CloudFormation Custom Resources: When Built-in Just Isn’t Enough

Ever hit a wall with CloudFormation because the resource you need simply doesn’t exist as a native type? Or maybe you need to run some complex logic during provisioning that CloudFormation can’t express on its own? That’s exactly where custom resources come in.

So, What Are Custom Resources?

Think of custom resources as an escape hatch. They let you plug your own code into the CloudFormation lifecycle so whenever a stack is created, updated, or deleted, your logic runs automatically.

You define them in your template using either:

• AWS::CloudFormation::CustomResource
• Custom::MyCustomResourceTypeName(a friendlier, descriptive name)

The only required piece? A service token — basically the address CloudFormation should call when it needs to do something. That’s either a Lambda function ARN or an SNS topic ARN.

How Does It Actually Work?

Here’s the flow, step by step:

1. Someone writes the logic
A custom resource provider writes the code that handles CloudFormation’s requests, what to do on create, update, or delete.

2. They wire it up
They deploy a Lambda function or SNS topic (must be in the same region as your stack) and hand you the ARN.