@aws-cdk/custom-resources
Advanced tools
AWS CloudFormation custom resources are extension points to the provisioning engine. When CloudFormation needs to create, update or delete a custom resource, it sends a lifecycle event notification to a custom resource provider. The provider handles the event (e.g. creates a resource) and sends back a response to CloudFormation.
The @aws-cdk/custom-resources.Provider construct is a "mini-framework" for
implementing providers for AWS CloudFormation custom resources. The framework offers a high-level API which makes it easier to implement robust
and powerful custom resources and includes the following capabilities:
The following code shows how the Provider construct is used in conjunction
with a CustomResource and a user-provided AWS Lambda function which implements
the actual handler.
import { CustomResource } from '@aws-cdk/core';
import * as logs from '@aws-cdk/aws-logs';
import * as cr from '@aws-cdk/custom-resources';
const onEvent = new lambda.Function(this, 'MyHandler', { /* ... */ });
const myProvider = new cr.Provider(this, 'MyProvider', {
onEventHandler: onEvent,
isCompleteHandler: isComplete, // optional async "waiter"
logRetention: logs.RetentionDays.ONE_DAY // default is INFINITE
});
new CustomResource(this, 'Resource1', { serviceToken: myProvider.serviceToken });
new CustomResource(this, 'Resource2', { serviceToken: myProvider.serviceToken });
Providers are implemented through AWS Lambda functions that are triggered by the provider framework in response to lifecycle events.
At the minimum, users must define the onEvent handler, which is invoked by the
framework for all resource lifecycle events (Create, Update and Delete)
and returns a result which is then submitted to CloudFormation.
The following example is a skeleton for a Python implementation of onEvent:
def on_event(event, context):
print(event)
request_type = event['RequestType']
if request_type == 'Create': return on_create(event)
if request_type == 'Update': return on_update(event)
if request_type == 'Delete': return on_delete(event)
raise Exception("Invalid request type: %s" % request_type)
def on_create(event):
props = event["ResourceProperties"]
print("create new resource with props %s" % props)
# add your create code here...
physical_id = ...
return { 'PhysicalResourceId': physical_id }
def on_update(event):
physical_id = event["PhysicalResourceId"]
props = event["ResourceProperties"]
print("update resource %s with props %s" % (physical_id, props))
# ...
def on_delete(event):
physical_id = event["PhysicalResourceId"]
print("delete resource %s" % physical_id)
# ...
Users may also provide an additional handler called isComplete, for cases
where the lifecycle operation cannot be completed immediately. The
isComplete handler will be retried asynchronously after onEvent until it
returns IsComplete: true, or until the total provider timeout has expired.
The following example is a skeleton for a Python implementation of isComplete:
def is_complete(event, context):
physical_id = event["PhysicalResourceId"]
request_type = event["RequestType"]
# check if resource is stable based on request_type
is_ready = ...
return { 'IsComplete': is_ready }
The user-defined onEvent AWS Lambda function is invoked whenever a resource
lifecycle event occurs. The function is expected to handle the event and return
a response to the framework that, at least, includes the physical resource ID.
If onEvent returns successfully, the framework will submit a "SUCCESS" response
to AWS CloudFormation for this resource operation. If the provider is
asynchronous (isCompleteHandler is
defined), the framework will only submit a response based on the result of
isComplete.
If onEvent throws an error, the framework will submit a "FAILED" response to
AWS CloudFormation.
The input event includes the following fields derived from the Custom Resource Provider Request:
| Field | Type | Description |
|---|---|---|
RequestType | String | The type of lifecycle event: Create, Update or Delete. |
LogicalResourceId | String | The template developer-chosen name (logical ID) of the custom resource in the AWS CloudFormation template. |
PhysicalResourceId | String | This field will only be present for Update and Delete events and includes the value returned in PhysicalResourceId of the previous operation. |
ResourceProperties | JSON | This field contains the properties defined in the template for this custom resource. |
OldResourceProperties | JSON | This field will only be present for Update events and contains the resource properties that were declared previous to the update request. |
ResourceType | String | The resource type defined for this custom resource in the template. A provider may handle any number of custom resource types. |
RequestId | String | A unique ID for the request. |
StackId | String | The ARN that identifies the stack that contains the custom resource. |
The return value from onEvent must be a JSON object with the following fields:
| Field | Type | Required | Description |
|---|---|---|---|
PhysicalResourceId | String | No | The allocated/assigned physical ID of the resource. If omitted for Create events, the event's RequestId will be used. For Update, the current physical ID will be used. If a different value is returned, CloudFormation will follow with a subsequent Delete for the previous ID (resource replacement). For Delete, it will always return the current physical resource ID, and if the user returns a different one, an error will occur. |
Data | JSON | No | Resource attributes, which can later be retrieved through Fn::GetAtt on the custom resource object. |
| any | any | No | Any other field included in the response will be passed through to isComplete. This can sometimes be useful to pass state between the handlers. |
It is not uncommon for the provisioning of resources to be an asynchronous operation, which means that the operation does not immediately finish, and we need to "wait" until the resource stabilizes.
The provider framework makes it easy to implement "waiters" by allowing users to
specify an additional AWS Lambda function in isCompleteHandler.
The framework will repeatedly invoke the handler every queryInterval. When
isComplete returns with IsComplete: true, the framework will submit a
"SUCCESS" response to AWS CloudFormation. If totalTimeout expires and the
operation has not yet completed, the framework will submit a "FAILED" response
with the message "Operation timed out".
If an error is thrown, the framework will submit a "FAILED" response to AWS CloudFormation.
The input event to isComplete includes all request fields, combined with all
fields returned from onEvent. If PhysicalResourceId has not been explicitly
returned from onEvent, it's value will be calculated based on the heuristics
described above.
The return value must be a JSON object with the following fields:
| Field | Type | Required | Description |
|---|---|---|---|
IsComplete | Boolean | Yes | Indicates if the operation has finished or not. |
Data | JSON | No | May only be sent if IsComplete is true and includes additional resource attributes. These attributes will be merged with the ones returned from onEvent |
Every resource in CloudFormation has a physical resource ID. When a resource is
created, the PhysicalResourceId returned from the Create operation is stored
by AWS CloudFormation and assigned to the logical ID defined for this resource
in the template. If a Create operation returns without a PhysicalResourceId,
the framework will use RequestId as the default. This is sufficient for
various cases such as "pseudo-resources" which only query data.
For Update and Delete operations, the resource event will always include the
current PhysicalResourceId of the resource.
When an Update operation occurs, the default behavior is to return the current
physical resource ID. if the onEvent returns a PhysicalResourceId which is
different from the current one, AWS CloudFormation will treat this as a
resource replacement, and it will issue a subsequent Delete operation for
the old resource.
As a rule of thumb, if your custom resource supports configuring a physical name
(e.g. you can specify a BucketName when you define an AWS::S3::Bucket), you
must return this name in PhysicalResourceId and make sure to handle
replacement properly. The S3File example demonstrates this
through the objectKey property.
As mentioned above, if any of the user handlers fail (i.e. throws an exception) or times out (due to their AWS Lambda timing out), the framework will trap these errors and submit a "FAILED" response to AWS CloudFormation, along with the error message.
Since errors can occur in multiple places in the provider (framework, onEvent,
isComplete), it is important to know that there could situations where a
resource operation fails even though the operation technically succeeded (i.e.
isComplete throws an error).
When AWS CloudFormation receives a "FAILED" response, it will attempt to roll back the stack to it's last state. This has different meanings for different lifecycle events:
Create event fails, the resource provider framework will automatically
ignore the subsequent Delete operation issued by AWS CloudFormation. The
framework currently does not support customizing this behavior (see
https://github.com/aws/aws-cdk/issues/5524).Update event fails, CloudFormation will issue an additional Update
with the previous properties.Delete event fails, CloudFormation will abandon this resource.Similarly to any AWS Lambda function, if the user-defined handlers require
access to AWS resources, you will have to define these permissions
by calling "grant" methods such as myBucket.grantRead(myHandler)), using myHandler.addToRolePolicy
or specifying an initialPolicy when defining the function.
Bear in mind that in most cases, a single provider will be used for multiple resource instances. This means that the execution policy of the provider must have the appropriate privileges.
The following example grants the onEvent handler s3:GetObject* permissions
to all buckets:
new lambda.Function(this, 'OnEventHandler', {
// ...
initialPolicy: [
new iam.PolicyStatement({ actions: [ 's3:GetObject*' ], resources: [ '*' ] })
]
});
Users are responsible to define the timeouts for the AWS Lambda functions for user-defined handlers. It is recommended not to exceed a 14 minutes timeout, since all framework functions are configured to time out after 15 minutes, which is the maximal AWS Lambda timeout.
If your operation takes over 14 minutes, the recommended approach is to
implement an asynchronous provider, and
then configure the timeouts for the asynchronous retries through the
queryInterval and the totalTimeout options.
This module includes a few examples for custom resource implementations:
Provisions an object in an S3 bucket with textual contents. See the source code for the construct and handler.
The following example will create the file folder/file1.txt inside myBucket
with the contents hello!.
new S3File(this, 'MyFile', {
bucket: myBucket,
objectKey: 'folder/file1.txt', // optional
content: 'hello!',
public: true // optional
});
This sample demonstrates the following concepts:
isComplete is not defined)objectKey is not definedChecks that the textual contents of an S3 object matches a certain value. The check will be retried for 5 minutes as long as the object is not found or the value is different. See the source code for the construct and handler.
The following example defines an S3Assert resource which waits until
myfile.txt in myBucket exists and includes the contents foo bar:
new S3Assert(this, 'AssertMyFile', {
bucket: myBucket,
objectKey: 'myfile.txt',
expectedContent: 'foo bar'
});
This sample demonstrates the following concepts:
Sometimes a single API call can fill the gap in the CloudFormation coverage. In
this case you can use the AwsCustomResource construct. This construct creates
a custom resource that can be customized to make specific API calls for the
CREATE, UPDATE and DELETE events. Additionally, data returned by the API
call can be extracted and used in other constructs/resources (creating a real
CloudFormation dependency using Fn::GetAtt under the hood).
The physical id of the custom resource can be specified or derived from the data returned by the API call.
The AwsCustomResource uses the AWS SDK for JavaScript. Services, actions and
parameters can be found in the API documentation.
Path to data must be specified using a dot notation, e.g. to get the string value
of the Title attribute for the first item returned by dynamodb.query it should
be Items.0.Title.S.
To make sure that the newest API calls are available the latest AWS SDK v2 is installed
in the Lambda function implementing the custom resource. The installation takes around 60
seconds. If you prefer to optimize for speed, you can disable the installation by setting
the installLatestAwsSdk prop to false.
You must provide the policy property defining the IAM Policy that will be applied to the API calls.
The library provides two factory methods to quickly configure this:
AwsCustomResourcePolicy.fromSdkCalls - Use this to auto-generate IAM Policy statements based on the configured SDK calls.
Note that you will have to either provide specific ARN's, or explicitly use AwsCustomResourcePolicy.ANY_RESOURCE to allow access to any resource.AwsCustomResourcePolicy.fromStatements - Use this to specify your own custom statements.The custom resource also implements iam.IGrantable, making it possible to use the grantXxx() methods.
As this custom resource uses a singleton Lambda function, it's important to note that the function's role will eventually accumulate the permissions/grants from all resources.
Chained API calls can be achieved by creating dependencies:
const awsCustom1 = new AwsCustomResource(this, 'API1', {
onCreate: {
service: '...',
action: '...',
physicalResourceId: PhysicalResourceId.of('...')
},
policy: AwsCustomResourcePolicy.fromSdkCalls({resources: AwsCustomResourcePolicy.ANY_RESOURCE})
});
const awsCustom2 = new AwsCustomResource(this, 'API2', {
onCreate: {
service: '...',
action: '...'
parameters: {
text: awsCustom1.getResponseField('Items.0.text')
},
physicalResourceId: PhysicalResourceId.of('...')
},
policy: AwsCustomResourcePolicy.fromSdkCalls({resources: AwsCustomResourcePolicy.ANY_RESOURCE})
})
Some AWS APIs may require passing the physical resource id in as a parameter for doing updates and deletes. You can pass it by using PhysicalResourceIdReference.
const awsCustom = new AwsCustomResource(this, '...', {
onCreate: {
service: '...',
action: '...'
parameters: {
text: '...'
},
physicalResourceId: PhysicalResourceId.of('...')
},
onUpdate: {
service: '...',
action: '...'.
parameters: {
text: '...',
resourceId: new PhysicalResourceIdReference()
}
},
policy: AwsCustomResourcePolicy.fromSdkCalls({resources: AwsCustomResourcePolicy.ANY_RESOURCE})
})
Every error produced by the API call is treated as is and will cause a "FAILED" response to be submitted to CloudFormation.
You can ignore some errors by specifying the ignoreErrorCodesMatching property, which accepts a regular expression that is
tested against the code property of the response. If matched, a "SUCCESS" response is submitted.
Note that in such a case, the call response data and the Data key submitted to CloudFormation would both be an empty JSON object.
Since a successful resource provisioning might or might not produce outputs, this presents us with some limitations:
PhysicalResourceId.fromResponse - Since the call response data might be empty, we cannot use it to extract the physical id.getResponseField and getResponseFieldReference - Since the Data key is empty, the resource will not have any attributes, and therefore, invoking these functions will result in an error.In both the cases, you will get a synth time error if you attempt to use it in conjunction with ignoreErrorCodesMatching.
Use the role, timeout, logRetention and functionName properties to customize
the Lambda function implementing the custom resource:
new AwsCustomResource(this, 'Customized', {
// other props here
role: myRole, // must be assumable by the `lambda.amazonaws.com` service principal
timeout: cdk.Duration.minutes(10) // defaults to 2 minutes
logRetention: logs.RetentionDays.ONE_WEEK // defaults to never delete logs
functionName: 'my-custom-name', // defaults to a CloudFormation generated name
})
const verifyDomainIdentity = new AwsCustomResource(this, 'VerifyDomainIdentity', {
onCreate: {
service: 'SES',
action: 'verifyDomainIdentity',
parameters: {
Domain: 'example.com'
},
physicalResourceId: PhysicalResourceId.fromResponse('VerificationToken') // Use the token returned by the call as physical id
},
policy: AwsCustomResourcePolicy.fromSdkCalls({resources: AwsCustomResourcePolicy.ANY_RESOURCE})
});
new route53.TxtRecord(this, 'SESVerificationRecord', {
zone,
recordName: `_amazonses.example.com`,
values: [verifyDomainIdentity.getResponseField('VerificationToken')]
});
const getParameter = new AwsCustomResource(this, 'GetParameter', {
onUpdate: { // will also be called for a CREATE event
service: 'SSM',
action: 'getParameter',
parameters: {
Name: 'my-parameter',
WithDecryption: true
},
physicalResourceId: PhysicalResourceId.of(Date.now().toString()) // Update physical id to always fetch the latest version
},
policy: AwsCustomResourcePolicy.fromSdkCalls({resources: AwsCustomResourcePolicy.ANY_RESOURCE})
});
// Use the value in another construct with
getParameter.getResponseField('Parameter.Value')
This module is part of the AWS Cloud Development Kit project.
1.88.0 (2021-02-03)
addVirtualService has been removed from IMeshstack.exportValue() can be used to solve "deadly embrace" (#12778) (3b66088), closes #7602 #2036FAQs
Constructs for implementing CDK custom resources
We found that @aws-cdk/custom-resources demonstrated a not healthy version release cadence and project activity because the last version was released a year ago. It has 4 open source maintainers collaborating on the project.
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