How to Trigger an AWS Lambda Function in Another Account Using Amazon S3

As a data scientist or software engineer, you might often face scenarios where you need to trigger an AWS Lambda function in another account using Amazon S3. This blog post aims to guide you on how to set up this cross-account access effectively and securely.

By Saturn Cloud | Tuesday, August 08, 2023 | Miscellaneous

How to Trigger an AWS Lambda Function in Another Account Using Amazon S3

What are Amazon S3 and AWS Lambda?

To ensure we’re on the same page, let’s first briefly define our key technologies:

Amazon S3 (Simple Storage Service) is an object storage service that offers industry-leading scalability, data availability, security, and performance.

AWS Lambda is a serverless computing service that lets you run your code without provisioning or managing servers.

The combination of these two powerful AWS services allows you to automate tasks and create reactive, decoupled architectures.

How to Set Up Cross-Account Access

Let’s go through the steps to set up an Amazon S3 bucket in one AWS account to trigger a Lambda function in another AWS account.

Step 1: Setup the Lambda Function

Create your Lambda function in Account B, which will be triggered by the S3 event. Write your function according to your needs, whether that’s data processing, alerting, or any other task.

Step 2: Define IAM Role

In Account B, define an IAM role that has the necessary permissions for the Lambda function and can be assumed by the S3 service in Account A. This role should have a trust relationship that allows s3.amazonaws.com to assume the role, and a policy that allows “lambda:InvokeFunction” on the specific Lambda function.

{
  "Version": "2012-10-17",
  "Statement": [
    {
      "Effect": "Allow",
      "Principal": {
        "Service": "s3.amazonaws.com"
      },
      "Action": "sts:AssumeRole"
    }
  ]
}

Step 3: S3 Bucket Notifications

In Account A, set up the bucket notification configuration to send events to the Lambda function. The bucket policy should also allow S3 to assume the role created in Account B.

In the AWS Management Console, navigate to your bucket, select “Properties”, and under “Advanced settings”, select “Events”. Add a notification with the event type that should trigger the Lambda function (for example, “All object create events”), and specify the ARN of the Lambda function in Account B.

Step 4: Testing and Validation

Once everything is set up, you can test the setup by performing the operation that should trigger the Lambda function from the S3 bucket. Check the CloudWatch logs in Account B to verify that the Lambda function was triggered and executed successfully.

Conclusion

Cross-account access between Amazon S3 and AWS Lambda allows you to create flexible, decoupled architectures that can react to events across different AWS accounts. By following these steps, you can securely setup this architecture to automate tasks and improve your systems' reactivity.

Remember, security should always be your priority. Always adhere to the principle of least privilege, only granting the necessary permissions needed for the task.

This setup is just one example of how powerful and flexible AWS services can be when combined. Explore, experiment, and don’t be afraid to build complex systems - AWS has the tools to support you.

How to Trigger an AWS Lambda Function in Another Account Using Amazon S3

How to Trigger an AWS Lambda Function in Another Account Using Amazon S3

What are Amazon S3 and AWS Lambda?

How to Set Up Cross-Account Access

Step 1: Setup the Lambda Function

Step 2: Define IAM Role

Step 3: S3 Bucket Notifications

Step 4: Testing and Validation

Conclusion

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