How synthesis and bootstrapping work together Configure CDK synthesis How to synthesize a CDK stack How synthesis works by default Customize CDK stack synthesis

Configure and perform CDK stack synthesis

Before you can deploy an AWS Cloud Development Kit (AWS CDK) stack, it must first be synthesized. Stack synthesis is the process of producing an AWS CloudFormation template and deployment artifacts from a CDK stack. The template and artifacts are known as the cloud assembly. The cloud assembly is what gets deployed to provision your resources on AWS. For more information on how deployments work, see How AWS CDK deployments work.

How synthesis and bootstrapping work together

For your CDK apps to properly deploy, the CloudFormation templates produced during synthesis must correctly specify the resources created during bootstrapping. Therefore, bootstrapping and synthesis must complement one another for a deployment to be successful:

Bootstrapping is a one-time process of setting up an AWS environment for AWS CDK deployments. It configures specific AWS resources in your environment that are used by the CDK for deployments. These are commonly referred to as bootstrap resources. For instructions on bootstrapping, see Bootstrap your environment for use with the AWS CDK.
CloudFormation templates produced during synthesis include information on which bootstrap resources to use. During synthesis, the CDK CLI doesn't know specifically how your AWS environment has been bootstrapped. Instead, the CDK CLI produces CloudFormation templates based on the synthesizer you configure for each CDK stack. For a deployment to be successful, the synthesizer must produce CloudFormation templates that reference the correct bootstrap resources to use.

The CDK comes with a default synthesizer and bootstrapping configuration that are designed to work together. If you customize one, you must apply relevant customizations to the other.

How to configure CDK stack synthesis

You configure CDK stack synthesis using the synthesizer property of your Stack instance. This property specifies how your CDK stacks will be synthesized. You provide an instance of a class that implements IStackSynthesizer or IReusableStackSynthesizer. Its methods will be invoked every time an asset is added to the stack or when the stack is synthesized. The following is a basic example of using this property within your stack:

You can also configure a synthesizer for all CDK stacks in your CDK app using the defaultStackSynthesizer property of your App instance:

TypeScript


import { App, Stack, DefaultStackSynthesizer } from 'aws-cdk-lib';

const app = new App({
  // Configure for all stacks in this app
  defaultStackSynthesizer: new DefaultStackSynthesizer({
    /* ... */
  }),
});

JavaScript


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

const app = new App({
  // Configure for all stacks in this app
  defaultStackSynthesizer: new DefaultStackSynthesizer({
    /* ... */
  }),
});

Python


from aws_cdk import App, Stack, DefaultStackSynthesizer

app = App(
    default_stack_synthesizer=DefaultStackSynthesizer(
        # Configure for all stacks in this app
        # ...
    )
)

Java


import software.amazon.awscdk.App;
import software.amazon.awscdk.Stack;
import software.amazon.awscdk.DefaultStackSynthesizer;

public class Main {
    public static void main(final String[] args) {
        App app = new App(AppProps.builder()
            // Configure for all stacks in this app
            .defaultStackSynthesizer(DefaultStackSynthesizer.Builder.create().build())
            .build()
        );
    }
}

C#


using Amazon.CDK;
using Amazon.CDK.Synthesizers;

namespace MyNamespace
{
    sealed class Program
    {
        public static void Main(string[] args)
        {
            var app = new App(new AppProps
            {
                // Configure for all stacks in this app
                DefaultStackSynthesizer = new DefaultStackSynthesizer(new DefaultStackSynthesizerProps
                {
                    // ...
                })
            });
        }
    }
}

Go


package main

import (
    "github.com/aws/aws-cdk-go/awscdk/v2"
    "github.com/aws/constructs-go/constructs/v10"
    "github.com/aws/jsii-runtime-go"
)

func main() {
    defer jsii.Close()
    
    app := awscdk.NewApp(&awscdk.AppProps{
        // Configure for all stacks in this app
        DefaultStackSynthesizer: awscdk.NewDefaultStackSynthesizer(&awscdk.DefaultStackSynthesizerProps{
            // ...
        }),
    })
}

By default, the AWS CDK uses DefaultStackSynthesizer. If you don’t configure a synthesizer, this synthesizer will be used.

If you don't modify bootstrapping, such as making changes to the bootstrap stack or template, you don’t have to modify stack synthesis. You don’t even have to provide a synthesizer. The CDK will use the default DefaultStackSynthesizer class to configure CDK stack synthesis to properly interact with your bootstrap stack.

How to synthesize a CDK stack

To synthesize a CDK stack, use the AWS CDK Command Line Interface (AWS CDK CLI) cdk synth command. For more information about this command, including options that you can use with this command, see cdk synthesize.

If your CDK app contains a single stack, or to synthesize all stacks, you don't have to provide the CDK stack name as an argument. By default, the CDK CLI will synthesize your CDK stacks into AWS CloudFormation templates. A json formatted template for each stack is saved to the cdk.out directory. If your app contains a single stack, a yaml formatted template is printed to stdout. The following is an example:


$ cdk synth
Resources:
  CDKMetadata:
    Type: AWS::CDK::Metadata
    Properties:
      Analytics: v2:deflate64:H4sIAAAAAAAA/unique-identifier
    Metadata:
      aws:cdk:path: CdkAppStack/CDKMetadata/Default
    Condition: CDKMetadataAvailable
    ...

If your CDK app contains multiple stacks, you can provide the logical ID of a stack to synthesize a single stack. The following is an example:


$ cdk synth MyStackName

If you don’t synthesize a stack and run cdk deploy, the CDK CLI will automatically synthesize your stack before deployment.

How synthesis works by default

Generated logical IDs in your AWS CloudFormation template

When you synthesize a CDK stack to produce a CloudFormation template, logical IDs are generated from the following sources, formatted as <construct-path><construct-ID><unique-hash>:

Construct path – The entire path to the construct in your CDK app. This path excludes the ID of the L1 construct, which is always Resource or Default, and the ID of the top-level stack that it's a part of.
Construct ID – The ID that you provide as the second argument when instantiating your construct.
Unique hash – The AWS CDK generates an 8 character unique hash using a deterministic hashing algorithm. This unique hash helps to ensure that logical ID values in your template are unique from one another. The deterministic behavior of this hash generation ensures that the generated logical ID value for each construct remains the same every time that you perform synthesis. The hash value will only change if you modify specific construct values such as your construct's ID or its path.

Logical IDs have a maximum length of 255 characters. Therefore, the AWS CDK will truncate the construct path and construct ID if necessary to keep within that limit.

The following is an example of a construct that defines an Amazon Simple Storage Service (Amazon S3) bucket. Here, we pass myBucket as the ID for our construct:

TypeScript


import * as cdk from 'aws-cdk-lib';
import { Construct} from 'constructs';
import * as s3 from 'aws-cdk-lib/aws-s3';

export class MyCdkAppStack extends cdk.Stack {
  constructor(scope: cdk.Construct, id: string, props?: cdk.StackProps) {
    super(scope, id, props);

    // Define the S3 bucket
    new s3.Bucket(this, 'myBucket', {
      versioned: true,
      removalPolicy: cdk.RemovalPolicy.DESTROY,
    });
  }
}

JavaScript


const cdk = require('aws-cdk-lib');
const s3 = require('aws-cdk-lib/aws-s3');

class MyCdkAppStack extends cdk.Stack {

  constructor(scope, id, props) {
    super(scope, id, props);
    
    new s3.Bucket(this, 'myBucket', {
      versioned: true,
      removalPolicy: cdk.RemovalPolicy.DESTROY,
    });
  }
}

module.exports = { MyCdkAppStack }

Python


import aws_cdk as cdk
from constructs import Construct
from aws_cdk import Stack
from aws_cdk import aws_s3 as s3

class MyCdkAppStack(Stack):
  def __init__(self, scope: Construct, construct_id: str, **kwargs) -> None:
    super().__init__(scope, construct_id, **kwargs)
          
    s3.Bucket(self, 'MyBucket',
      versioned=True,
      removal_policy=cdk.RemovalPolicy.DESTROY
    )

Java


package com.myorg;

import software.constructs.Construct;
import software.amazon.awscdk.Stack;
import software.amazon.awscdk.StackProps;
import software.amazon.awscdk.services.s3.Bucket;
import software.amazon.awscdk.services.s3.BucketProps;
import software.amazon.awscdk.RemovalPolicy;

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

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

        Bucket.Builder.create(this, "myBucket")
            .versioned(true)
            .removalPolicy(RemovalPolicy.DESTROY)
            .build();
    }
}

C#


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

namespace MyCdkApp
{
    public class MyCdkAppStack : Stack
    {
        public MyCdkAppStack(Construct scope, string id, IStackProps props = null) : base(scope, id, props)
        {
            new Bucket(this, "myBucket", new BucketProps
            {
                Versioned = true,
                RemovalPolicy = RemovalPolicy.DESTROY
            });
        }
    }
}

Go


package main
            
import (
    "github.com/aws/aws-cdk-go/awscdk/v2"
    "github.com/aws/aws-cdk-go/awscdk/v2/awss3"
    "github.com/aws/constructs-go/constructs/v10"
    "github.com/aws/jsii-runtime-go"
)

type MyCdkAppStackProps struct {
    awscdk.StackProps
}

func NewMyCdkAppStack(scope constructs.Construct, id string, props *MyCdkAppStackProps) awscdk.Stack {
    var sprops awscdk.StackProps
    if props != nil {
        sprops = props.StackProps
    }
    stack := awscdk.NewStack(scope, &id, &sprops)
    
    awss3.NewBucket(stack, jsii.String("myBucket"), &awss3.BucketProps{
      Versioned: jsii.Bool(true),
      RemovalPolicy: awscdk.RemovalPolicy_DESTROY,
    })
    
    return stack
}

// ...

When we run cdk synth, a logical ID in the format of myBucketunique-hash gets generated. The following is an example of this resource in the generated AWS CloudFormation template:


Resources:
  myBucket5AF9C99B:
    Type: AWS::S3::Bucket
    Properties:
      VersioningConfiguration:
        Status: Enabled
    UpdateReplacePolicy: Delete
    DeletionPolicy: Delete
    Metadata:
      aws:cdk:path: S3BucketAppStack/myBucket/Resource

The following is an example of a custom construct named Bar that defines an Amazon S3 bucket. The Bar construct includes the custom construct Foo in its path:

TypeScript


import * as cdk from 'aws-cdk-lib';
import { Construct } from 'constructs';
import * as s3 from 'aws-cdk-lib/aws-s3';

// Define the Bar construct
export class Bar extends Construct {
  constructor(scope: Construct, id: string) {
    super(scope, id);

    // Define an S3 bucket inside of Bar
    new s3.Bucket(this, 'Bucket', {
       versioned: true,
       removalPolicy: cdk.RemovalPolicy.DESTROY,
      } );
  }
}

// Define the Foo construct
export class Foo extends Construct {
  constructor(scope: Construct, id: string) {
    super(scope, id);

    // Create an instance of Bar inside Foo
    new Bar(this, 'Bar');
  }
}

// Define the CDK stack
export class MyCustomAppStack extends cdk.Stack {
  constructor(scope: Construct, id: string, props?: cdk.StackProps) {
    super(scope, id, props);

    // Instantiate Foo construct in the stack
    new Foo(this, 'Foo');
  }
}

JavaScript


const cdk = require('aws-cdk-lib');
const s3 = require('aws-cdk-lib/aws-s3');
const { Construct } = require('constructs');

// Define the Bar construct
class Bar extends Construct {
  constructor(scope, id) {
    super(scope, id);

    // Define an S3 bucket inside of Bar
    new s3.Bucket(this, 'Bucket', {
      versioned: true,
      removalPolicy: cdk.RemovalPolicy.DESTROY,
    });
  }
}

// Define the Foo construct
class Foo extends Construct {
  constructor(scope, id) {
    super(scope, id);

    // Create an instance of Bar inside Foo
    new Bar(this, 'Bar');
  }
}

// Define the CDK stack
class MyCustomAppStack extends cdk.Stack {
  constructor(scope, id, props) {
    super(scope, id, props);

    // Instantiate Foo construct in the stack
    new Foo(this, 'Foo');
  }
}

module.exports = { MyCustomAppStack }

Python


import aws_cdk as cdk
from constructs import Construct
from aws_cdk import (
    Stack,
    aws_s3 as s3,
    RemovalPolicy,
)

# Define the Bar construct
class Bar(Construct):
    def __init__(self, scope: Construct, id: str) -> None:
        super().__init__(scope, id)

        # Define an S3 bucket inside of Bar
        s3.Bucket(self, 'Bucket',
            versioned=True,
            removal_policy=RemovalPolicy.DESTROY
        )

# Define the Foo construct
class Foo(Construct):
    def __init__(self, scope: Construct, id: str) -> None:
        super().__init__(scope, id)

        # Create an instance of Bar inside Foo
        Bar(self, 'Bar')

# Define the CDK stack
class MyCustomAppStack(Stack):
    def __init__(self, scope: Construct, id: str, **kwargs) -> None:
        super().__init__(scope, id, **kwargs)

        # Instantiate Foo construct in the stack
        Foo(self, 'Foo')

Java

In my-custom-app/src/main/java/com/myorg/Bar.java:


package com.myorg;

import software.constructs.Construct;
import software.amazon.awscdk.services.s3.Bucket;
import software.amazon.awscdk.services.s3.BucketProps;
import software.amazon.awscdk.RemovalPolicy;

public class Bar extends Construct {
    public Bar(final Construct scope, final String id) {
        super(scope, id);

        // Define an S3 bucket inside Bar
        Bucket.Builder.create(this, "Bucket")
            .versioned(true)
            .removalPolicy(RemovalPolicy.DESTROY)
            .build();
    }
}

In my-custom-app/src/main/java/com/myorg/Foo.java:


package com.myorg;
							
import software.constructs.Construct;

public class Foo extends Construct {
    public Foo(final Construct scope, final String id) {
        super(scope, id);

        // Create an instance of Bar inside Foo
        new Bar(this, "Bar");
    }
}

In my-custom-app/src/main/java/com/myorg/MyCustomAppStack.java:


package com.myorg;

import software.constructs.Construct;
import software.amazon.awscdk.Stack;
import software.amazon.awscdk.StackProps;

public class MyCustomAppStack extends Stack {
    public MyCustomAppStack(final Construct scope, final String id, final StackProps props) {
        super(scope, id, props);

        // Instantiate Foo construct in the stack
        new Foo(this, "Foo");
    }

    // Overload constructor in case StackProps is not provided
    public MyCustomAppStack(final Construct scope, final String id) {
        this(scope, id, null);
    }
}

C#


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

namespace MyCustomApp
{
    // Define the Bar construct
    public class Bar : Construct
    {
        public Bar(Construct scope, string id) : base(scope, id)
        {
            // Define an S3 bucket inside Bar
            new Bucket(this, "Bucket", new BucketProps
            {
                Versioned = true,
                RemovalPolicy = RemovalPolicy.DESTROY
            });
        }
    }

    // Define the Foo construct
    public class Foo : Construct
    {
        public Foo(Construct scope, string id) : base(scope, id)
        {
            // Create an instance of Bar inside Foo
            new Bar(this, "Bar");
        }
    }

    // Define the CDK Stack
    public class MyCustomAppStack : Stack
    {
        public MyCustomAppStack(Construct scope, string id, StackProps props = null) : base(scope, id, props)
        {
            // Instantiate Foo construct in the stack
            new Foo(this, "Foo");
        }
    }
}

Go


package main

import (
	"github.com/aws/aws-cdk-go/awscdk/v2"
	"github.com/aws/aws-cdk-go/awscdk/v2/awss3"
	"github.com/aws/constructs-go/constructs/v10"
	"github.com/aws/jsii-runtime-go"
)

// Define the Bar construct
type Bar struct {
	constructs.Construct
}

func NewBar(scope constructs.Construct, id string) constructs.Construct {
	bar := constructs.NewConstruct(scope, &id)

	// Define an S3 bucket inside Bar
	awss3.NewBucket(bar, jsii.String("Bucket"), &awss3.BucketProps{
		Versioned:     jsii.Bool(true),
		RemovalPolicy: awscdk.RemovalPolicy_DESTROY,
	})

	return bar
}

// Define the Foo construct
type Foo struct {
	constructs.Construct
}

func NewFoo(scope constructs.Construct, id string) constructs.Construct {
	foo := constructs.NewConstruct(scope, &id)

	// Create an instance of Bar inside Foo
	NewBar(foo, "Bar")

	return foo
}

// Define the CDK Stack
type MyCustomAppStackProps struct {
	awscdk.StackProps
}

func NewMyCustomAppStack(scope constructs.Construct, id string, props *MyCustomAppStackProps) awscdk.Stack {
	stack := awscdk.NewStack(scope, &id, &props.StackProps)

	// Instantiate Foo construct in the stack
	NewFoo(stack, "Foo")

	return stack
}

// Define the CDK App
func main() {
	app := awscdk.NewApp(nil)

	NewMyCustomAppStack(app, "MyCustomAppStack", &MyCustomAppStackProps{
		StackProps: awscdk.StackProps{},
	})

	app.Synth(nil)
}

When we run cdk synth, a logical ID in the format of FooBarBucketunique-hash gets generated. The following is an example of this resource in the generated AWS CloudFormation template:


Resources:
  FooBarBucketBA3ED1FA:
    Type: AWS::S3::Bucket
    Properties:
      VersioningConfiguration:
        Status: Enabled
    UpdateReplacePolicy: Delete
    DeletionPolicy: Delete
    # ...

Customize CDK stack synthesis

If the default CDK synthesis behavior doesn't suit your needs, you can customize CDK synthesis. To do this, you modify DefaultStackSynthesizer, use other available built-in synthesizers, or create your own synthesizer. For instructions, see Customize CDK stack synthesis.

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Document Conventions

Define permissions for L2 constructs

Customize CDK synthesis