AWS Construct library Composition Initialization Apps and stacks Using L1 constructs Using L2 constructs Configuration Interacting with constructs Writing your own constructs The construct tree

Constructs

Constructs are the basic building blocks of AWS CDK apps. A construct represents a "cloud component" and encapsulates everything AWS CloudFormation needs to create the component.

Note

Constructs are part of the Construct Programming Model (CPM). They're also used by other tools such as CDK for Terraform (CDKtf), CDK for Kubernetes (CDK8s), and Projen.

A construct can represent a single AWS resource, such as an Amazon Simple Storage Service (Amazon S3) bucket. A construct can also be a higher-level abstraction consisting of multiple related AWS resources. Examples of such components include a worker queue with its associated compute capacity, or a scheduled job with monitoring resources and a dashboard.

The AWS CDK includes a collection of constructs called the AWS Construct Library, containing constructs for every AWS service. Construct Hub is a resource to help you discover additional constructs from AWS, third parties, and the open-source CDK community.

Important

In AWS CDK v1, the Construct base class was in the CDK core module. In CDK v2, there is a separate module called constructs that contains this class.

AWS Construct library

The AWS CDK includes the AWS Construct Library, which contains constructs representing AWS resources.

This library includes constructs that represent all the resources available on AWS. For example, the s3.Bucket class represents an Amazon S3 bucket, and the dynamodb.Table class represents an Amazon DynamoDB table.

L1 constructs

There are three different levels of constructs in this library, beginning with low-level constructs, which we call CFN Resources (or L1, short for "layer 1"). These constructs directly represent all resources available in AWS CloudFormation. CFN Resources are periodically generated from the AWS CloudFormation Resource Specification. They are named CfnXyz, where Xyz is name of the resource. For example, CfnBucket represents the AWS::S3::Bucket AWS CloudFormation resource. When you use Cfn resources, you must explicitly configure all resource properties. This requires a complete understanding of the details of the underlying AWS CloudFormation resource model.

L2 constructs

The next level of constructs, L2, also represent AWS resources, but with a higher-level, intent-based API. They provide similar functionality, but incorporate the defaults, boilerplate, and glue logic you'd be writing yourself with a CFN Resource construct. AWS constructs offer convenient defaults and reduce the need to know all the details about the AWS resources they represent. They also provide convenience methods that make it simpler to work with the resource. For example, the s3.Bucket class represents an Amazon S3 bucket with additional properties and methods, such as bucket.addLifeCycleRule(), which adds a lifecycle rule to the bucket.

L3 constructs

Finally, the AWS Construct Library includes L3 constructs, which we call patterns. These constructs are designed to help you complete common tasks in AWS, often involving multiple kinds of resources. For example, the aws-ecs-patterns.ApplicationLoadBalancedFargateService construct represents an architecture that includes an AWS Fargate container cluster employing an Application Load Balancer. The aws-apigateway.LambdaRestApi construct represents an Amazon API Gateway API that's backed by an AWS Lambda function.

For more information about how to navigate the library and discover constructs that can help you build your apps, see the API Reference.

Composition

Composition is the key pattern for defining higher-level abstractions through constructs. A high-level construct can be composed from any number of lower-level constructs. In turn, those could be composed from even lower-level constructs, which eventually are composed from AWS resources.

From a bottom-up perspective, you use constructs to organize the individual AWS resources that you want to deploy. You use whatever abstractions are convenient for your purpose, with as many layers as you need.

Composition lets you define reusable components and share them like any other code. For example, a team can define a construct that implements the company's best practice for a DynamoDB table with backup, global replication, automatic scaling, and monitoring. The team can share the construct with other teams in their organization, or publicly.

Teams can use this construct in their preferred programming language like they would use any other library package to define their tables and comply with best practices. When the library is updated, developers will get access to the new version's bug fixes and improvements through the workflows they already have for their other types of code.

Initialization

Constructs are implemented in classes that extend the Construct base class. You define a construct by instantiating the class. All constructs take three parameters when they are initialized:

scope — The construct's parent or owner, either a stack or another construct, which determines its place in the construct tree. You should usually pass this (or self in Python), which represents the current object, for the scope.
id — An identifier that must be unique within this scope. The identifier serves as a namespace for everything that's defined within the current construct. It's used to generate unique identifiers such as resource names and AWS CloudFormation logical IDs.
props — A set of properties or keyword arguments, depending upon the language, that define the construct's initial configuration. In most cases, constructs provide sensible defaults, and if all props elements are optional, you can omit the props parameter completely.

Identifiers need only be unique within a scope. This lets you instantiate and reuse constructs without concern for the constructs and identifiers they might contain, and enables composing constructs into higher-level abstractions. In addition, scopes make it possible to refer to groups of constructs all at once. Examples include for tagging, or specifying where the constructs will be deployed.

Apps and stacks

We call your CDK application an app, which is represented by the AWS CDK class App. The following example defines an app with a single stack that contains a single Amazon S3 bucket with versioning enabled:

TypeScript


import { App, Stack, StackProps } from 'aws-cdk-lib';
import * as s3 from 'aws-cdk-lib/aws-s3';

class HelloCdkStack extends Stack {
  constructor(scope: App, id: string, props?: StackProps) {
    super(scope, id, props);

    new s3.Bucket(this, 'MyFirstBucket', {
      versioned: true
    });
  }
}

const app = new App();
new HelloCdkStack(app, "HelloCdkStack");

JavaScript


const { App , Stack } = require('aws-cdk-lib');
const s3 = require('aws-cdk-lib/aws-s3');

class HelloCdkStack extends Stack {
  constructor(scope, id, props) {
    super(scope, id, props);

    new s3.Bucket(this, 'MyFirstBucket', {
      versioned: true
    });
  }
}

const app = new App();
new HelloCdkStack(app, "HelloCdkStack");

Python


from aws_cdk import App, Stack
import aws_cdk.aws_s3 as s3
from constructs import Construct

class HelloCdkStack(Stack):

    def __init__(self, scope: Construct, id: str, **kwargs) -> None:
        super().__init__(scope, id, **kwargs)

        s3.Bucket(self, "MyFirstBucket", versioned=True)

app = App()
HelloCdkStack(app, "HelloCdkStack")

Java


import software.amazon.awscdk.*;
import software.amazon.awscdk.services.s3.*;

public class HelloCdkStack extends Stack {
    public HelloCdkStack(final Construct scope, final String id) {
        this(scope, id, null);
    }

    public HelloCdkStack(final Construct scope, final String id, final StackProps props) {
        super(scope, id, props);

        Bucket.Builder.create(this, "MyFirstBucket")
                .versioned(true).build();
    }
}

C#


using Amazon.CDK;
using Amazon.CDK.AWS.S3;

namespace HelloCdkApp
{
    internal static class Program
    {
        public static void Main(string[] args)
        {
            var app = new App();
            new HelloCdkStack(app, "HelloCdkStack");
            app.Synth();
        }
    }
    
    public class HelloCdkStack : Stack
    {
        public HelloCdkStack(Construct scope, string id, IStackProps props=null) : base(scope, id, props)
        {
            new Bucket(this, "MyFirstBucket", new BucketProps { Versioned = true });
        }
    }
}

Go


func NewHelloCdkStack(scope constructs.Construct, id string, props *HelloCdkStackProps) awscdk.Stack {
	var sprops awscdk.StackProps
	if props != nil {
		sprops = props.StackProps
	}
	stack := awscdk.NewStack(scope, &id, &sprops)

	awss3.NewBucket(stack, jsii.String("MyFirstBucket"), &awss3.BucketProps{
		Versioned: jsii.Bool(true),
	})

	return stack
}

As you can see, you need a scope within which to define your bucket. Resources eventually need to be deployed as part of an AWS CloudFormation stack into an AWS environment. The environment covers a specific AWS account and AWS Region. AWS constructs, such as s3.Bucket, must be defined within the scope of a Stack.

Stacks in AWS CDK apps extend the Stack base class, as shown in the previous example. The following example is a common pattern when creating a stack within your AWS CDK app. It shows how to extend the Stack class, define a constructor that accepts scope, id, and props, and invoke the base class constructor via super with the received scope, id, and props.

Using L1 constructs

Once you have defined a stack, you can populate it with resources by instantiating constructs. First, we'll do it with an L1 construct.

L1 constructs are exactly the resources defined by AWS CloudFormation—no more, no less. You must provide the resource's required configuration yourself. Here, for example, is how to create an Amazon S3 bucket using the CfnBucket class. (You'll see a similar definition using the Bucket class in the next section.)

Construct properties that aren't simple Booleans, strings, numbers, or containers are handled differently in the supported languages.

TypeScript


const bucket = new s3.CfnBucket(this, "MyBucket", {
  bucketName: "MyBucket",
  corsConfiguration: {
    corsRules: [{
          allowedOrigins: ["*"],
          allowedMethods: ["GET"]
    }]
  }
});

JavaScript


const bucket = new s3.CfnBucket(this, "MyBucket", {
  bucketName: "MyBucket",
  corsConfiguration: {
    corsRules: [{
          allowedOrigins: ["*"],
          allowedMethods: ["GET"]
    }]
  }
});

Python

In Python, these properties are represented by types defined as inner classes of the L1 construct. For example, the optional property cors_configuraiton of a CfnBucket requires a wrapper of type CfnBucket.CorsConfigurationProperty. Here we are defining cors_configuration on a CfnBucket instance.


bucket = CfnBucket(self, "MyBucket", bucket_name="MyBucket",
    cors_configuration=CfnBucket.CorsConfigurationProperty(
        cors_rules=[CfnBucket.CorsRuleProperty(
            allowed_origins=["*"],
            allowed_methods=["GET"]
        )]
    )
)

Java

In Java, these properties are represented by types defined as inner classes of the L1 construct. For example, the optional property corsConfiguration of a CfnBucket requires a wrapper of type CfnBucket.CorsConfigurationProperty. Here we are defining corsConfiguration on a CfnBucket instance.


CfnBucket bucket = CfnBucket.Builder.create(this, "MyBucket")
                        .bucketName("MyBucket")
                        .corsConfiguration(new CfnBucket.CorsConfigurationProperty.Builder()
                            .corsRules(Arrays.asList(new CfnBucket.CorsRuleProperty.Builder()
                                .allowedOrigins(Arrays.asList("*"))
                                .allowedMethods(Arrays.asList("GET"))
                                .build()))
                            .build())
                        .build();

C#

In C#, these properties are represented by types defined as inner classes of the L1 construct. For example, the optional property CorsConfiguration of a CfnBucket requires a wrapper of type CfnBucket.CorsConfigurationProperty. Here we are defining CorsConfiguration on a CfnBucket instance.


var bucket = new CfnBucket(this, "MyBucket", new CfnBucketProps
{
    BucketName = "MyBucket",
    CorsConfiguration = new CfnBucket.CorsConfigurationProperty
    {
        CorsRules = new object[] {
            new CfnBucket.CorsRuleProperty
            {
                AllowedOrigins = new string[] { "*" },
                AllowedMethods = new string[] { "GET" },
            }
        }
    }
});

Go

In Go, these types are named using the name of the L1 construct, an underscore, and the property name. For example, the optional property CorsConfiguration of a CfnBucket requires a wrapper of type CfnBucket_CorsConfigurationProperty. Here we are defining CorsConfiguration on a CfnBucket instance.


	awss3.NewCfnBucket(stack, jsii.String("MyBucket"), &awss3.CfnBucketProps{
		BucketName: jsii.String("MyBucket"),
		CorsConfiguration: &awss3.CfnBucket_CorsConfigurationProperty{
			CorsRules: []awss3.CorsRule{
				awss3.CorsRule{
					AllowedOrigins: jsii.Strings("*"),
					AllowedMethods: &[]awss3.HttpMethods{"GET"},
				},
			},
		},
	})

Important

You can't use L2 property types with L1 constructs, or vice versa. When working with L1 constructs, always use the types defined for the L1 construct you're using. Do not use types from other L1 constructs (some may have the same name, but they are not the same type).

Some of our language-specific API references currently have errors in the paths to L1 property types, or don't document these classes at all. We hope to fix this soon. In the meantime, remember that such types are always inner classes of the L1 construct they are used with.

Using L2 constructs

The following example defines an Amazon S3 bucket by creating an instance of the Bucket class, an L2 construct.

The AWS Construct Library includes constructs that represent many AWS resources.

Note

MyFirstBucket is not the name of the bucket that AWS CloudFormation creates. It is a logical identifier given to the new construct. See Physical Names for details.

Configuration

Most constructs accept props as their third argument (or in Python, keyword arguments), a name/value collection that defines the construct's configuration. The following example defines a bucket with AWS Key Management Service (AWS KMS) encryption and static website hosting enabled. Since it does not explicitly specify an encryption key, the Bucket construct defines a new kms.Key and associates it with the bucket.

AWS constructs are designed around the concept of "sensible defaults." Most constructs have a minimal required configuration, enabling you to quickly get started while also providing full control over the configuration when you need it.

Interacting with constructs

Constructs are classes that extend the base Construct class. After you instantiate a construct, the construct object exposes a set of methods and properties that let you interact with the construct and pass it around as a reference to other parts of the system.

The AWS CDK framework doesn't put any restrictions on the APIs of constructs. Authors can define any API they want. However, the AWS constructs that are included with the AWS Construct Library, such as s3.Bucket, follow guidelines and common patterns. This provides a consistent experience across all AWS resources.

Most AWS constructs have a set of grant methods that you can use to grant AWS Identity and Access Management (IAM) permissions on that construct to a principal. The following example grants the IAM group data-science permission to read from the Amazon S3 bucket raw-data.

Another common pattern is for AWS constructs to set one of the resource's attributes from data supplied elsewhere. Attributes can include Amazon Resource Names (ARNs), names, or URLs.

The following code defines an AWS Lambda function and associates it with an Amazon Simple Queue Service (Amazon SQS) queue through the queue's URL in an environment variable.

TypeScript


const jobsQueue = new sqs.Queue(this, 'jobs');
const createJobLambda = new lambda.Function(this, 'create-job', {
  runtime: lambda.Runtime.NODEJS_14_X,
  handler: 'index.handler',
  code: lambda.Code.fromAsset('./create-job-lambda-code'),
  environment: {
    QUEUE_URL: jobsQueue.queueUrl
  }
});

JavaScript


const jobsQueue = new sqs.Queue(this, 'jobs');
const createJobLambda = new lambda.Function(this, 'create-job', {
  runtime: lambda.Runtime.NODEJS_14_X,
  handler: 'index.handler',
  code: lambda.Code.fromAsset('./create-job-lambda-code'),
  environment: {
    QUEUE_URL: jobsQueue.queueUrl
  }
});

Python


jobs_queue = sqs.Queue(self, "jobs")
create_job_lambda = lambda_.Function(self, "create-job",
    runtime=lambda_.Runtime.NODEJS_14_X,
    handler="index.handler",
    code=lambda_.Code.from_asset("./create-job-lambda-code"),
    environment=dict(
        QUEUE_URL=jobs_queue.queue_url
    )
)

Java


final Queue jobsQueue = new Queue(this, "jobs");
Function createJobLambda = Function.Builder.create(this, "create-job")
                .handler("index.handler")
                .code(Code.fromAsset("./create-job-lambda-code"))
                .environment(java.util.Map.of(   // Map.of is Java 9 or later
                    "QUEUE_URL", jobsQueue.getQueueUrl())
                .build();

C#


var jobsQueue = new Queue(this, "jobs");
var createJobLambda = new Function(this, "create-job", new FunctionProps
{
    Runtime = Runtime.NODEJS_14_X,
    Handler = "index.handler",
    Code = Code.FromAsset(@".\create-job-lambda-code"),
    Environment = new Dictionary<string, string>
    {
        ["QUEUE_URL"] = jobsQueue.QueueUrl
    }
});

Go


	createJobLambda := awslambda.NewFunction(stack, jsii.String("create-job"), &awslambda.FunctionProps{
		Runtime: awslambda.Runtime_NODEJS_14_X(),
		Handler: jsii.String("index.handler"),
		Code:    awslambda.Code_FromAsset(jsii.String(".\\create-job-lambda-code"), nil),
		Environment: &map[string]*string{
			"QUEUE_URL": jsii.String(*jobsQueue.QueueUrl()),
		},
	})

For information about the most common API patterns in the AWS Construct Library, see Resources.

Writing your own constructs

In addition to using existing constructs like s3.Bucket, you can also write your own constructs, and then anyone can use them in their apps. All constructs are equal in the AWS CDK. An AWS CDK construct (such as s3.Bucket or sns.Topic) behaves the same as a construct from a third-party library published via NPM, Maven, or PyPI. Constructs published to your company's internal package repository also behave the same way.

To declare a new construct, create a class that extends the Construct base class, in the constructs package, then follow the pattern for initializer arguments.

The following example shows how to declare a construct that represents an Amazon S3 bucket. The S3 bucket sends an Amazon Simple Notification Service (Amazon SNS) notification every time someone uploads a file into it.

TypeScript


export interface NotifyingBucketProps {
  prefix?: string;
}

export class NotifyingBucket extends Construct {
  constructor(scope: Construct, id: string, props: NotifyingBucketProps = {}) {
    super(scope, id);
    const bucket = new s3.Bucket(this, 'bucket');
    const topic = new sns.Topic(this, 'topic');
    bucket.addObjectCreatedNotification(new s3notify.SnsDestination(topic),
      { prefix: props.prefix });
  }
}

JavaScript


class NotifyingBucket extends Construct {
  constructor(scope, id, props = {}) {
    super(scope, id);
    const bucket = new s3.Bucket(this, 'bucket');
    const topic = new sns.Topic(this, 'topic');
    bucket.addObjectCreatedNotification(new s3notify.SnsDestination(topic),
      { prefix: props.prefix });
  }
}

module.exports = { NotifyingBucket }

Python


class NotifyingBucket(Construct):

    def __init__(self, scope: Construct, id: str, *, prefix=None):
        super().__init__(scope, id)
        bucket = s3.Bucket(self, "bucket")
        topic = sns.Topic(self, "topic")
        bucket.add_object_created_notification(s3notify.SnsDestination(topic),
            s3.NotificationKeyFilter(prefix=prefix))

Java


public class NotifyingBucket extends Construct {

    public NotifyingBucket(final Construct scope, final String id) {
        this(scope, id, null, null);
    }
    
    public NotifyingBucket(final Construct scope, final String id, final BucketProps props) {
        this(scope, id, props, null);
    }
    
    public NotifyingBucket(final Construct scope, final String id, final String prefix) {
        this(scope, id, null, prefix);
    }

    public NotifyingBucket(final Construct scope, final String id, final BucketProps props, final String prefix) {
        super(scope, id);

        Bucket bucket = new Bucket(this, "bucket");
        Topic topic = new Topic(this, "topic");
        if (prefix != null)
            bucket.addObjectCreatedNotification(new SnsDestination(topic),
                NotificationKeyFilter.builder().prefix(prefix).build());
     }
}

C#


public class NotifyingBucketProps : BucketProps
{
    public string Prefix { get; set; }
}

public class NotifyingBucket : Construct
{
    public NotifyingBucket(Construct scope, string id, NotifyingBucketProps props = null) : base(scope, id)
    {
        var bucket = new Bucket(this, "bucket");
        var topic = new Topic(this, "topic");
        bucket.AddObjectCreatedNotification(new SnsDestination(topic), new NotificationKeyFilter
        {
            Prefix = props?.Prefix
        });
    }
}

Go


type NotifyingBucketProps struct {
	awss3.BucketProps
	Prefix *string
}

func NewNotifyingBucket(scope constructs.Construct, id *string, props *NotifyingBucketProps) awss3.Bucket {
	var bucket awss3.Bucket
	if props == nil {
		bucket = awss3.NewBucket(scope, jsii.String(*id+"Bucket"), nil)
	} else {
		bucket = awss3.NewBucket(scope, jsii.String(*id+"Bucket"), &props.BucketProps)
	}
	topic := awssns.NewTopic(scope, jsii.String(*id+"Topic"), nil)
	if props == nil {
		bucket.AddObjectCreatedNotification(awss3notifications.NewSnsDestination(topic))
	} else {
		bucket.AddObjectCreatedNotification(awss3notifications.NewSnsDestination(topic), &awss3.NotificationKeyFilter{
			Prefix: props.Prefix,
		})
	}
	return bucket
}

Note

Our NotifyingBucket construct inherits not from Bucket but rather from Construct. We are using composition, not inheritance, to bundle an Amazon S3 bucket and an Amazon SNS topic together. In general, composition is preferred over inheritance when developing AWS CDK constructs.

The NotifyingBucket constructor has a typical construct signature: scope, id, and props. The last argument, props, is optional (gets the default value {}) because all props are optional. (The base Construct class does not take a props argument.) You could define an instance of this construct in your app without props, for example:

Or you could use props (in Java, an additional parameter) to specify the path prefix to filter on, for example:

Typically, you would also want to expose some properties or methods on your constructs. It's not very useful to have a topic hidden behind your construct, because users of your construct aren't able to subscribe to it. Adding a topic property lets consumers access the inner topic, as shown in the following example:

TypeScript


export class NotifyingBucket extends Construct {
  public readonly topic: sns.Topic;

  constructor(scope: Construct, id: string, props: NotifyingBucketProps) {
    super(scope, id);
    const bucket = new s3.Bucket(this, 'bucket');
    this.topic = new sns.Topic(this, 'topic');
    bucket.addObjectCreatedNotification(new s3notify.SnsDestination(this.topic), { prefix: props.prefix });
  }
}

JavaScript


class NotifyingBucket extends Construct {

  constructor(scope, id, props) {
    super(scope, id);
    const bucket = new s3.Bucket(this, 'bucket');
    this.topic = new sns.Topic(this, 'topic');
    bucket.addObjectCreatedNotification(new s3notify.SnsDestination(this.topic), { prefix: props.prefix });
  }
}

module.exports = { NotifyingBucket };

Python


class NotifyingBucket(Construct):

    def __init__(self, scope: Construct, id: str, *, prefix=None, **kwargs):
        super().__init__(scope, id)
        bucket = s3.Bucket(self, "bucket")
        self.topic = sns.Topic(self, "topic")
        bucket.add_object_created_notification(s3notify.SnsDestination(self.topic),
            s3.NotificationKeyFilter(prefix=prefix))

Java


public class NotifyingBucket extends Construct {

    public Topic topic = null;
    
    public NotifyingBucket(final Construct scope, final String id) {
        this(scope, id, null, null);
    }
    
    public NotifyingBucket(final Construct scope, final String id, final BucketProps props) {
        this(scope, id, props, null);
    }
    
    public NotifyingBucket(final Construct scope, final String id, final String prefix) {
        this(scope, id, null, prefix);
    }

    public NotifyingBucket(final Construct scope, final String id, final BucketProps props, final String prefix) {
        super(scope, id);

        Bucket bucket = new Bucket(this, "bucket");
        topic = new Topic(this, "topic");
        if (prefix != null)
            bucket.addObjectCreatedNotification(new SnsDestination(topic),
                NotificationKeyFilter.builder().prefix(prefix).build());
     }
}

C#


public class NotifyingBucket : Construct
{
    public readonly Topic topic;

    public NotifyingBucket(Construct scope, string id, NotifyingBucketProps props = null) : base(scope, id)
    {
        var bucket = new Bucket(this, "bucket");
        topic = new Topic(this, "topic");
        bucket.AddObjectCreatedNotification(new SnsDestination(topic), new NotificationKeyFilter
        {
            Prefix = props?.Prefix
        });
    }
}

Go

To do this in Go, we'll need a little extra plumbing. Our original NewNotifyingBucket function returned an awss3.Bucket. We'll need to extend Bucket to include a topic member by creating a NotifyingBucket struct. Our function will then return this type.


type NotifyingBucket struct {
	awss3.Bucket
	topic awssns.Topic
}

func NewNotifyingBucket(scope constructs.Construct, id *string, props *NotifyingBucketProps) NotifyingBucket {
	var bucket awss3.Bucket
	if props == nil {
		bucket = awss3.NewBucket(scope, jsii.String(*id+"Bucket"), nil)
	} else {
		bucket = awss3.NewBucket(scope, jsii.String(*id+"Bucket"), &props.BucketProps)
	}
	topic := awssns.NewTopic(scope, jsii.String(*id+"Topic"), nil)
	if props == nil {
		bucket.AddObjectCreatedNotification(awss3notifications.NewSnsDestination(topic))
	} else {
		bucket.AddObjectCreatedNotification(awss3notifications.NewSnsDestination(topic), &awss3.NotificationKeyFilter{
			Prefix: props.Prefix,
		})
	}
	var nbucket NotifyingBucket
	nbucket.Bucket = bucket
	nbucket.topic = topic
	return nbucket
}

Now, consumers can subscribe to the topic, for example:

The construct tree

As we've already seen, in AWS CDK apps, you define constructs "inside" other constructs using the scope argument passed to every construct. In this way, an AWS CDK app defines a hierarchy of constructs known as the construct tree.

The root of this tree is your app—that is, an instance of the App class. Within the app, you instantiate one or more stacks. Within stacks, you instantiate either AWS CloudFormation resources or higher-level constructs, which may themselves instantiate resources or other constructs, and so on down the tree.

Constructs are always explicitly defined within the scope of another construct, so there is no doubt about the relationships between constructs. Almost always, you should pass this (in Python, self) as the scope, indicating that the new construct is a child of the current construct. The intended pattern is that you derive your construct from Construct, then instantiate the constructs it uses in its constructor.

Passing the scope explicitly allows each construct to add itself to the tree, with this behavior entirely contained within the Construct base class. It works the same way in every language supported by the AWS CDK and does not require introspection or other "magic."

Important

Technically, it's possible to pass some scope other than this when instantiating a construct. You can add constructs anywhere in the tree, or even in another stack in the same app. For example, you could write a mixin-style function that adds constructs to a scope passed in as an argument. The practical difficulty here is that you can't easily ensure that the IDs you choose for your constructs are unique within someone else's scope. The practice also makes your code more difficult to understand, maintain, and reuse. It is almost always better to find a way to express your intent without resorting to abusing the scope argument.

The AWS CDK uses the IDs of all constructs in the path from the tree's root to each child construct to generate the unique IDs required by AWS CloudFormation. This approach means that construct IDs only need to be unique within their scope, rather than within the entire stack as in native AWS CloudFormation. However, if you move a construct to a different scope, its generated stack-unique ID changes, and AWS CloudFormation won't consider it the same resource.

The construct tree is separate from the constructs that you define in your AWS CDK code. However, it's accessible through any construct's node attribute, which is a reference to the node that represents that construct in the tree. Each node is a Node instance, the attributes of which provide access to the tree's root and to the node's parent scopes and children.

node.children – The direct children of the construct.
node.id – The identifier of the construct within its scope.
node.path – The full path of the construct including the IDs of all of its parents.
node.root – The root of the construct tree (the app).
node.scope – The scope (parent) of the construct, or undefined if the node is the root.
node.scopes – All parents of the construct, up to the root.
node.uniqueId – The unique alphanumeric identifier for this construct within the tree (by default, generated from node.path and a hash).

The construct tree defines an implicit order in which constructs are synthesized to resources in the final AWS CloudFormation template. Where one resource must be created before another, AWS CloudFormation or the AWS Construct Library generally infers the dependency. They then make sure that the resources are created in the right order.

You can also add an explicit dependency between two nodes by using node.addDependency(). For more information, see Dependencies in the AWS CDK API Reference.

The AWS CDK provides a simple way to visit every node in the construct tree and perform an operation on each one. For more information, see Aspects.

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Document Conventions

Concepts

Apps