🚀 How to Integrate a Cross-Account Lambda with a API Gateway REST API 🌐

4 min readOct 17, 2024

Hey there! 👋 Recently, I ran into a cool challenge: How do you hook up a Lambda function from one AWS account with a REST API in another? 🤔 As more organizations go the multi-account route for better security and structure, connecting cross-account Lambdas with REST or HTTP APIs is becoming super common! 💻✨

So, if you’re in that boat 🛳️, here’s a fun step-by-step guide on how to do it! 🎉 Let’s dive in! 🏊‍♂️

🎯 Audience:
Hey new joiners! 👋 This article is just for YOU! 🎉 If you’re new to aws or curious about how to set up a REST API with cross-account Lambda integration, you’re in the right place! 🚀 Whether you’re just getting started or want to sharpen your skills, this guide will walk you through everything step-by-step. 🛠️ Let’s get you up and running in no time! 💡🔥

🎯 What You’ll Need:

2 AWS accounts (we’ll call them Account A & Account B 🏠🏠)
IAM permissions to create Lambda, API Gateway, and IAM roles 🔑

🛠️ Step 1: Create a Lambda Function in Account B

First, we need to whip up a Lambda function in Account B. 🧑‍💻 Here’s a simple code to start with:

export const handler = async (event) => {
  // TODO implement
  const response = {
    statusCode: 200,
    body: JSON.stringify('Hello from lambda!'),
  };
  return response;
};

Once that’s done, copy the ARN of the Lambda function. We’ll need this later! 📝✨

🌐 Step 2: Set Up an API Gateway in Account A

Hop over to Account A 🏃‍♂️ and create a shiny new API Gateway!

✨ After that, add a method and paste the ARN of the Lambda function from Account B into the Lambda function section. 💡

Don’t forget to copy the ARN of the API Gateway resource — you’ll need it in the next step! 📋

⚙️ Step 3: Permissions Setup

Okay, time to test!

🎯 But… 🛑 Oops! You’ll probably get an error like:

❌ Execution failed due to configuration error: Invalid permissions on Lambda function 😬

No worries, let’s fix this! 🧑‍🔧

Go back to Account B, navigate to your Lambda function, and head to the Permissions section. 🔐 Scroll to Resource-based policy statements and click Add permission.

Choose API Gateway as the AWS service
Paste the API Gateway ARN you copied earlier (you can use a wildcard * for the method)
For Action, select lambda:InvokeFunction 🛠️

Save your changes, and you’re good to go! ✅

🚀 Step 4: Test It!

Back in Account A, let’s give it another whirl! 🎯 Run the API call, and… 🎉 You should see a successful response from your Lambda now! 🙌

And that’s it! You’ve successfully connected your cross-account Lambda with a REST API! 🎉🎉 Super cool, right? Now go and wow your team with this seamless integration! 💡💪