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Contributing to the AWS Cloud Development Kit

Thanks for your interest in contributing to the AWS CDK! ❤️

We highly value contributions, with roughly half of all commits to the CDK coming from the community. We want to recognize all your hard work by getting your code merged as quickly as we can, so please read the guidance here carefully to make sure the review process goes smoothly.

The CDK is released under the Apache license. Any code you submit will be released under that license.

This document describes how to set up a development environment and submit your changes. Please let us know if it's not up-to-date (even better, submit a PR with your corrections ;-)).

Getting Started

The following steps describe how to set up the AWS CDK repository on your local machine. The alternative is to use Gitpod, a Cloud IDE for your development. See Gitpod section on how to set up the CDK repo on Gitpod.

Setup

The following tools need to be installed on your system prior to installing the CDK:

First fork the repository, and then run the following commands to clone the repository locally.

$ git clone https://github.com/{your-account}/aws-cdk.git
$ cd aws-cdk
$ yarn install

We recommend that you use Visual Studio Code to work on the CDK. We use eslint to keep our code consistent in terms of style and reducing defects. We recommend installing the eslint extension as well.

Repo Layout

The AWS CDK is a NPM project written in typescript. More specifically, it is a monorepo managed using lerna. If you're unfamiliar with any of these technologies, it is useful to learn about them and will make understanding the AWS CDK codebase easier but strictly not necessary for simple contributions.

The CDK uses jsii as its primary build system. jsii enables us to write typescript-compliant source code and produce polyglot libraries, such as, in Java, .NET, Python and Go.

The repo contains packages/ directory that contains the CDK public modules. The source code for the IAM module in the CDK can be found at the location packages/@aws-cdk/aws-iam. The repo also contains the tools/ directory that holds custom build tooling (modeled as private npm packages) specific to the CDK.

Build

The full build of the CDK takes a long time to complete; 1-2 hours depending on the performance of the build machine. However, most first time contributions will require changing only one CDK module, sometimes two. A full build of the CDK is not required in these cases.

If you want to work on the @aws-cdk/aws-ec2 module, the following command will build just the EC2 module and any necessary dependencies.

$ cd packages/@aws-cdk/aws-ec2
$ ../../../scripts/buildup

Note: The buildup command is resumable. If your build fails, you can fix the issue and run buildup --resume to resume.

At this point, you can run build and test the aws-ec2 module by running

$ cd packages/@aws-cdk/aws-ec2
$ yarn build
$ yarn test

However, if you wish to build the entire repository, the following command will achieve this.

cd <root of the CDK repo>
scripts/foreach.sh yarn build

Note: The foreach command is resumable by default; you must supply -r or --reset to start a new session.

You are now ready to start contributing to the CDK. See the Pull Requests section on how to make your changes and submit it as a pull request.

Pack

As called out in the above sections, the AWS CDK uses jsii to produce polyglot targets. This means that each CDK module produces artifact in all of its target languages.

Packing involves generating CDK code in the various target languages and packaging them up to be published to their respective package managers. Once in a while, these will need to be generated either to test the experience of a new feature, or reproduce a packaging failure.

To package a specific module, say the @aws-cdk/aws-ec2 module:

$ cd <root-of-cdk-repo>
$ docker run --rm --net=host -it -v $PWD:$PWD -w $PWD jsii/superchain:1-buster-slim
docker$ cd packages/@aws-cdk/aws-ec2
docker$ ../../../scripts/foreach.sh --up yarn run package
docker$ exit

The dist/ folder within each module contains the packaged up language artifacts.

Dev Container

The AWS CDK provides a VS Code Dev Container with all dependencies pre-installed. Please follow the setup instructions to configure VS Code.

With VS Code setup, you will be prompted to open the aws-cdk repo in a Dev Container, or you can choos "Dev Containers: Reopen in Container" from the VS Code command palette.

Gitpod (Alternative)

You may also set up your local development environment using Gitpod - a service that allows you to spin up an in-browser Visual Studio Code-compatible editor, with everything set up and ready to go for CDK development. Just click the button below to create your private workspace:

Open in Gitpod

This will start a new Gitpod workspace, with the CDK repository pre-built. You can now work on your CDK repository, as described in the Getting Started section.

Gitpod is free for 50 hours per month - make sure to stop your workspace when you're done (you can always resume it later, and it won't need to run the build again).

For Gitpod users only! The best way to supply CDK with your AWS credentials is to add them as persisting environment variables. Adding them works as follows via terminal:

eval $(gp env -e AWS_ACCESS_KEY_ID=XXXXXXXXX)
eval $(gp env -e AWS_SECRET_ACCESS_KEY=YYYYYYY)
eval $(gp env -e AWS_DEFAULT_REGION=ZZZZZZZZ)
eval $(gp env -e)

Pull Requests

Below is a flow chart that describes how your PR may be treated by repository maintainers:

graph TD
    A[Incoming PR] -->B[Is an issue attached?]
    B -->|Yes - labels copied from issue| C[Is it labeled P1?]
    B -->|No - auto-labeled as P2| D["Is the effort small?"]
    C -->|Yes - P1| E[Is the PR build succeeding?]
    C -->|No - it is P2| D
    D -->|Yes| E
    D -->|No| F[Can you break down the PR into smaller chunks?]
    F --->|Yes| I[Please do. This will help get traction on your PR.]
    F -->|No| J[Try to garner community support on the issue you are <br/> trying to solve. With 20 +1s, the issue will be relabeled as P1.]
    E --->|Yes| G[We will review your PR as soon as we can]
    E -->|No| H[If the build is failing for more than 4 weeks <br/> without any work on it, we will close the PR.]
Loading

Note that, if we do not have time to review your PR, it is not the end of the road. We are asking for more community support on the attached issue before we focus our attention there. Any P2 issue with 20 or more +1s will be automatically upgraded from P2to P1.

Step 1: Find something to work on

If you want to contribute a specific feature or fix you have in mind, look to see if an issue already exists in our backlog. If not, please contribute a feature request or bug report prior to contributing the PR. We will triage this issue promptly, and the priority of the issue (P1 or P2) will give indication of how much attention your PR may get.

It's not required to submit an issue first, but PRs that come in without attached issues will be automatically labeled as P2.

On the other hand, if you are here looking for an issue to work on, explore our backlog of issues and find something that piques your interest. We have labeled all of our issues for easy searching. If you are looking for your first contribution, the 'good first issue' label will be of help.

It's a good idea to keep the priority of issues in mind when deciding what to work on. If we have labelled an issue as P2, it means it's something we won't get to soon, and we're waiting on more feedback from the community (in the form of +1s and comments) to give it a higher priority. A PR for a P2 issue may be closed by a maintainer, especially if it involves a complex implementation. P1 issues impact a significant number of customers, so we are much more likely to give a PR for those issues prompt attention.

Step 2: Design

In some cases, it is useful to seek feedback by iterating on a design document. This is useful when you plan a big change or feature, or you want advice on what would be the best path forward.

In many cases, the GitHub issue is sufficient for such discussions, and can be sufficient to get clarity on what you plan to do. If the changes are significant or intrusive to the existing CDK experience, and especially for a brand new L2 construct implementation, please write an RFC in our RFC repository before jumping into the code base. L2 construct implementation pull requests will not be reviewed without linking an approved RFC.

Step 3: Work your Magic

Work your magic. Here are some guidelines:

  • Coding style.
    • If your change introduces a new construct, take a look at our design guidelines for construct libraries. We also have an example construct library that showcases a simple construct library with a single construct.
    • We have a number of linters that run during standard build that will enforce coding consistency and correctness. Watch out for their error messages and adjust your code accordingly.
  • Every change requires a unit test
  • If you change APIs, make sure to update the module's README file
    • When you add new examples to the module's README file, you must also ensure they compile - the PR build will fail if they do not. To learn more about how to ensure that they compile, see Documentation.
  • Try to maintain a single feature/bugfix per pull request. It's okay to introduce a little bit of housekeeping changes along the way, but try to avoid conflating multiple features. Eventually, all these are going to go into a single commit, so you can use that to frame your scope.

Integration Tests

Integration tests perform a few functions in the CDK code base -

  1. Acts as a regression detector. It does this by running cdk synth on the integration test and comparing it against the *.snapshot directory. This highlights how a change affects the synthesized stacks.
  2. Allows for a way to verify if the stacks are still valid CloudFormation templates, as part of an intrusive change. This is done by running yarn integ which will run cdk deploy across all of the integration tests in that package. If you are developing a new integration test or for some other reason want to work on a single integration test over and over again without running through all the integration tests you can do so using yarn integ integ.test-name.js Remember to set up AWS credentials before doing this.
  3. (Optionally) Acts as a way to validate that constructs set up the CloudFormation resources as expected. A successful CloudFormation deployment does not mean that the resources are set up correctly.

When are integration tests required?

The following list contains common scenarios where we know that integration tests are required. This is not an exhaustive list and we will, by default, require integration tests for all new features and all fixes unless there is a good reason why one is not needed.

  1. Adding a new feature
  2. Adding a fix to an existing feature
  3. Involves configuring resource types across services (i.e. integrations)
  4. Adding a new supported version (e.g. a new AuroraMysqlEngineVersion)
  5. Adding any functionality via a Custom Resource

All integration tests going forward should use the IntegTest construct. Over time we will be updating all of our existing tests to use this construct. It allows for more control over configuring each tests as well as the ability to perform assertions against the deployed infrastructure.

/*
 * Stack verification steps:
 * * <step-1>
 * * <step-2>
 */

Examples:

What do do if you cannot run integration tests

If you are working on a PR that requires an update to an integration test and you are unable to run the cdk-integ tool to perform a real deployment, please call this out on the pull request so a maintainer can run the tests for you. Please do not run the cdk-integ tool with --dry-run or manually update the snapshot.

See the integration test guide for a more complete guide on running CDK integration tests.

yarn watch (Optional)

We've added a watch feature to the CDK that builds your code as you type it. Start this by running yarn watch for each module that you are modifying.

For example, watch the EC2 and IAM modules in a second terminal session:

$ cd packages/@aws-cdk/aws-ec2
$ yarn watch & # runs in the background
$ cd packages/@aws-cdk/aws-iam
$ yarn watch & # runs in the background

Step 4: Pull Request

  • Create a commit with your changes and push them to a fork.

    Note: CDK core members can push to a branch on the AWS CDK repo (naming convention: <user>/<feature-bug-name>).

  • Create a pull request on Github.

  • Pull request title and message (and PR title and description) must adhere to conventionalcommits.

    • The title must begin with feat(module): title, fix(module): title, refactor(module): title or chore(module): title.
      • feat: indicates a feature added (requires tests and README updates in principle, but can be suppressed)
      • fix: indicates a bug fixes (requires tests in principle, but can be suppressed)
      • docs: indicates updated documentation (docstrings or Markdown files)
      • refactor: indicates a feature-preserving refactoring
      • chore: something without directly visible user benefit (does not end up in the CHANGELOG). Typically used for build scripts, config, or changes so minor they don't warrant showing up the CHANGELOG.
    • Titles for feat and fix PRs end up in the change log. Think about what makes most sense for users reading the changelog while writing them.
      • feat: describe the feature (not the action of creating the commit or PR, for example, avoid words like "added" or "changed")
      • fix: describe the bug (not the solution)
    • Title should be lowercase.
    • No period at the end of the title.

  • Pull request body should describe motivation. Think about your code reviewers and what information they need in order to understand what you did. If it's a big commit (hopefully not), try to provide some good entry points so it will be easier to follow.

    • For bugs, describe bug, root cause, solution, potential alternatives considered but discarded.
    • For features, describe use case, most salient design aspects (especially if new), potential alternatives.

  • Pull request message should indicate which issues are fixed: fixes #<issue> or closes #<issue>.

  • Shout out to collaborators.

  • Call out any new unconventional dependencies that are created as part of your PR.

  • If not obvious (i.e. from unit tests), describe how you verified that your change works.

  • If this PR includes breaking changes, they must be listed at the end in the following format (notice how multiple breaking changes should be formatted):

    BREAKING CHANGE: Description of what broke and how to achieve this behavior now
* **module-name:** Another breaking change
* **module-name:** Yet another breaking change

    Breaking changes are only allowed in experimental libraries. Experimental libraries are published with an -alpha suffix, and have the stability property set to experimental in their package.json.

  • Once the pull request is submitted, a reviewer will be assigned by the maintainers.

  • If the PR build is failing, update the PR with fixes until the build succeeds. You may have trouble getting attention from maintainers if your build is failing, and after 4 weeks of staleness, your PR will be automatically closed.

  • Discuss review comments and iterate until you get at least one "Approve". When iterating, push new commits to the same branch. Usually all these are going to be squashed when you merge to main. The commit messages should be hints for you when you finalize your merge commit message.

  • Make sure to update the PR title/description if things change. The PR title/description are going to be used as the commit title/message and will appear in the CHANGELOG, so maintain them all the way throughout the process.

Adding new unconventional dependencies

For the aws-cdk an unconventional dependency is defined as any dependency that is not managed via the module's package.json file.

Sometimes constructs introduce new unconventional dependencies. Any new unconventional dependency that is introduced needs to have an auto upgrade process in place. The recommended way to update dependencies is through dependabot. You can find the dependabot config file here.

An example of this is the @aws-cdk/lambda-layer-awscli module. This module creates a lambda layer that bundles the AWS CLI. This is considered an unconventional dependency because the AWS CLI is bundled into the CDK as part of the build, and the version of the AWS CLI that is bundled is not managed by the package.json file.

In order to automatically update the version of the AWS CLI, a custom build process was created that takes upgrades into consideration. You can take a look at the files in packages/@aws-cdk/lambda-layer-awscli/layer to see how the build works, but at a high level a requirements.txt file was created to manage the version. This file was then added to dependabot.yml so that dependabot will automatically upgrade the version as new versions are released.

If you think your PR introduces a new unconventional dependency, make sure to call it out in the description so that we can discuss the best way to manage that dependency.

Step 5: Merge

  • Make sure your PR builds successfully (we have CodeBuild setup to automatically build all PRs).
  • Once approved and tested, one of our bots will squash-merge to main and will use your PR title/description as the commit message.

Breaking Changes

NOTE: Starting with version 2.0.0 of the AWS CDK, all modules and members vended as part of the main CDK library (aws-cdk-lib) will always be stable; we are committing to never introduce breaking changes in a non-major bump. Breaking changes are only allowed on pre-released (experimental or dev preview) modules (those with a stability of experimental in their respective package.json files). For v1, each module is separately released. For v2, only stable modules are released as part of the main aws-cdk-lib release, and all experimental modules are released independently as -alpha versions, and not included in the main CDK library.

Whenever you are making changes, there is a chance for those changes to be breaking existing users of the library. A change is breaking if there are programs that customers could have been writing against the current version of the CDK, that will no longer "work correctly" with the proposed new version of the CDK.

Breaking changes are not allowed in stable libraries. They are permitted in experimental libraries, unless the maintainer of the module decides that it should be avoided. Breaking changes require explicit callouts in the bodies of Pull Requests that introduce them.

Breaking changes come in two flavors:

  • API surface changes
  • Behavior changes

API surface changes

This encompasses any changes that affect the shape of the API. Changes that will make existing programs fail to compile are not allowed. Typical examples of that are:

  • Renaming classes or methods
  • Adding required properties to a struct that is used as an input to a constructor or method. This also includes changing a type from nullable to non-nullable.
  • Removing properties from a struct that is returned from a method, or removing properties from a class. This also includes changing a type from non-nullable to nullable.

To see why the latter is a problem, consider the following class:

class SomeClass {
  public readonly count: number;
  //               ❓ let's say I want to change this to 'count?: number',
  //                  i.e. make it optional.
}

// Someone could have written the following code:
const obj = new SomeClass();
console.log(obj.count + 1);

// After the proposed change, this code that used to compile fine will now throw:
console.log(obj.count + 1);
//          ~~~~~~~~~ Error: Object is possibly 'undefined'.

CDK comes with build tooling to check whether changes you made introduce breaking changes to the API surface. In a package directory, run:

$ yarn build
$ yarn compat

The only case where it is legitimate to break a public API is if the existing API is a bug that blocked the usage of a feature. This means that by breaking this API we will not break anyone, because they weren't able to use it. The file allowed-breaking-changes.txt in the root of the repo is an exclusion file that can be used in these cases.

Dealing with breaking API surface changes

If you need to change the type of some API element, introduce a new API element and mark the old API element as @deprecated.

If you need to pretend to have a value for the purposes of implementing an API and you don't actually have a useful value to return, it is acceptable to make the property a getter and throw an exception (keeping in mind to write error messages that will be useful to a user of your construct):

class SomeClass implements ICountable {
  constructor(private readonly _count?: number) {
  }

  public get count(): number {
    if (this._count === undefined) {
      // ✅ DO: throw a descriptive error that tells the user what to do
      throw new Error('This operation requires that a \'count\' is specified when SomeClass is created.');
      // ❌ DO NOT: just throw an error like 'count is missing'
    }
    return this._count;
  }
}

Behavior changes

These are changes that do not directly affect the compilation of programs written against the previous API, but may change their meaning. In practice, even though the user didn't change their code, the CloudFormation template that gets synthesized is now different.

Not all template changes are breaking changes! Consider a user that has created a Stack using the previous version of the library, has updated their version of the CDK library and is now deploying an update. A behavior change is breaking if:

  • The update cannot be applied at all
  • The update can be applied but causes service interruption or data loss.

Data loss happens when the Logical ID of a stateful resource changes, or one of the resource properties that requires replacement is modified. In both of these cases, CloudFormation will delete the resource, and if it was a stateful resource like a database the data in it is now gone.

If a change applies cleanly and does not cause any service interruption, it is not breaking. Nevertheless, it might still be wise to avoid those kinds of changes as users are understandably wary of unexpected template changes, will scrutinize them heavily, and we don't want to cause unnecessary panic and churn in our use base.

Determining whether or not behavioral changes are breaking requires expertise and judgement on the part of the library owner, and testing.

Dealing with breaking behavior changes

Most of the time, behavioral changes will arise because we want to change the default value or default behavior of some property (i.e., we want to change the interpretation of what it means if the value is missing).

If the new behavior is going to be breaking, the user must opt in to it, either by:

  • Adding a new API element (class, property, method, ...) to have users explicitly opt in to the new behavior at the source code level (potentially @deprecateing the old API element); or
  • Use the feature flag mechanism to have the user opt in to the new behavior without changing the source code.

Of these two, the first one is preferred if possible (as feature flags have non-local effects which can cause unintended effects).

Adding new experimental ("preview") APIs

To make sure we can keep adding features fast, while keeping our commitment to not release breaking changes, we are introducing a new model - API Previews. APIs that we want to get in front of developers early, and are not yet finalized, will be added to the AWS CDK with a specific suffix: BetaX. APIs with the preview suffix will never be removed, instead they will be deprecated and replaced by either the stable version (without the suffix), or by a newer preview version. For example, assume we add the method grantAwesomePowerBeta1:

/**
 * This methods grants awesome powers
 */
grantAwesomePowerBeta1();

Times goes by, we get feedback that this method will actually be much better if it accepts a Principal. Since adding a required property is a breaking change, we will add grantAwesomePowerBeta2() and deprecate grantAwesomePowerBeta1:

/**
* This methods grants awesome powers to the given principal
*
* @param grantee The principal to grant powers to
*/
grantAwesomePowerBeta2(grantee: iam.IGrantable)

/**
* This methods grants awesome powers
* @deprecated use grantAwesomePowerBeta2
*/
grantAwesomePowerBeta1()

When we decide its time to graduate the API, the latest preview version will be deprecated and the final version - grantAwesomePower will be added.

Documentation

Every module's README is rendered as the landing page of the official documentation. For example, this is the README for the aws-ec2 module - https://docs.aws.amazon.com/cdk/api/latest/docs/aws-ec2-readme.html.

Rosetta

The README file contains code snippets written as typescript code. Code snippets typed in fenced code blocks (such as ```ts) will be automatically extracted, compiled and translated to other languages when the during the pack step. We call this feature 'rosetta'.

You can run rosetta on the EC2 module (or any other module) by running:

$ cd packages/@aws-cdk/aws-ec2
$ yarn rosetta:extract --strict

To successfully do that, they must be compilable. The easiest way to do that is using a fixture, which looks like this:

```ts fixture=with-bucket
bucket.addLifecycleTransition({ ...props });
```

While processing the examples, the tool will look for a file called rosetta/with-bucket.ts-fixture in the package directory. This file will be treated as a regular TypeScript source file, but it must also contain the text /// here, at which point the example will be inserted. The complete file must compile properly.

Before the /// here marker, the fixture should import the necessary packages and initialize the required variables.

When no fixture is specified, the fixture with the name rosetta/default.ts-fixture will be used if present. nofixture can be used to opt out of that behavior.

In an @example block, which is unfenced, additional information pertaining to the example can be provided via the @exampleMetadata tag:

/**
 * @exampleMetadata fixture=with-bucket
 * @example
 *   /// fixture=with-bucket
 *   bucket.addLifecycleTransition({ ...props });
 */

For a practical example of how making sample code compilable works, see the aws-ec2 package.

⚠️ NOTE: README files often contain code snippets that refer to modules that are consumers of the current module, and hence not present in the current module's dependency closure. Compilation of these snippets will fail if the module referenced has not been built. For the best experience when working on snippets, a full build of the CDK repo is required. However, it may be prudent to "build up" these modules as required.

Recommendations

In order to offer a consistent documentation style throughout the AWS CDK codebase, example code should follow the following recommendations (there may be cases where some of those do not apply - good judgement is to be applied):

  • Types from the documented module should be un-qualified:

    // An example in the @aws-cdk/core library, which defines Duration
Duration.minutes(15);

  • Types from other modules should be qualified:

    // An example in the @aws-cdk/core library, using something from @aws-cdk/aws-s3
const bucket = new s3.Bucket(this, 'Bucket');
// ...rest of the example...

  • Make use of declare statements directly in examples for values that are necessary for compilation but unimportant to the example:

    // An example about adding a stage to a pipeline in the @aws-cdk/pipelines library
declare const pipeline: pipelines.CodePipeline;
declare const myStage: Stage;
pipeline.addStage(myStage);

  • Utilize the default.ts-fixture that already exists rather than writing new .ts-fixture files. This is because values stored in .ts-fixture files do not surface to the examples visible in the docs, so while they help successful compilation, they do not help users understand the example.

Tools (Advanced)

scripts/foreach.sh

This wonderful tool allows you to execute a command for all modules in this repo in topological order, but has the incredible property of being stateful. This means that if a command fails, you can fix the issue and resume from where you left off.

To start a session, run:

$ scripts/foreach.sh COMMAND

This will execute "COMMAND" for each module in the repo (cwd will be the directory of the module). If a task fails, it will stop. To resume, simply run foreach.sh again (with or without the same command).

To reset the session (either when all tasks finished or if you wish to run a different session), run:

$ scripts/foreach.sh --reset

If you wish to run a command only against a module's dependency closure, use:

$ cd packages/my-module
$ ../scripts/foreach.sh --up COMMAND

This will execute COMMAND against my-module and all its deps (in a topological order, of course).

Consequently, there are two useful scripts that are built on top of foreach.sh, and lets you build modules.

  • scripts/buildup: builds the current module and all of its dependencies (in topological order).
  • scripts/builddown: builds the current module and all of its consumers (in topological order).

Linters

All linters are executed automatically as part of the build script, yarn build.

They can also be executed independently of the build script. From the root of a specific package (e.g. packages/@aws-cdk/aws-ec2), run the following command to execute all the linters on that package -

yarn lint

The following linters are used:

eslint

All packages in the repo use a standard base configuration found at eslintrc.js. This can be customized for any package by modifying the .eslintrc file found at its root.

If you're using the VS Code and would like to see eslint violations on it, install the eslint extension.

pkglint

The pkglint tool "lints" package.json files across the repo according to rules.ts.

To evaluate (and attempt to fix) all package linting issues in the repo, run the following command from the root of the repository (after bootstrapping):

$ lerna run pkglint

You can also do that per package:

$ lr pkglint

awslint

awslint is a linter for the AWS Construct Library APIs. It is executed as a part of the build of all AWS modules in the project and enforces the AWS Construct Library Design Guidelines.

For more information about this tool, see the awslint README.

Generally speaking, if you make any changes which violate an awslint rule, build will fail with appropriate messages. All rules are documented and explained in the guidelines.

Here are a few useful commands:

  • yarn awslint in every module will run awslint for that module.
  • yarn awslint list prints all rules (details and rationale in the guidelines doc).
  • scripts/foreach.sh yarn awslint will start linting the entire repo, progressively. Rerun scripts/foreach.sh after fixing to continue.
  • lerna run awslint --no-bail --stream 2> awslint.txt will run awslint in all modules and collect all results into awslint.txt
  • lerna run awslint -- -i <RULE> will run awslint throughout the repo and evaluate only the rule specified awslint README for details on include/exclude rule patterns.

cfn2ts

This tool is used to generate our low-level CloudFormation resources (L1/CfnFoo). It is executed as part of the build step of all modules in the AWS Construct Library.

The tool consults the cdk-build.cloudformation key in package.json to determine which CloudFormation namespace this library represents (e.g. AWS::EC2 is the namespace for aws-ec2). We maintain strict 1:1 relationship between those.

Each module also has an npm script called cfn2ts:

  • yarn cfn2ts: generates L1 for a specific module
  • lerna run cfn2ts: generates L1 for the entire repo

Jetbrains support (WebStorm/IntelliJ)

This project uses lerna and utilizes symlinks inside nested node_modules directories. You may encounter an issue during indexing where the IDE attempts to index these directories and keeps following links until the process runs out of available memory and crashes. To fix this, you can run node ./scripts/jetbrains-remove-node-modules.js to exclude these directories.

Linking against this repository

If you are developing your own CDK application or library and want to use the locally checked out version of the AWS CDK, instead of the version of npm, the ./link-all.sh script will help here.

This script symlinks the built modules from the local AWS CDK repo under the node_modules/ folder of the CDK app or library.

$ cd <your own CDK app>
$ <path to the AWS CDK repo>/link-all.sh

Running integration tests in parallel

See the Integration testing guide

Visualizing dependencies in a CloudFormation Template

Use GraphViz with template-deps-to-dot:

$ cdk -a some.app.js synth | $awscdk/scripts/template-deps-to-dot | dot -Tpng > deps.png

Find dependency cycles between packages

You can use find-cycles to print a list of internal dependency cycles:

$ scripts/find-cycles.sh
Cycle: @aws-cdk/aws-iam => @aws-cdk/assert => aws-cdk => @aws-cdk/aws-s3 => @aws-cdk/aws-kms => @aws-cdk/aws-iam
Cycle: @aws-cdk/assert => aws-cdk => @aws-cdk/aws-s3 => @aws-cdk/aws-kms => @aws-cdk/assert
Cycle: @aws-cdk/aws-iam => @aws-cdk/assert => aws-cdk => @aws-cdk/aws-s3 => @aws-cdk/aws-iam
Cycle: @aws-cdk/assert => aws-cdk => @aws-cdk/aws-s3 => @aws-cdk/assert
Cycle: @aws-cdk/assert => aws-cdk => @aws-cdk/aws-cloudformation => @aws-cdk/assert
Cycle: @aws-cdk/aws-iam => @aws-cdk/assert => aws-cdk => @aws-cdk/util => @aws-cdk/aws-iam
Cycle: @aws-cdk/aws-sns => @aws-cdk/aws-lambda => @aws-cdk/aws-codecommit => @aws-cdk/aws-sns
Cycle: @aws-cdk/aws-sns => @aws-cdk/aws-lambda => @aws-cdk/aws-codecommit => @aws-cdk/aws-codepipeline => @aws-cdk/aws-sns

Running CLI integration tests

The CLI package (packages/aws-cdk) has some integration tests that aren't run as part of the regular build, since they have some particular requirements. See the CLI CONTRIBUTING.md file for more information on running those tests.

Building aws-cdk-lib

In AWS CDK v2, all stable libraries are packaged into a single monolithic package and published as aws-cdk-lib. In most cases, you can iterate on a single module's directory as previously described in this document (e.g. packages/@aws-cdk/aws-s3). In some cases, you might need to build aws-cdk-lib:

# Generate all of the L1s first. If you have already done a full build in the repository, you can skip this.
cd <CDK repo root>/
./scripts/gen.sh

# Generate and build `aws-cdk-lib`
cd packages/aws-cdk-lib
yarn build

The commands above perform the following steps:

  1. Run yarn install to install all dependencies
  2. Generate .generated.ts files in each packages/@aws-cdk/aws-<service> directory. These files contain TypeScript source code for all of the L1 (Cfn) Constructs, and are generated from the CloudFormation Resource Specification.
  3. Copy the .ts source code files from each packages/@aws-cdk/aws-<service> directory to the corresponding packages/aws-cdk-lib/aws-<service> directory.
  4. Compile aws-cdk-lib.

Running unit tests and integration tests still has to be performed in each module's packages/@aws-cdk directory.

Building and testing v2 -alpha packages

In AWS CDK v2, all experimental libraries are published separately with an -alpha suffix. In most cases, you can iterate on a single module's directory as already described in this document (e.g. packages/@aws-cdk/aws-amplify). If you need to generate and iterate on the alpha package, here are the steps. The main differences between the alpha package is naming of the package, and import statements.

First, make sure the following packages are built:

  • packages/@aws-cdk/assert
  • packages/aws-cdk-lib
  • tools/individual-pkg-gen

The following command will create all of the alpha packages by copying files from their source directories under packages/@aws-cdk/aws-<service>, and it will build and run unit tests for all of them. This is sometimes too much for a developer machine or laptop.

<CDK repo root>/scripts/transform.sh

To only copy and transform the source files, and then build and test one alpha package at a time, use the following:

<CDK repo root>/scripts/transform.sh --skip-build
cd packages/individual-packages/aws-<service>
yarn build+test

Changing Cloud Assembly Schema

If you plan on making changes to the cloud-assembly-schema package, make sure you familiarize yourself with its own contribution guide

Feature Flags

Sometimes we want to introduce new breaking behavior because we believe this is the correct default behavior for the CDK. The problem of course is that breaking changes are only allowed in major versions and those are rare.

To address this need, we have a feature flags pattern/mechanism. It allows us to introduce new breaking behavior which is disabled by default (so existing projects will not be affected) but enabled automatically for new projects created through cdk init. Existing users can selectively opt in to new behavior on their own schedule.

Whenever a change leads to CloudFormation template differences that cause any of the following during an update, it is not safe to apply the new behavior automatically, and we have to use a feature flag:

  • Resources replacement leading to service disruption; or
  • Users could have taken assumptions on the old setup and the change will break them.

Adding a new flag looks as follows:

  1. Define a new const under cx-api/lib/features.ts with the name of the context key that enables this new feature (for example, ENABLE_STACK_NAME_DUPLICATES). The context key should be in the form module.Type:feature (e.g. @aws-cdk/core:enableStackNameDuplicates).

  2. Use FeatureFlags.of(construct).isEnabled(cxapi.ENABLE_XXX) to check if this feature is enabled in your code. If it is not defined, revert to the legacy behavior.

  3. Add your feature flag to the FLAGS map in cx-api/lib/features.ts. In your description, be sure to cover the following:

    • Consciously pick the type of feature flag. Can the flag be removed in a future major version, or not?
    • Motivate why the feature flag exists. What is the change to existing infrastructure and why is it not safe?
    • In case of a "default change flag", describe what the user needs to do to restore the old behavior.

  4. Add an entry for your feature flag in the README file.

  5. In your tests, ensure that you test your feature with and without the feature flag enabled. You can do this by passing the feature flag to the context property when instantiating an App.

    const myFeatureFlag = { [cxapi.MY_FEATURE_FLAG]: true };
const app = new App({
   context: myFeatureFlag,
}),
const stackUnderTest = new Stack(app);

  6. In your PR title (which goes into CHANGELOG), add a (under feature flag) suffix. e.g:

    fix(core): impossible to use the same physical stack name for two stacks (under feature flag)

Versioning and Release

See release.md for details on how CDK versions are maintained and how to trigger a new release

Troubleshooting

Most build issues can be solved by doing a full clean rebuild:

$ git clean -fqdx .
$ yarn build

However, this will be time consuming. In this section we'll describe some common issues you may encounter and some more targeted commands you can run to resolve your issue.

The compiler is throwing errors on files that I renamed/it's running old tests that I meant to remove/code coverage is low and I didn't change anything.

If you switch to a branch in which .ts files got renamed or deleted, the generated .js and .d.ts files from the previous compilation run are still around and may in some cases still be picked up by the compiler or test runners.

Run the following to clear out stale build artifacts:

$ scripts/clean-stale-files.sh

I added a dependency but it's not being picked up by the build

You need to tell Lerna to update all dependencies:

$ node_modules/.bin/lerna bootstrap

I added a dependency but it's not being picked up by a watch background compilation run.

No it's not. After re-bootstrapping you need to restart the watch command.

I added a dependency but it's not being picked up by Visual Studio Code (I still get red underlines).

The TypeScript compiler that's running has cached your dependency tree. After re-bootstrapping, restart the TypeScript compiler.

Hit F1, type > TypeScript: Restart TS Server.

I'm doing refactorings between packages and compile times are killing me/I need to switch between differently-verionsed branches a lot and rebuilds because of version errors are taking too long.

Our build steps for each package do a couple of things, such as generating code and generating JSII assemblies. If you've done a full build at least once to generate all source files, you can do a quicker TypeScript-only rebuild of the entire source tree by doing the following:

# Only works after at least one full build to generate source files
$ scripts/build-typescript.sh

# Also works to start a project-wide watch compile
$ scripts/build-typescript.sh -w

This does not do code generation and it does not do JSII checks and JSII assembly generation. Instead of doing a package-by-package ordered build, it compiles all .ts files in the repository all at once. This takes about the same time as it does to compile the biggest package all by itself, and on my machine is the difference between a 15 CPU-minute build and a 20 CPU-second build. If you use this methods of recompiling and you want to run the test, you have to disable the built-in rebuild functionality of lerna run test:

$ CDK_TEST_BUILD=false lr test

Debugging

Connecting the VS Code Debugger

Note: This applies to typescript CDK application only.

To debug your CDK application along with the CDK repository,

  1. Clone the CDK repository locally and build the repository. See Workflows section for the different build options.

  2. Build the CDK application using the appropriate npm script (typically, yarn build) and then run the link-all.sh script as follows:

    cd /path/to/cdk/app
/path/to/aws-cdk/link-all.sh

  3. Open the CDK application (assume it's hello-cdk in these steps) and the CDK repository as a VS code multi-root workspace.

  4. Open the workspace settings file and verify that the following two folders must already exist

{
  "folders": [
    { "path": "<path-to-cdk-repo>/aws-cdk" },
    { "path": "<path-to-cdk-app>/hello-cdk" }
  ],
}
  1. Add the following launch configuration to the settings file -
"launch": {
  "configurations": [{
    "type": "node",
    "request": "launch",
    "name": "Debug hello-cdk",
    "program": "${workspaceFolder:hello-cdk}/bin/hello-cdk.js",
    "cwd": "${workspaceFolder:hello-cdk}",
    "console": "internalConsole",
    "sourceMaps": true,
    "skipFiles": [ "<node_internals>/**/*" ],
    "outFiles": [
      "${workspaceFolder:aws-cdk}/**/*.js",
      "${workspaceFolder:hello-cdk}/**/*.js",
    ],
  }]
}

NOTE: Go here for more about launch configurations.

  1. The debug view, should now have a launch configuration called 'Debug hello-cdk' and launching that will start the debugger.
  2. Any time you modify the CDK app or any of the CDK modules, they need to be re-built and depending on the change the link-all.sh script from step#2, may need to be re-run. Only then, would VS code recognize the change and potentially the breakpoint.

Run a CDK unit test in the debugger

If you want to run the VSCode debugger on unit tests of the CDK project itself, do the following:

  1. Set a breakpoint inside your unit test.
  2. In your terminal, depending on the type of test, run either:
# (For tests names test.xxx.ts)
$ node --inspect-brk /path/to/aws-cdk/node_modules/.bin/nodeunit -t 'TESTNAME'

# (For tests names xxxx.test.ts)
$ node --inspect-brk /path/to/aws-cdk/node_modules/.bin/jest -i -t 'TESTNAME'
  1. On the Run pane of VSCode, select the run configuration Attach to NodeJS and click the button.

Badges (Pilot Program)

CDK Merit Badges is a Pilot Program. The badges you get are experimental and may change.

CDK Merit Badges is a program aimed at enhancing the CDK contributor experience. When you submit new pull requests to the CDK repository, you will receive a merit badge that reflects how many prior successful contributions you have to the repository. Right now, these badges are just for fun and are meant as a small incentive to continued contributions to the CDK.

The badges have the following meaning:

  • beginning-contributor: contributed between 0-2 PRs to the CDK
  • repeat-contributor: contributed between 3-5 PRs to the CDK
  • valued-contributor: contributed between 6-12 PRs to the CDK
  • admired-contributor: contributed between 13-24 PRs to the CDK
  • star-contributor: contributed between 25-49 PRs to the CDK
  • distinguished-contributor: contributed 50+ PRs to the CDK

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