However, LocalStack was instrumental with providing rapid feedback on my configurations and allowing me to iterate quickly without waiting on cloud deployment cycles. This fast feedback loop was critical in debunking my initial configurations and narrowing down the solution.

I guess they had a lot of questions regarding this topic, and they made it clear now:

Clarifying the Misconceptions

It turned out that the integration_http_method is not about the HTTP methods (GET, POST, DELETE, etc.) that the Lambda is supposed to handle. Rather, it is about how the API Gateway communicates with the Lambda function. It specifies the method used by the API Gateway to invoke the Lambda function, which should invariably be POST, regardless of the HTTP methods your API exposes. This is a subtle yet significant distinction that impacts how services are integrated and interact within AWS.

Concluding Thoughts

This experience underscored the importance of understanding the underlying mechanisms of cloud services and infrastructure as code tools. The ability to simulate environments with LocalStack before moving to actual AWS deployments can save countless hours of debugging and reconfiguration. For fellow developers venturing into similar territories, remember to check the details, and tools like LocalStack are invaluable in getting your IaC right.