Way to access S3 Bucket's name from cloudformation/boto3?

Question

My team wants to create an S3 bucket in a cloudformation template without assigning it a bucket name (to let cloudformation name it itself).

When putting a file into the S3 bucket from my lambda function, is there a way for me to get the S3 bucket's name without having to manually look at the AWS console and check what name was created?

Answer 1

Using the Serverless Application Model (SAM) you can include environment variables with the function properties

AWSTemplateFormatVersion: '2010-09-09'
Description: "Demo"
Transform: 'AWS::Serverless-2016-10-31'
Resources:
  MyLambdaFunction:
    Type: 'AWS::Serverless::Function'
    Properties:
      Runtime: nodejs10.x
      Handler: index.handler
      CodeUri: ./src
      Policies:
      - Version: '2012-10-17'
        Statement:
        - Action:
          - s3:PutObject
          Effect: 'Allow'
          Resource: !Sub ${MyS3Bucket.Arn}/*
      Environment:
        Variables:
          BUCKET_NAME: !Ref MyS3Bucket
  MyS3Bucket:
    Type: 'AWS::S3::Bucket'

Then your function can access the environment variables using process.env.BUCKET_NAME in node.js. In python I think you'd use os.environ['BUCKET_NAME']

const aws = require('aws-sdk');
const s3 = new aws.S3();

exports.handler = async (event) => {
    const params = {
        Body: 'The Body',
        Bucket: process.env.BUCKET_NAME,
        Key: 'abc123',
    }

    return s3.putObject(params).promise();
}

I would assume this works for any CloudFormation templates which aren't using the SAM transform too.

Answer 2

You can use Fn::GetAtt to get the values from your newly created S3 bucket. You can check it here. S3 Ref & Get Attribute Documentation

The problem is how to pass the value to lambda function. Here is the step that might be works.

• Use the function Get Attribute above to get the s3 bucket name that cloudformation created.
• Insert the value into a file, you can use the UserData, or use Metadata if you are using cloud-init already.
• Store the file into existed s3 bucket (or any other storage that lambda can access), you can using the cloud formation template bucket, that always been created when you launch a cloudformation template (usually named cf-template...).
• Add a code to your lambda to access the s3 and get the file. Now you get the data of the s3 bucket that your cloudformation has been created and can use it on lambda.

Hope this help.