aws-cdk.core 0.38.0

AWS Cloud Development Kit Core Library

AWS Cloud Development Kit Core Library

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This is a developer preview (public beta) module. Releases might lack important features and might have future breaking changes.

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This library includes the basic building blocks of the AWS Cloud Development Kit (AWS CDK).

Aspects

Aspects are a mechanism to extend the CDK without having to directly impact the class hierarchy. We have implemented aspects using the Visitor Pattern.

An aspect in the CDK is defined by this interface

Aspects can be applied to any construct. During the tree "prepare" phase the aspect will visit each construct in the tree once. Aspects are invoked in the order they were added to the construct. They traverse the construct tree in a breadth first order starting at the App ending at the leaf nodes (most commonly the CloudFormation Resource). Aspect authors implement the visit(IConstruct) function and can inspect the Construct for specific characteristics. Such as, is this construct a CloudFormation Resource?

Tagging

Tags are implemented using aspects.

Tags can be applied to any construct. Tags are inherited, based on the scope. If you tag construct A, and A contains construct B, construct B inherits the tag. The Tag API supports:

• Tag add (apply) a tag, either to specific resources or all but specific resources
• RemoveTag remove a tag, again either from specific resources or all but specific resources

A simple example, if you create a stack and want anything in the stack to receive a tag:

import cdk = require('@aws-cdk/core');

const app = new cdk.App();
const theBestStack = new cdk.Stack(app, 'MarketingSystem');
theBestStack.node.apply(new cdk.Tag('StackType', 'TheBest'));

// any resources added that support tags will get them

The goal was to enable the ability to define tags in one place and have them applied consistently for all resources that support tagging. In addition the developer should not have to know if the resource supports tags. The developer defines the tagging intents for all resources within a path. If the resources support tags they are added, else no action is taken.

Tag Example with ECS

We are going to use the ECS example as starting point.

For the purposes of example, this ECS cluster is for the Marketing Department. Marketing has two core groups Business to Business (B2B) and Business to Consumer (B2C). However, the Marketing team relies on the Platform team to help build the common components across businesses and separates costs to match. The goal here is tag the Platform team resources, the Marketing Department and then Marketing groups to enable proper cost allocations.

We have modified the example and the code is located: examples/cdk-examples-typescript/hello-cdk-ecs-tags

When the example is run the following tags are created:

We are omitting the default tags for VPC components.

Construct Path	Tag Key	Tag Value
MarketingSystem/MarketingVpc	CostCenter	Platform
MarketingSystem/MarketingVpc/PublicSubnet1	CostCenter	Platform
MarketingSystem/MarketingVpc/PublicSubnet1/RouteTable	CostCenter	Platform
MarketingSystem/MarketingVpc/PublicSubnet1/NATGateway	CostCenter	Platform
MarketingSystem/MarketingVpc/PublicSubnet2	CostCenter	Platform
MarketingSystem/MarketingVpc/PublicSubnet2/RouteTable	CostCenter	Platform
MarketingSystem/MarketingVpc/PublicSubnet2/NATGateway	CostCenter	Platform
MarketingSystem/MarketingVpc/PublicSubnet3	CostCenter	Platform
MarketingSystem/MarketingVpc/PublicSubnet3/RouteTable	CostCenter	Platform
MarketingSystem/MarketingVpc/PublicSubnet3/NATGateway	CostCenter	Platform
MarketingSystem/MarketingVpc/PrivateSubnet1	CostCenter	Platform
MarketingSystem/MarketingVpc/PrivateSubnet1/RouteTable	CostCenter	Platform
MarketingSystem/MarketingVpc/PrivateSubnet2	CostCenter	Platform
MarketingSystem/MarketingVpc/PrivateSubnet2/RouteTable	CostCenter	Platform
MarketingSystem/MarketingVpc/PrivateSubnet3	CostCenter	Platform
MarketingSystem/MarketingVpc/PrivateSubnet3/RouteTable	CostCenter	Platform
MarketingSystem/MarketingVpc/IGW	CostCenter	Platform
MarketingSystem/B2BService/Service/SecurityGroup/Resource	CostCenter	Marketing
MarketingSystem/B2BService/LB/Resource	CostCenter	Marketing
MarketingSystem/B2BService/LB/SecurityGroup/Resource	CostCenter	Marketing
MarketingSystem/B2BService/LB/PublicListener/ECSGroup/Resource	CostCenter	Marketing
MarketingSystem/B2CService/Service/SecurityGroup/Resource	CostCenter	Marketing
MarketingSystem/B2CService/LB/Resource	CostCenter	Marketing
MarketingSystem/B2CService/LB/SecurityGroup/Resource	CostCenter	Marketing
MarketingSystem/B2CService/LB/PublicListener/ECSGroup/Resource	CostCenter	Marketing

As you can see many tags are generated with only a few intent based directives. The CDK does default some additional tags for suggested Name keys. If you want to remove those tags you can do so by using the RemoveTag aspect, see below:

// snip //
const vpc = new ec2.Vpc(marketingStack, 'MarketingVpc', {
  maxAZs: 3 // Default is all AZs in region
  });
// override the VPC tags with Platform
// this will tag the VPC, Subnets, Route Tables, IGW, and NatGWs
vpc.node.apply(new cdk.Tag(COST_CENTER_KEY, 'Platform'));
vpc.node.apply(new cdk.RemoveTag('Name'));
// snip //

This will remove the name tags from the VPC, subnets, route tables and NAT gateways. If you've been following closely, this may lead you to ask how does remove work when the tag is actually applied closer to the resource? The Tag API has a few features that are covered later to explain how this works.

API

In order to enable additional controls a Tag can specifically include or exclude a CloudFormation Resource Type, propagate tags for an autoscaling group, and use priority to override the default precedence. See the TagProps interface for more details.

Tags can be configured by using the properties for the AWS CloudFormation layer resources or by using the tag aspects described here. The aspects will always take precedence over the AWS CloudFormation layer in the event of a name collision. The tags will be merged otherwise. For the aspect based tags, the tags applied closest to the resource will take precedence, given an equal priority. A higher priority tag will always take precedence over a lower priority tag.

applyToLaunchedInstances

This property is a boolean that defaults to true. When true and the aspect visits an AutoScalingGroup resource the PropagateAtLaunch property is set to true. If false the property is set accordingly.

// ... snip
const vpc = new ec2.Vpc(this, 'MyVpc', { ... });
vpc.node.apply(new cdk.Tag('MyKey', 'MyValue', { applyToLaunchedInstances: false }));
// ... snip

includeResourceTypes

Include is an array property that contains strings of CloudFormation Resource Types. As the aspect visits nodes it only takes action if node is one of the resource types in the array. By default the array is empty and an empty array is interpreted as apply to any resource type.

// ... snip
const vpc = new ec2.Vpc(this, 'MyVpc', { ... });
vpc.node.apply(new cdk.Tag('MyKey', 'MyValue', { includeResourceTypes: ['AWS::EC2::Subnet']}));
// ... snip

excludeResourceTypes

Exclude is the inverse of include. Exclude is also an array of CloudFormation Resource Types. As the aspect visit nodes it will not take action if the node is one of the resource types in the array. By default the array is empty and an empty array is interpreted to match no resource type. Exclude takes precedence over include in the event of a collision.

// ... snip
const vpc = new ec2.Vpc(this, 'MyVpc', { ... });
vpc.node.apply(new cdk.Tag('MyKey', 'MyValue', { exludeResourceTypes: ['AWS::EC2::Subnet']}));
// ... snip

priority

Priority is used to control precedence when the default pattern does not work. In general users should try to avoid using priority, but in some situations it is required. In the example above, this is how RemoveTag works. The default setting for removing tags uses a higher priority than the standard tag.

// ... snip
const vpc = new ec2.Vpc(this, 'MyVpc', { ... });
vpc.node.apply(new cdk.Tag('MyKey', 'MyValue', { priority: 2 }));
// ... snip

Secrets

To help avoid accidental storage of secrets as plain text we use the SecretValue type to represent secrets.

The best practice is to store secrets in AWS Secrets Manager and reference them using SecretValue.secretsManager:

const secret = SecretValue.secretsManager('secretId', {
  jsonField: 'password' // optional: key of a JSON field to retrieve (defaults to all content),
  versionId: 'id'       // optional: id of the version (default AWSCURRENT)
  versionStage: 'stage' // optional: version stage name (default AWSCURRENT)
});

Using AWS Secrets Manager is the recommended way to reference secrets in a CDK app. However, SecretValue supports the following additional options:

• SecretValue.plainText(secret): stores the secret as plain text in your app and the resulting template (not recommended).
• SecretValue.ssmSecure(param, version): refers to a secret stored as a SecureString in the SSM Parameter Store.
• SecretValue.cfnParameter(param): refers to a secret passed through a CloudFormation parameter (must have NoEcho: true).
• SecretValue.cfnDynamicReference(dynref): refers to a secret described by a CloudFormation dynamic reference (used by ssmSecure and secretsManager).

AWS CloudFormation features

A CDK stack synthesizes to AWS CloudFormation templates. This section explains how this module allows users to access low-level CloudFormation features when needed.

Template Parameters

CloudFormation templates support the use of Parameters to customize a template. They enable CloudFormation users to input custom values to a template each time a stack is created or updated. While the CDK design philosophy favors using build-time parameterization, users may need to use CloudFormation in a number of cases (for example, when migrating an existing stack to the AWS CDK).

Template parameters can be added to a stack by using the CfnParameter class:

// "this" is the current Construct scope
new CfnParameter(this, 'MyParameter');

// - or -
new CfnParameter(this, 'MyParameter', {
  type: 'Number',
  default: 1337,
  // See the API reference for more configuration props
});

The value of parameters can then be obtained using one of the value methods. As parameters are only resolved at deployment time, the values obtained are placeholder tokens for the real value (Token.isUnresolved would return true for those):

const param = new CfnParameter(this, 'ParameterName', { /* config */ });

// If the parameter is a String
param.valueAsString;

// If the parameter is a Number
param.valueAsNumber;

// If the parameter is a List
param.valueAsList;

Pseudo Parameters

CloudFomration supports a number of pseudo parameters, which resolve to useful values at deployment time. CloudFormation pseudo parameters can be obtained from static members of the Aws class.

It is generally recommended to access pseudo parameters from the scope's stack instead, which guarantees the values produced are qualifying the designated stack, which is essential in cases where resources are shared cross-stack:

// "this" is the current construct
const stack = Stack.of(this);

stack.account; // Returns the AWS::AccountId for this stack (or the literal value if known)
stack.region;  // Returns the AWS::Region for this stack (or the literal value if known)
stack.partition;

Intrinsic Functions and Condition Expressions

CloudFormation supports intrinsic functions. These functions can be accessed from the Fn class, which provides type-safe methods for each intrinsic function as well as condition expressions:

// To use Fn::Base64
Fn.base64('SGVsbG8gQ0RLIQo=');

// To compose condition expressions:
const environmentParameter = new CfnParameter(this, 'Environment');
Fn.conditionAnd(
  // The "Environment" CloudFormation template parameter evaluates to "Production"
  Fn.conditionEquals('Production', environmentParameter),
  // The AWS::Region pseudo-parameter value is NOT equal to "us-east-1"
  Fn.conditionNot(Fn.conditionEquals('us-east-1', Aws.REGION)),
);

When working with deploy-time values (those for which Token.isUnresolved returns true), idiomatic conditionals from the programming language cannot be used (the value will not be known until deployment time). When conditional logic needs to be expressed with un-resolved values, it is necessary to use CloudFormation conditions, thanks to the CfnCondition class:

const environmentParameter = new CfnParameter(this, 'Environment');
const isProd = new CfnCondition(this, 'IsProduction', {
  expression: Fn.conditionEquals('Production', environmentParameter),
});

// Configuration value that is different based on IsProduction
Fn.conditionIf(isProd.logicalId, '1337', 'YOLO').toString();

// Make a CfnResource creation conditional to IsProduction
const resource = new CfnResource(this, 'RawResource', { /* ... */ });
resource.cfnOptions.condition = isProd;

Mappings

CloudFormation mappings are created and queried using the CfnMappings class:

const mapping = new CfnMapping(this, 'MappingTable', {
  mapping: {
    regionName: {
      'us-east-1': 'US East (N. Virginia)',
      'us-east-2': 'US East (Ohio)',
      // ...
    },
    // ...
  }
});

mapping.findInMap('regionName', Aws.REGION);

Dynamic References

CloudFormation supports dynamically resolving values for SSM parameters (including secure strings) and Secrets Manager. Encoding such references is done using the CfnDynamicReference class:

new CfnDynamicReference(this, 'SecureStringValue', {
  service: CfnDynamicReferenceService.SECRETS_MANAGER,
  referenceKey: 'secret-id:secret-string:json-key:version-stage:version-id',
});

Stack Outputs

CloudFormation stack outputs and exports are created using the CfnOutput class:

new CfnOutput(this, 'OutputName', {
  value: bucket.bucketName,
  description: 'The name of an S3 bucket', // Optional
  exportName: 'Global.BucketName', // Registers a CloudFormation export
});

Template Options & Transform

CloudFormation templates support a number of options, including which Macros or Transforms to use when deploying the stack. Those can be configured using the stack.templateOptions property:

const stack = new Stack(app, 'StackName');

stack.templateOptions.description = 'This will appear in the AWS console';
stack.templateOptions.transform = 'AWS::Serverless';
stack.templateOptions.metadata = {
  metadataKey: 'MetadataValue',
};

Resource Options

Similar to template options, CloudFormation resources can also specify resource attributes. The CfnResource class allows accessing those though the cfnOptions property:

const rawBucket = new s3.CfnBucket(this, 'Bucket', { /* ... */ });
rawBucket.condition = new CfnCondition(this, 'EnableBucket', { /* ... */ });
rawBucket.cfnOptions.metadata = {
  metadataKey: 'MetadataValue',
};

Resource dependencies (the DependsOn attribute) is modified using the cfnResource.addDependsOn method:

const resourceA = new CfnResource(this, 'ResourceA', { /* ... */ });
const resourceB = new CfnResource(this, 'ResourceB', { /* ... */ });

resourceB.addDependsOn(resourceA);

Emitting Raw Resources

The CfnResource class allows emitting arbitrary entries in the [Resources][cfn-resources] section of the CloudFormation template.

new CfnResource(this, 'ResourceId', {
  type: 'AWS::S3::Bucket',
  properties: {
    BucketName: 'bucket-name'
  },
});

As for any other resource, the logical ID in the CloudFormation template will be generated by the AWS CDK, but the type and properties will be copied verbatim in the synthesized template.

Including raw CloudFormation template fragments

When migrating a CloudFormation stack to the AWS CDK, it can be useful to include fragments of an existing template verbatim in the synthesized template. This can be achieved using the CfnInclude class.

new CfnInclude(this, 'ID', {
  template: {
    Resources: {
      Bucket: {
        Type: 'AWS::S3::Bucket',
        Properties: {
          BucketName: 'my-shiny-bucket'
        }
      }
    }
  },
});