cfn-variable

Purpose

Provides a function to use arbitrary JSON variables in CloudFormation, by simply taking input Properties on the Resource (omitting ServiceToken) and returning them in new Buffer(JSON.stringify(Properties)).toString('base64') form. You can use this to quickly add reusability to your template Resource definitions.

CloudFormation converts all primitives to String representations, account for this.

Natively supported by cfn-lambda's ability to use __default__ expansion in JSON Properties objects (documented here)

This package on NPM
This package on GitHub

Implementation

This Lambda makes use of the Lambda-Backed CloudFormation Custom Resource flow module, cfn-lambda (GitHub / NPM).

Usage

See ./example.template.json for a sample CloudFormation template. The example uses Condition statements, Parameters, and dynamic ServiceToken generation fully.

"VariableLogicalIdInResourcesObject": {
  "Type": "Type": "Custom::Variable",
  "Properties": {
    "ServiceToken": "arn:aws:lambda:<cfn-region-id>:<your-account-id>:function:<this-deployed-lambda-name>",
    "VariableValue": { // Arbitrary JSON or String or Number or Array
      "FooProperty": {...},
      "BarProperty": "can by anything"
    }
  }
}

CloudFormation converts all primitives to String representations, account for this.

Ref and Fn::GetAtt

To reference the base64(JSON.stringify(Properties)) value, use "Fn::GetAtt":

{
  "Fn::GetAtt": [
    "MyVariableResource",
    "Value"
  ]
}

Ref simply returns the SHA256(JSON.stringify(Properties)), and is used to make sure the template knows when to propagate value changes.

Installation of the Resource Service Lambda

Using the Provided Instant Install Script

The way that takes 10 seconds...

# Have aws CLI installed + permissions for IAM and Lamdba
$ npm run cfn-lambda-deploy

You will have this resource installed in every supported Region globally!

Using the AWS Console

... And the way more difficult way.

IMPORTANT: With this method, you must install this custom service Lambda in each AWS Region in which you want CloudFormation to be able to access the Variable custom resource!

1. Go to the AWS Lambda Console Create Function view:

• us-east-1 / N. Virginia
• us-west-2 / Oregon
• eu-west-1 / Ireland
• ap-northeast-1 / Tokyo

2. Zip this repository into /tmp/Variable.zip

   $ cd $REPO_ROOT && zip -r /tmp/Variable.zip;

3. Enter a name in the Name blank. I suggest: CfnLambdaResouce-Variable

4. Enter a Description (optional).

5. Toggle Code Entry Type to "Upload a .ZIP file"

6. Click "Upload", navigate to and select /tmp/Variable.zip

7. Set the Timeout under Advanced Settings to 10 sec

8. Click the Role dropdown then click "Basic Execution Role". This will pop out a new window.

9. Select IAM Role, then select option "Create a new IAM Role"

10. Name the role lambda_cfn_api_gateway_resource (or something descriptive)

11. Click "View Policy Document", click "Edit" on the right, then hit "OK"

12. Copy and paste the ./execution-policy.json document.

13. Hit "Allow". The window will close. Go back to the first window if you are not already there.

14. Click "Create Function". Finally, done! Now go to Usage or see the example template. Next time, stick to the instant deploy script.

Miscellaneous

Collaboration & Requests

Submit pull requests or Tweet @ayetempleton if you want to get involved with roadmap as well, or if you want to do this for a living :)

License

MIT

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Work is (extremely) active, published here:
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