CloudFormation custom resource request and response reference - AWS CloudFormation
Template setupRequest objectResponse object
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CloudFormation custom resource request and response reference

CloudFormation manages custom resources through a request-response protocol that communicates with your custom resource provider. Each request includes a request type (Create, Update, or Delete) and follows this high-level workflow:

  1. A template developer defines a custom resource with a ServiceToken and ServiceTimeout in the template and initiates a stack operation.

  2. CloudFormation sends a JSON request to the custom resource provider through SNS or Lambda.

  3. The custom resource provider processes the request and returns a JSON response to a presigned Amazon S3 bucket URL before the timeout period expires.

  4. CloudFormation reads the response and proceeds with the stack operation. If no response is received before the timeout period ends, the request is considered unsuccessful, and the stack operation fails.

For more information, see How custom resources work.

This section describes the structure, parameters, and expected responses for each request type.

Note

The total size of the response body can't exceed 4096 bytes.

Template setup

When defining a custom resource in a template, the template developer uses AWS::CloudFormation::CustomResource with the following properties:

ServiceToken

An Amazon SNS topic ARN or Lambda function ARN from the same Region as the stack.

Required: Yes

Type: String

ServiceTimeout

The maximum time, in seconds, before a custom resource operation times out. It must be a value between 1 and 3600. Default: 3600 seconds (1 hour).

Required: No

Type: String

Additional resource properties are supported. Resource properties will be included as ResourceProperties in the request. The custom resource provider must determine which properties are valid and their acceptable values.

Request object

Create

When the template developer creates a stack containing a custom resource, CloudFormation sends a request with RequestType set to Create.

Create requests contain the following fields:

RequestType

Create.

Required: Yes

Type: String

RequestId

A unique ID for the request.

Combining the StackId with the RequestId forms a value that you can use to uniquely identify a request on a particular custom resource.

Required: Yes

Type: String

StackId

The Amazon Resource Name (ARN) that identifies the stack that contains the custom resource.

Combining the StackId with the RequestId forms a value that you can use to uniquely identify a request on a particular custom resource.

Required: Yes

Type: String

ResponseURL

The response URL identifies a presigned S3 bucket that receives responses from the custom resource provider to CloudFormation.

Required: Yes

Type: String

ResourceType

The template developer-chosen resource type of the custom resource in the CloudFormation template. Custom resource type names can be up to 60 characters long and can include alphanumeric and the following characters: _@-.

Required: Yes

Type: String

LogicalResourceId

The template developer-chosen name (logical ID) of the custom resource in the CloudFormation template.

Required: Yes

Type: String

ResourceProperties

This field contains the contents of the Properties object sent by the template developer. Its contents are defined by the custom resource provider.

Required: No

Type: JSON object

Example

{
   "RequestType" : "Create",
   "RequestId" : "unique-request-id",
   "StackId" : "arn:aws-eusc:cloudformation:us-west-2:123456789012:stack/mystack/id",
   "ResponseURL" : "pre-signed-url-for-create-response",
   "ResourceType" : "Custom::MyCustomResourceType",
   "LogicalResourceId" : "resource-logical-id",
   "ResourceProperties" : {
      "key1" : "string",
      "key2" : [ "list" ],
      "key3" : { "key4" : "map" }
   }
}
Update

When the template developer makes changes to the properties of a custom resource within the template and updates the stack, CloudFormation sends a request to the custom resource provider with RequestType set to Update. This means that your custom resource code doesn't have to detect changes in resources because it knows that its properties have changed when the request type is Update.

Update requests contain the following fields:

RequestType

Update.

Required: Yes

Type: String

RequestId

A unique ID for the request.

Combining the StackId with the RequestId forms a value that you can use to uniquely identify a request on a particular custom resource.

Required: Yes

Type: String

StackId

The Amazon Resource Name (ARN) that identifies the stack that contains the custom resource.

Combining the StackId with the RequestId forms a value that you can use to uniquely identify a request on a particular custom resource.

Required: Yes

Type: String

ResponseURL

The response URL identifies a presigned S3 bucket that receives responses from the custom resource provider to CloudFormation.

Required: Yes

Type: String

ResourceType

The template developer-chosen resource type of the custom resource in the CloudFormation template. Custom resource type names can be up to 60 characters long and can include alphanumeric and the following characters: _@-. You can't change the type during an update.

Required: Yes

Type: String

LogicalResourceId

The template developer-chosen name (logical ID) of the custom resource in the CloudFormation template.

Required: Yes

Type: String

PhysicalResourceId

A custom resource provider-defined physical ID that is unique for that provider.

Required: Yes

Type: String

ResourceProperties

This field contains the contents of the Properties object sent by the template developer. Its contents are defined by the custom resource provider.

Required: No

Type: JSON object

OldResourceProperties

Used only for Update requests. The resource property values that were previously declared by the template developer in the CloudFormation template.

Required: Yes

Type: JSON object

Example

{
   "RequestType" : "Update",
   "RequestId" : "unique-request-id",
   "StackId" : "arn:aws-eusc:cloudformation:us-west-2:123456789012:stack/mystack/id",
   "ResponseURL" : "pre-signed-url-for-update-response",
   "ResourceType" : "Custom::MyCustomResourceType",
   "LogicalResourceId" : "resource-logical-id",
   "PhysicalResourceId" : "provider-defined-physical-id",
   "ResourceProperties" : {
      "key1" : "new-string",
      "key2" : [ "new-list" ],
      "key3" : { "key4" : "new-map" }
   },
   "OldResourceProperties" : {
      "key1" : "string",
      "key2" : [ "list" ],
      "key3" : { "key4" : "map" }
   }
}
Delete

When the template developer deletes the stack or removes the custom resource from the template and then updates the stack, CloudFormation sends a request with RequestType set to Delete.

Delete requests contain the following fields:

RequestType

Delete.

Required: Yes

Type: String

RequestId

A unique ID for the request.

Required: Yes

Type: String

StackId

The Amazon Resource Name (ARN) that identifies the stack that contains the custom resource.

Required: Yes

Type: String

ResponseURL

The response URL identifies a presigned S3 bucket that receives responses from the custom resource provider to CloudFormation.

Required: Yes

Type: String

ResourceType

The template developer-chosen resource type of the custom resource in the CloudFormation template. Custom resource type names can be up to 60 characters long and can include alphanumeric and the following characters: _@-.

Required: Yes

Type: String

LogicalResourceId

The template developer-chosen name (logical ID) of the custom resource in the CloudFormation template.

Required: Yes

Type: String

PhysicalResourceId

A custom resource provider-defined physical ID that is unique for that provider.

Required: Yes

Type: String

ResourceProperties

This field contains the contents of the Properties object sent by the template developer. Its contents are defined by the custom resource provider.

Required: No

Type: JSON object

Example

{
   "RequestType" : "Delete",
   "RequestId" : "unique-request-id",
   "StackId" : "arn:aws-eusc:cloudformation:us-west-2:123456789012:stack/mystack/id",
   "ResponseURL" : "pre-signed-url-for-delete-response",
   "ResourceType" : "Custom::MyCustomResourceType",
   "LogicalResourceId" : "resource-logical-id",
   "PhysicalResourceId" : "provider-defined-physical-id",
   "ResourceProperties" : {
      "key1" : "string",
      "key2" : [ "list" ],
      "key3" : { "key4" : "map" }
   }
}

Response object

The custom resource provider sends a response to the pre-signed URL for all request types. If the custom resource provider doesn't send a response, CloudFormation waits until the operation times out.

The response must be a JSON object with the following fields:

Status

Must be either SUCCESS or FAILED.

Required: Yes

Type: String

RequestId

A unique ID for the request. Copy this value exactly as it appears in the request.

Required: Yes

Type: String

StackId

The Amazon Resource Name (ARN) that identifies the stack that contains the custom resource. Copy this value exactly as it appears in the request.

Required: Yes

Type: String

LogicalResourceId

The template developer-chosen name (logical ID) of the custom resource in the CloudFormation template. Copy this value exactly as it appears in the request.

Required: Yes

Type: String

PhysicalResourceId

This value should be an identifier unique to the custom resource vendor, and can be up to 1 KB in size. The value must be a non-empty string and must be identical for all responses for the same resource.

When updating custom resources, the value returned for PhysicalResourceId determines the update behavior. If the value remains the same, CloudFormation considers it a normal update. If the value changes, CloudFormation interprets the update as a replacement and sends a delete request to the old resource. For more information, see AWS::CloudFormation::CustomResource.

Required: Yes

Type: String

Reason

Describes the reason for a failure response.

Required if Status is FAILED. It's optional otherwise.

Required: Conditional

Type: String

NoEcho

Indicates whether to mask the output of the custom resource when retrieved by using the Fn::GetAtt function. If set to true, all returned values are masked with asterisks (*****), except for those stored in the Metadata section of the template. CloudFormation does not transform, modify, or redact any information you include in the Metadata section. The default value is false.

For more information about using NoEcho to mask sensitive information, see the Do not embed credentials in your templates best practice.

Available for Create and Update responses only. Not supported for Delete responses.

Required: No

Type: Boolean

Data

The custom resource provider-defined name-value pairs to send with the response. You can access the values provided here by name in the template with Fn::GetAtt.

Available for Create and Update responses only. Not supported for Delete responses.

Important

If the name-value pairs contain sensitive information, you should use the NoEcho field to mask the output of the custom resource. Otherwise, the values are visible through APIs that surface property values (such as DescribeStackEvents).

Required: No

Type: JSON object

Success Response Examples

Create and Update Response

{
   "Status": "SUCCESS",
   "RequestId": "unique-request-id",
   "StackId": "arn:aws-eusc:cloudformation:us-west-2:123456789012:stack/name/id",
   "LogicalResourceId": "resource-logical-id", 
   "PhysicalResourceId": "provider-defined-physical-id",
   "NoEcho": true,
   "Data": {
      "key1": "value1",
      "key2": "value2"
   }
}

Delete Response

{
   "Status": "SUCCESS",
   "RequestId": "unique-request-id",
   "StackId": "arn:aws-eusc:cloudformation:us-west-2:123456789012:stack/name/id",
   "LogicalResourceId": "resource-logical-id", 
   "PhysicalResourceId": "provider-defined-physical-id"
}

Failed Response Example

{
   "Status": "FAILED",
   "RequestId": "unique-request-id",
   "StackId": "arn:aws-eusc:cloudformation:us-west-2:123456789012:stack/name/id",
   "LogicalResourceId": "resource-logical-id",
   "PhysicalResourceId": "provider-defined-physical-id",
   "Reason": "Required failure reason string"
}