The problem described is a resource dependency issue where AWS CloudFormation is not aware that the custom resource requires the EC2 instance to be available before it can execute successfully. The DependsOn attribute is the explicit mechanism within a CloudFormation template to define such dependencies. By adding DependsOn to the custom resource and referencing the logical ID of the EC2 instance, you instruct CloudFormation to fully create and configure the EC2 instance before it proceeds to create the custom resource. This enforces the correct operational sequence and resolves the timing-related failure.

B. The ServiceToken property links the custom resource to its backing Lambda function. The issue is not an invalid link but the execution order of the resources.

C. The cfn-response module is used by the Lambda function to signal its success or failure back to CloudFormation. This does not influence when CloudFormation initially invokes the function.

D. The Fn::If intrinsic function is used for conditional resource creation based on template parameters. It cannot be used to control the sequence or timing of resource creation.

1. AWS CloudFormation User Guide

DependsOn attribute: "The DependsOn attribute can be used on any resource to specify that the creation of a specific resource follows another. When you add a DependsOn attribute to a resource

that resource is created only after the resource specified in the DependsOn attribute is successfully created." (AWS CloudFormation User Guide

Template anatomy

Resources

DependsOn attribute section).

2. AWS CloudFormation User Guide

Custom resources: This section details the workflow of custom resources

including the invocation by CloudFormation and the requirement to send a response. It implicitly highlights that CloudFormation controls the invocation timing

which is managed by dependencies. (AWS CloudFormation User Guide

Template reference

Custom resources).

3. AWS CloudFormation User Guide

Intrinsic function reference

Fn::If: "The intrinsic function Fn::If returns one of two values based on a condition. [...] Fn::If is often used in the Resources and Outputs sections of a template." This documentation confirms its purpose is conditional logic

not dependency management. (AWS CloudFormation User Guide

Intrinsic functions reference

Fn::If).