A package that facilitates working with existing CloudFormation templates in the CDK
Project description
Include CloudFormation templates in the CDK
---The APIs of higher level constructs in this module are in developer preview before they become stable. We will only make breaking changes to address unforeseen API issues. Therefore, these APIs are not subject to Semantic Versioning, and breaking changes will be announced in 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 This means that while you may use them, you may need to update your source code when upgrading to a newer version of this package.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 your template uses YAML:
# 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"
)
Note: different YAML parsers sometimes don't agree on what exactly constitutes valid YAML. If you get a YAML exception when including your template, try converting it to JSON, and including that file instead. If you're downloading your template from the CloudFormation AWS Console, you can easily get it in JSON format by clicking the 'View in Designer' button on the 'Template' tab - once in Designer, select JSON in the "Choose template language" radio buttons on the bottom pane.
This will add all resources from my-template.json
/ my-template.yaml
into the CDK application,
preserving their original logical IDs from the template file.
Note that this including process will not execute any CloudFormation transforms - including the Serverless transform.
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")
Note that any resources not present in the latest version of the CloudFormation schema
at the time of publishing the version of this module that you depend on,
including Custom Resources,
will be returned as instances of the class CfnResource
,
and so cannot be cast to a different resource type.
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:
# Example automatically generated without compilation. See https://github.com/aws/jsii/issues/826
bucket = s3.Bucket.from_bucket_name(self, "L2Bucket", cfn_bucket.ref)
Non-resource template elements
In addition to resources, you can also retrieve and mutate all other template elements:
-
# 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") # mutating the parameter param.default = "MyDefault"
-
# 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") # mutating the condition condition.expression = core.Fn.condition_equals(1, 2)
-
# Example automatically generated without compilation. See https://github.com/aws/jsii/issues/826 import aws_cdk.core as core mapping = cfn_template.get_mapping("MyMapping") # mutating the mapping mapping.set_value("my-region", "AMI", "ami-04681a1dbd79675a5")
-
Service Catalog template Rules:
# Example automatically generated without compilation. See https://github.com/aws/jsii/issues/826 import aws_cdk.core as core rule = cfn_template.get_rule("MyRule") # mutating the rule rule.add_assertion(core.Fn.condition_contains(["m1.small"], my_parameter.value), "MyParameter has to be m1.small")
-
# 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") # mutating the output output.value = cfn_bucket.attr_arn
-
Hooks for blue-green deployments:
# Example automatically generated without compilation. See https://github.com/aws/jsii/issues/826 import aws_cdk.core as core hook = cfn_template.get_hook("MyOutput") # mutating the hook code_deploy_hook = hook code_deploy_hook.service_role = my_role.role_arn
Parameter replacement
If your existing template uses CloudFormation Parameters,
you may want to remove them in favor of build-time values.
You can do that using the parameters
property:
# Example automatically generated without compilation. See https://github.com/aws/jsii/issues/826
inc.CfnInclude(self, "includeTemplate",
template_file="path/to/my/template",
parameters={
"MyParam": "my-value"
}
)
This will replace all references to MyParam
with the string 'my-value'
,
and MyParam
will be removed from the 'Parameters' section of the template.
Nested Stacks
This module also supports 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-stack.json"
}
}
}
}
where the child template pointed to by https://my-s3-template-source.s3.amazonaws.com/child-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(self, "ParentStack",
template_file="path/to/my-parent-template.json",
load_nested_stacks={
"ChildStack": {
"template_file": "path/to/my-nested-template.json"
}
}
)
Here, path/to/my-nested-template.json
represents the path on disk to the downloaded template file from the original template URL of the nested stack
(https://my-s3-template-source.s3.amazonaws.com/child-stack.json
).
In the CDK application,
this file will be turned into an Asset,
and the TemplateURL
property of the nested stack resource
will be modified to point to that asset.
The included nested stack can be accessed with the getNestedStack
method:
# 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_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
cfn_bucket = child_template.get_resource("MyBucket")
cfn_bucket.bucket_name = "my-new-bucket-name"
role = iam.Role(child_stack, "MyRole",
assumed_by=iam.AccountRootPrincipal()
)
role.add_to_policy(iam.PolicyStatement(
actions=["s3:GetObject*", "s3:GetBucket*", "s3:List*"
],
resources=[cfn_bucket.attr_arn]
))
You can also include the nested stack after the CfnInclude
object was created,
instead of doing it on construction:
# Example automatically generated without compilation. See https://github.com/aws/jsii/issues/826
included_child_stack = parent_template.load_nested_stack("ChildTemplate",
template_file="path/to/my-nested-template.json"
)
Vending CloudFormation templates as Constructs
In many cases, there are existing CloudFormation templates that are not entire applications,
but more like specialized fragments, implementing a particular pattern or best practice.
If you have templates like that,
you can use the CfnInclude
class to vend them as CDK Constructs:
# Example automatically generated without compilation. See https://github.com/aws/jsii/issues/826
import path as path
class MyConstruct(Construct):
def __init__(self, scope, id):
super().__init__(scope, id)
# include a template inside the Construct
cfn_inc.CfnInclude(self, "MyConstruct",
template_file=path.join(__dirname, "my-template.json"),
preserve_logical_ids=False
)
Notice the preserveLogicalIds
parameter -
it makes sure the logical IDs of all the included template elements are re-named using CDK's algorithm,
guaranteeing they are unique within your application.
Without that parameter passed,
instantiating MyConstruct
twice in the same Stack would result in duplicated logical IDs.
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