AWS Cloud Development Kit (AWS CDK)
Developer Guide (Version latest)

Assets

Assets are local files, directories, or Docker images that can be bundled into AWS CDK libraries and apps; for example, a directory that contains the handler code for an AWS Lambda function. Assets can represent any artifact that the app needs to operate.

You typically reference assets through APIs that are exposed by specific AWS constructs. For example, when you define a lambda.Function construct, the code property enables you to pass an asset (directory). Function uses assets to bundle the contents of the directory and use it for the function’s code. Similarly, ecs.ContainerImage.fromAsset uses a Docker image built from a local directory when defining an Amazon ECS task definition.

Assets in Detail

When you refer to an asset in your app, the cloud assembly synthesized from your application includes metadata information with instructions for the AWS CDK CLI on where to find the asset on the local disk, and what type of bundling to perform based on the type of asset, such as a directory to compress (zip) or a Docker image to build.

The AWS CDK generates a source hash for assets and can be used at construction time to determine whether the contents of an asset have changed.

By default, the AWS CDK creates a copy of the asset in the cloud assembly directory, which defaults to cdk.out, under the source hash. This is so that the cloud assembly is self-contained and moved over to a different host for deployment. See Cloud Assemblies for details.

The AWS CDK also synthesizes AWS CloudFormation parameters that the AWS CDK CLI specifies during deployment. The AWS CDK uses those parameters to refer to the deploy-time values of the asset.

When the AWS CDK deploys an app that references assets (either directly by the app code or through a library), the AWS CDK CLI first prepares and publishes them to Amazon S3 or Amazon ECR, and only then deploys the stack. The AWS CDK specifies the locations of the published assets as AWS CloudFormation parameters to the relevant stacks, and uses that information to enable referencing these locations within an AWS CDK app.

This section describes the low-level APIs available in the framework.

Asset Types

The AWS CDK supports the following types of assets:

Amazon S3 Assets

These are local files and directories that the AWS CDK uploads to Amazon S3.

Docker Image

These are Docker images that the AWS CDK uploads to Amazon ECR.

These asset types are explained in the following sections.

Amazon S3 Assets

You can define local files and directories as assets, and the AWS CDK packages and uploads them to Amazon S3 through the aws-s3-assets module.

The following example defines a local directory asset and a file asset.

import { Asset } from '@aws-cdk/aws-s3-assets'; // Archived and uploaded to Amazon S3 as a .zip file const directoryAsset = new Asset(this, "SampleAsset", { path: path.join(__dirname, "sample-asset-directory") }); // Uploaded to Amazon S3 as-is const fileAsset = new Asset(this, 'SampleAsset', { path: path.join(__dirname, 'file-asset.txt') });

In most cases, you don’t need to directly use the APIs in the aws-s3-assets module. Modules that support assets, such as aws-lambda, have convenience methods that enable you to use assets. For Lambda functions, the asset property enables you to specify a directory or a .zip file in the local file system.

Lambda Function Example

A common use case is to create AWS Lambda functions with the handler code, which is the entry point for the function, as an Amazon S3 asset.

The following example uses an Amazon S3 asset to define a handler in the local directory handler and creates a Lambda function with the local directory asset as the code property.

def lambda_handler(event, context): message = 'Hello World!' return { 'message': message }

The code for the main AWS CDK app should look like the following.

import cdk = require('@aws-cdk/core'); import lambda = require('@aws-cdk/aws-lambda'); import path = require('path'); export class HelloAssetStack extends cdk.Stack { constructor(scope: cdk.Construct, id: string, props?: cdk.StackProps) { super(scope, id, props); new lambda.Function(this, 'myLambdaFunction', { code: lambda.Code.asset(path.join(__dirname, 'handler')), runtime: lambda.Runtime.PYTHON_3_6, handler: 'index.lambda_handler' }); } }

The Function method uses assets to bundle the contents of the directory and use it for the function’s code.

Deploy-Time Attributes Example

Amazon S3 asset types also expose deploy-time attributes that can be referenced in AWS CDK libraries and apps. The AWS CDK CLI command cdk synth displays asset properties as AWS CloudFormation parameters.

The following example uses deploy-time attributes to pass the location of an image asset into a Lambda function as environment variables.

const imageAsset = new Asset(this, "SampleAsset", { path: path.join(__dirname, “images/my-image.png") }); new lambda.Function(this, "myLambdaFunction", { code: lambda.Code.asset(path.join(__dirname, "handler")), runtime: lambda.Runtime.PYTHON_3_6, handler: "index.lambda_handler", environment: { 'S3_BUCKET_NAME': imageAsset.s3BucketName, 'S3_OBJECT_KEY': imageAsset.s3ObjectKey, 'S3_URL': imageAsset.s3Url } });

Permissions

If you use Amazon S3 assets directly through the aws-s3-assets module, IAM roles, users, or groups, and need to read assets in runtime, grant those assets IAM permissions through the asset.grantRead method.

The following example grants an IAM group read permissions on a file asset.

const asset = new Asset(this, 'MyFile', { path: path.join(__dirname, 'my-image.png') }); const group = new iam.Group(this, 'MyUserGroup'); asset.grantRead(group); }

Docker Image Assets

The AWS CDK supports bundling local Docker images as assets through the aws-ecr-assets module.

The following example defines a docker image that is built locally and pushed to Amazon ECR. Images are built from a local Docker context directory (with a Dockerfile) and uploaded to Amazon ECR by the AWS CDK CLI or your app's CI/CD pipeline, and can be naturally referenced in your AWS CDK app.

import { DockerImageAsset } from '@aws-cdk/aws-ecr-assets'; const asset = new DockerImageAsset(this, 'MyBuildImage', { directory: path.join(__dirname, 'my-image') });

The my-image directory must include a Dockerfile. The AWS CDK CLI builds a Docker image from my-image, pushes it to an Amazon ECR repository, and specifies the name of the repository as an AWS CloudFormation parameter to your stack. Docker image asset types expose deploy-time attributes that can be referenced in AWS CDK libraries and apps. The AWS CDK CLI command cdk synth displays asset properties as AWS CloudFormation parameters.

Amazon ECS Task Definition Example

A common use case is to create an Amazon ECS TaskDefinition to run docker containers. The following example specifies the location of a Docker image asset that the AWS CDK builds locally and pushes to Amazon ECR.

import ecs = require('@aws-cdk/aws-ecs'); import path = require('path'); const taskDefinition = new ecs.FargateTaskDefinition(this, "TaskDef", { memoryLimitMiB: 1024, cpu: 512 }); taskDefinition.addContainer("my-other-container", { image: ecs.ContainerImage.fromAsset(path.join(__dirname, "..", "demo-image")) });

Deploy-Time Attributes Example

The following example shows how to use the deploy-time attributes repository and imageUri to create an Amazon ECS task definition with the AWS Fargate launch type.

import ecs = require('@aws-cdk/aws-ecs'); import path = require('path'); import { DockerImageAsset } from '@aws-cdk/aws-ecr-assets'; const asset = new DockerImageAsset(this, 'my-image', { directory: path.join(__dirname, "..", "demo-image") }); const taskDefinition = new ecs.FargateTaskDefinition(this, "TaskDef", { memoryLimitMiB: 1024, cpu: 512 }); taskDefinition.addContainer("my-other-container", { image: ecs.ContainerImage.fromEcrRepository(asset.repository, asset.imageUri) });

Build Arguments Example

You can provide customized build arguments for the Docker build step through the buildArgs property option when the AWS CDK CLI builds the image during deployment.

const asset = new DockerImageAsset(this, 'MyBuildImage', { directory: path.join(__dirname, 'my-image'), buildArgs: { HTTP_PROXY: 'http://10.20.30.2:1234' } });

Permissions

If you use a module that supports Docker image assets, such as aws-ecs, the AWS CDK manages permissions for you when you use assets directly or through ContainerImage.fromEcrRepository. If you use Docker image assets directly, you need to ensure that the consuming principal has permissions to pull the image.

In most cases, you should use asset.repository.grantPull method. This modifies the IAM policy of the principal to enable it to pull images from this repository. If the principal that is pulling the image is not in the same account or is an AWS service, such as AWS CodeBuild, that does not assume a role in your account, you must grant pull permissions on the resource policy and not on the principal's policy. Use the asset.repository.addToResourcePolicy method to grant the appropriate principal permissions.

AWS CloudFormation Resource Metadata

Note

This section is relevant only for construct authors. In certain situations, tools need to know that a certain CFN resource is using a local asset. For example, you can use the AWS SAM CLI to invoke Lambda functions locally for debugging purposes. See SAM CLI for details.

To enable such use cases, external tools consult a set of metadata entries on AWS CloudFormation resources:

  • aws:asset:path – Points to the local path of the asset.

  • aws:asset:property – The name of the resource property where the asset is used.

Using these two metadata entries, tools can identify that assets are used by a certain resource, and enable advanced local experiences.

To add these metadata entries to a resource, use the asset.addResourceMetadata method.