@aws-cdk/cdk

AWS Cloud Development Kit Core Library

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/cdk');

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.VpcNetwork(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 Tags 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.

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