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A package that facilitates working with existing CloudFormation templates in the CDK

Project description

Include CloudFormation templates in the CDK

---

cdk-constructs: Experimental

The APIs of higher level constructs in this module are experimental and under active development. They are subject to non-backward compatible changes or removal in any future version. These are not subject to the Semantic Versioning model and breaking changes will be announced in the release notes. This means that while you may use them, you may need to update your source code when upgrading to a newer version of this package.


This module contains a set of classes whose goal is to facilitate working with existing CloudFormation templates in the CDK. It can be thought of as an extension of the capabilities of the CfnInclude class.

Basic usage

Assume we have a file with an existing template. It could be in JSON format, in a file my-template.json:

{
  "Resources": {
    "Bucket": {
      "Type": "AWS::S3::Bucket",
      "Properties": {
        "BucketName": "some-bucket-name"
      }
    }
  }
}

Or it could by in YAML format, in a file my-template.yaml:

Resources:
  Bucket:
    Type: AWS::S3::Bucket
    Properties:
      BucketName: some-bucket-name

It can be included in a CDK application with the following code:

# Example automatically generated without compilation. See https://github.com/aws/jsii/issues/826
import aws_cdk.cloudformation_include as cfn_inc

cfn_template = cfn_inc.CfnInclude(self, "Template",
    template_file="my-template.json"
)

Or, if our template is YAML, we can use

# Example automatically generated without compilation. See https://github.com/aws/jsii/issues/826
cfn_template = cfn_inc.CfnInclude(self, "Template",
    template_file="my-template.yaml"
)

This will add all resources from my-template.json into the CDK application, preserving their original logical IDs from the template file.

Any resource from the included template can be retrieved by referring to it by its logical ID from the template. If you know the class of the CDK object that corresponds to that resource, you can cast the returned object to the correct type:

# Example automatically generated without compilation. See https://github.com/aws/jsii/issues/826
import aws_cdk.aws_s3 as s3

cfn_bucket = cfn_template.get_resource("Bucket")

Any modifications made to that resource will be reflected in the resulting CDK template; for example, the name of the bucket can be changed:

# Example automatically generated without compilation. See https://github.com/aws/jsii/issues/826
cfn_bucket.bucket_name = "my-bucket-name"

You can also refer to the resource when defining other constructs, including the higher-level ones (those whose name does not start with Cfn), for example:

# Example automatically generated without compilation. See https://github.com/aws/jsii/issues/826
import aws_cdk.aws_iam as iam

role = iam.Role(self, "Role",
    assumed_by=iam.AnyPrincipal()
)
role.add_to_policy(iam.PolicyStatement(
    actions=["s3:*"],
    resources=[cfn_bucket.attr_arn]
))

If you need, you can also convert the CloudFormation resource to a higher-level resource by importing it by its name:

# Example automatically generated without compilation. See https://github.com/aws/jsii/issues/826
bucket = s3.Bucket.from_bucket_name(self, "L2Bucket", cfn_bucket.ref)

Note that Custom Resources will be of type CfnResource, and hence won't need to be casted. This holds for any resource that isn't in the CloudFormation schema.

Parameters

If your template uses [CloudFormation Parameters] (https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/parameters-section-structure.html), you can retrieve them from your template:

# Example automatically generated without compilation. See https://github.com/aws/jsii/issues/826
import aws_cdk.core as core

param = cfn_template.get_parameter("MyParameter")

The CfnParameter object is mutable, and any changes you make to it will be reflected in the resulting template:

# Example automatically generated without compilation. See https://github.com/aws/jsii/issues/826
param.default = "MyDefault"

Conditions

If your template uses CloudFormation Conditions, you can retrieve them from your template:

# Example automatically generated without compilation. See https://github.com/aws/jsii/issues/826
import aws_cdk.core as core

condition = cfn_template.get_condition("MyCondition")

The CfnCondition object is mutable, and any changes you make to it will be reflected in the resulting template:

# Example automatically generated without compilation. See https://github.com/aws/jsii/issues/826
condition.expression = core.Fn.condition_equals(1, 2)

Outputs

If your template uses CloudFormation Outputs, you can retrieve them from your template:

# Example automatically generated without compilation. See https://github.com/aws/jsii/issues/826
import aws_cdk.core as core

output = cfn_template.get_output("MyOutput")

The CfnOutput object is mutable, and any changes you make to it will be reflected in the resulting template:

# Example automatically generated without compilation. See https://github.com/aws/jsii/issues/826
output.value = cfn_bucket.attr_arn

Nested Stacks

This module also support templates that use nested stacks.

For example, if you have the following parent template:

{
  "Resources": {
    "ChildStack": {
      "Type": "AWS::CloudFormation::Stack",
      "Properties": {
        "TemplateURL": "https://my-s3-template-source.s3.amazonaws.com/child-import-stack.json"
      }
    }
  }
}

where the child template pointed to by https://my-s3-template-source.s3.amazonaws.com/child-import-stack.json is:

{
  "Resources": {
    "MyBucket": {
      "Type": "AWS::S3::Bucket"
      }
    }
  }
}

You can include both the parent stack and the nested stack in your CDK Application as follows:

# Example automatically generated without compilation. See https://github.com/aws/jsii/issues/826
parent_template = inc.CfnInclude(stack, "ParentStack",
    template_file="path/to/my-parent-template.json",
    nested_stacks={
        "ChildStack": {
            "template_file": "path/to/my-nested-template.json"
        }
    }
)

Now you can access the ChildStack nested stack and included template with:

# Example automatically generated without compilation. See https://github.com/aws/jsii/issues/826
included_child_stack = parent_template.get_nested_stack("ChildStack")
child_stack = included_child_stack.stack
child_stack_template = included_child_stack.included_template

Now you can reference resources from ChildStack and modify them like any other included template:

# Example automatically generated without compilation. See https://github.com/aws/jsii/issues/826
bucket = child_stack_template.get_resource("MyBucket")
bucket.bucket_name = "my-new-bucket-name"

bucket_read_role = iam.Role(child_stack, "MyRole",
    assumed_by=iam.AccountRootPrincipal()
)

bucket_read_role.add_to_policy(iam.PolicyStatement(
    actions=["s3:GetObject*", "s3:GetBucket*", "s3:List*"
    ],
    resources=[bucket.attr_arn]
))

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