Lambda with API Gateway pattern in AWS - Commands & Configuration

Introduction

Sometimes you want to run code in the cloud that responds to web requests without managing servers. AWS Lambda with API Gateway lets you do this by running your code only when needed and connecting it to web URLs.

When you want to create a simple web API without setting up servers.

When you need to run backend code that triggers on HTTP requests.

When you want to save costs by paying only for the code execution time.

When you want to quickly deploy a function that handles web requests.

When you want to build a serverless app that scales automatically.

Config File - template.yaml

template.yaml

AWSTemplateFormatVersion: '2010-09-09'
Transform: AWS::Serverless-2016-10-31
Resources:
  MyApiFunction:
    Type: AWS::Serverless::Function
    Properties:
      Handler: index.handler
      Runtime: nodejs18.x
      CodeUri: ./src/
      Events:
        ApiEvent:
          Type: Api
          Properties:
            Path: /hello
            Method: GET

This is an AWS SAM template that defines a Lambda function and an API Gateway endpoint.

• MyApiFunction: The Lambda function resource.
• Handler: The file and function name Lambda runs.
• Runtime: The language environment for the function.
• CodeUri: Location of the function code.
• Events: Defines API Gateway event to trigger Lambda on HTTP GET /hello.

Commands

Builds the Lambda function and prepares deployment artifacts.

Terminal

sam build

Expected Output

Building codeuri: ./src runtime: nodejs18.x metadata: {} architecture: x86_64 functions: ['MyApiFunction']

Running NodejsNpmBuilder:CopyNpmrc

Running NodejsNpmBuilder:Build

Build Succeeded

Deploys the Lambda function and API Gateway to AWS with guided prompts for configuration.

Terminal

sam deploy --stack-name my-api-stack --guided

Expected Output

Configuring SAM deploy

Stack Name [my-api-stack]: 

AWS Region [us-east-1]: 

Confirm changes before deploy [y/N]: N

Allow SAM CLI IAM role creation [Y/n]: Y

Save arguments to samconfig.toml [Y/n]: Y



Deploying with following values

Stack name: my-api-stack

Region: us-east-1

Confirm changeset: false

Deployment s3 bucket: sam-artifacts-bucket

Capabilities: ['CAPABILITY_IAM']



Waiting for stack create/update to complete

Successfully created/updated stack - my-api-stack



Outputs

Key                 MyApiFunctionApi

Description         API Gateway endpoint URL for Prod stage for MyApiFunction function

Value               https://abcdefghij.execute-api.us-east-1.amazonaws.com/Prod/hello

→

--stack-name - Names the CloudFormation stack for deployment

→

--guided - Runs interactive deployment setup

Sends an HTTP GET request to the deployed API Gateway URL to test the Lambda function response.

Terminal

curl https://abcdefghij.execute-api.us-east-1.amazonaws.com/Prod/hello

Expected Output

{"message":"Hello from Lambda!"}

Key Concept

If you remember nothing else from this pattern, remember: API Gateway connects web requests to Lambda functions so your code runs only when needed without managing servers.

Common Mistakes

Not setting the correct handler path in the Lambda function configuration.

Lambda will fail to find the function to run, causing errors on invocation.

Ensure the handler matches the file and exported function name exactly, like 'index.handler'.

Forgetting to deploy the API Gateway after updating the Lambda function.

Changes won't be available on the web URL, so you see old or no responses.

Always run 'sam deploy' after code or configuration changes.

Using the wrong HTTP method when testing the API (e.g., POST instead of GET).

API Gateway will return method not allowed errors.

Use the HTTP method defined in the API event, here GET.

Summary

Use an AWS SAM template to define a Lambda function triggered by API Gateway on HTTP requests.

Build the function with 'sam build' to prepare deployment files.

Deploy the stack with 'sam deploy --guided' to create the Lambda and API Gateway resources.

Test the API by sending an HTTP request to the API Gateway URL to invoke the Lambda function.