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Contributing to the AWS Cloud Development Kit

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

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

Getting Started

For day-to-day development and normal contributions, the following SDKs and tools are required:

The basic commands to get the repository cloned and built locally follow:

$ git clone https://github.com/aws/aws-cdk.git
$ cd aws-cdk
$ yarn install
$ yarn build

Alternatively, the Full Docker build workflow can be used so that you don't have to worry about installing all those tools on your local machine and instead only depend on having a working Docker install.

Pull Requests

Pull Request Checklist

  • Testing
    • Unit test added (prefer not to modify an existing test, otherwise, it's probably a breaking change)
    • CLI change?: coordinate update of integration tests with team
    • cdk-init template change?: coordinated update of integration tests with team
  • Docs
    • jsdocs: All public APIs documented
    • README: README and/or documentation topic updated
    • Design: For significant features, design document added to design folder
  • Title and Description
    • Change type: title prefixed with fix, feat and module name in parens, which will appear in changelog
    • Title: use lower-case and doesn't end with a period
    • Breaking?: last paragraph: "BREAKING CHANGE: <describe what changed + link for details>"
    • Issues: Indicate issues fixed via: "Fixes #xxx" or "Closes #xxx"
  • Sensitive Modules (requires 2 PR approvers)
    • IAM Policy Document (in @aws-cdk/aws-iam)
    • EC2 Security Groups and ACLs (in @aws-cdk/aws-ec2)
    • Grant APIs (only if not based on official documentation with a reference)

Step 1: Open Issue

If there isn't one already, open an issue describing what you intend to contribute. It's useful to communicate in advance, because sometimes, someone is already working in this space, so maybe it's worth collaborating with them instead of duplicating the efforts.

Step 2: Design (optional)

In some cases, it is useful to seek for 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.

Sometimes, the GitHub issue is sufficient for such discussions, and can be sufficient to get clarity on what you plan to do. Sometimes, a design document would work better, so people can provide iterative feedback.

In such cases, use the GitHub issue description to collect requirements and use cases for your feature.

Then, create a design document in markdown format under the design/ directory and request feedback through a pull request. Prefix the PR title with "RFC:" (request for comments).

Once the design is finalized, you can re-purpose this PR for the implementation, or open a new PR to that end.

Step 3: Work your Magic

Work your magic. Here are some guidelines:

  • Coding style (abbreviated):
    • In general, follow the style of the code around you
    • 2 space indentation
    • 120 characters wide
    • ATX style headings in markdown (e.g. ## H2 heading)
  • Every change requires a unit test
  • If you change APIs, make sure to update the module's README file
  • 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 *.expected.json file. 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. 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.

If you are working on a new feature that is using previously unused CloudFormation resource types, or involves configuring resource types across services, you need to write integration tests that use these resource types or features.

To the extent possible, include a section (like below) in the integration test file that specifies how the successfully deployed stack can be verified for correctness. Correctness here implies that the resources have been set up correctly. The steps here are usually AWS CLI commands but they need not be.

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

Examples:

Step 4: Commit

Create a commit with the proposed change changes:

  • Commit 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.
    • Title should be lowercase.
    • No period at the end of the title.

  • Commit message 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.

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

  • Shout out to collaborators.

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

  • If this commit 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

Step 5: Pull Request

  • Push to a GitHub fork or to a branch (naming convention: <user>/<feature-bug-name>)
  • Submit a Pull Requests on GitHub and assign the PR for a review to the "awslabs/aws-cdk" team.
  • Please follow the PR checklist written below. We trust our contributors to self-check, and this helps that process!
  • 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 master. 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.

Step 6: Merge

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

Tools

The CDK is a big project, and, at the moment, all of the CDK modules are mastered in a single monolithic repository (uses lerna). There are pros and cons to this approach, and it's especially valuable to maintain integrity in the early stage of the project where things constantly change across the stack. In the future we believe many of these modules will be extracted to their own repositories.

Another complexity is that the CDK is packaged using jsii to multiple programming languages. This means that when a full build is complete, there will be a version of each module for each supported language.

However, in many cases, you can probably get away with just building a portion of the project, based on areas that you want to work on.

We recommend that you use Visual Studio Code to work on the CDK. Be sure to install the tslint extension for it as well, since we have strict linting rules that will prevent your code from compiling, but with VSCode and this extension can be automatically fixed for you by hitting Ctrl-. when your cursor is on a red underline.

Main build scripts

The build process is divided into stages, so you can invoke them as needed from the root of the repo:

  • yarn build: runs the build and test commands in all modules (in topological order).
  • yarn pack: packages all modules to all supported languages and produces a dist/ directory with all the outputs (running this script requires that you installed the toolchains for all target languages on your system).

Partial build tools

There are also two useful scripts in the scripts directory that can help you build part of the repo:

  • 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).

Useful aliases

You can also add a few useful aliases to your shell profile:

# runs an npm script via lerna for a the current module
alias lr='lerna run --stream --scope $(node -p "require(\"./package.json\").name")'

# runs "yarn build" (build + test) for the current module
alias lb='lr build'
alias lt='lr test'

# runs "yarn watch" for the current module (recommended to run in a separate terminal session):
alias lw='lr watch'

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

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, and then 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 it's deps (in a topological order of course).

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.

Workflows

This section includes step-by-step descriptions of common workflows.

Full clean build

Clone the repo:

$ git clone https://github.com/aws/aws-cdk.git
$ cd aws-cdk

If you already have a local repo and you want a fresh build, run git clean -fdx from the root.

Install and build:

$ ./install.sh
$ yarn build

If you also wish to package to all languages, make sure you have all the toolchains and now run:

$ ./pack.sh

NOTE: in local builds, pack.sh will finish but will fail with an error indicating the build artifacts use the marker version (0.0.0). This is normal, and you can trust the output in dist/ despite the failure. This is a protection we have to make sure we don't accidentally release artifacts with the marker version.

Full Docker build

Clone the repo:

$ git clone https://github.com/aws/aws-cdk.git
$ cd aws-cdk

If you already have a local repo and you want a fresh build, run git clean -fdx from the root.

Build the docker image:

$ docker build -t aws-cdk .

This allows you to run the CDK in a CDK-compatible directory with a command like:

$ docker run -v $(pwd):/app -w /app aws-cdk <CDK ARGS>

Partial build

In many cases, you don't really need to build the entire project. Say you want to work on the @aws-cdk/aws-ec2 module:

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

Note that buildup uses foreach.sh, which means it's resumable. If your build fails and you wish to resume, just run buildup --resume. If you wish to restart, run buildup again.

Quick Iteration

After you've built the modules you want to work on once, use lr watch for each module that you are modifying.

Watch the EC2 and IAM modules in a second terminal session:

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

Code...

Now to test, you can either use lr test or invoke nodeunit directory (faster, since "test" will also build):

$ cd packages/@aws-cdk/aws-iam
$ nodeunit test/test.*.js
<BOOM>

Linking against this repository

The script ./link-all.sh can be used to generate symlinks to all modules in this repository under some node_module directory. This can be used to develop against this repo as a local dependency.

One can use the postinstall script to symlink this repo:

{
  "scripts": {
    "postinstall": "../aws-cdk/link-all.sh"
  }
}

This assumes this repo is a sibling of the target repo and will install the CDK as a linked dependency during yarn install.

Running integration tests in parallel

Integration tests may take a long time to complete. We can speed this up by running them in parallel in different regions.

# Install GNU parallel (may require uninstall 'moreutils' if you have it)
$ apt-get install parallel
$ brew install parallel

$ scripts/run-integ-parallel @aws-cdk/aws-ec2 @aws-cdk/aws-autoscaling ...

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

Adding Dependencies

The root package.json includes global devDependencies (see lerna docs) on the topic.

  • To add a global dependency, run yarn add <dep> --dev at the root.
  • To add a dependency for a specific module, run yarn add <dep> inside the module's directory.

Guidelines:

  • We cannot accept dependencies that use non-permissive open source licenses (Apache, MIT, etc).
  • Make sure dependencies are defined using caret ranges (e.g. ^1.2.3). This enables non-breaking updates to automatically be picked up.
  • Make sure yarn.lock is included in your commit.

Finding 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

Updating all Dependencies

To update all dependencies (without bumping major versions):

  1. Obtain a fresh clone from "master".
  2. Run yarn install
  3. Run ./scripts/update-dependencies.sh --mode full (use --mode semver to avoid bumping major versions)
  4. Submit a Pull Request.

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.

API Compatibility Checks

All stable APIs in the CDK go through a compatibility check during build using the jsii-diff tool. This tool downloads the latest released version from npm and verifies that the APIs in the current build have not changed in a breaking way.

Compatibility checks always run as part of a full build (yarn build).

You can use yarn compat to run compatibility checks for all modules:

(working directory is repo root)
$ yarn build
$ yarn compat

You can also run compat from individual package directories:

$ cd packages/@aws-cdk/aws-sns
$ 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.

Examples

Examples typed in fenced code blocks (looking like '''ts, but then with backticks instead of regular quotes) will be automatically extrated, compiled and translated to other languages when the bindings are generated.

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({ ... });
'''

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.

If 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, the first line of the example can contain three slashes to achieve the same effect:

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

When including packages in your examples (even the package you're writing the examples for), use the full package name (e.g. import s3 = require('@aws-cdk/aws-s3);). The example will be compiled in an environment where all CDK packages are available using their public names. In this way, it's also possible to import packages that are not in the dependency set of the current package.

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

Examples of all packages are extracted and compiled as part of the packaging step. If you are working on getting rid of example compilation errors of a single package, you can run scripts/compile-samples on the package by itself.

For now, non-compiling examples will not yet block the build, but at some point in the future they will.

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.

The pattern is simple:

  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 node.tryGetContext(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 FUTURE_FLAGS map in cx-api/lib/features.ts. This map is inserted to generated cdk.json files for new projects created through cdk init.

  4. 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)

  5. Under BREAKING CHANGES in your commit message describe this new behavior:

    BREAKING CHANGE: template file names for new projects created through "cdk init"
will use the template artifact ID instead of the physical stack name to enable
multiple stacks to use the same name. This is enabled through the flag
`@aws-cdk/core:enableStackNameDuplicates` in newly generated `cdk.json` files.

In the next major version of the CDK we will either remove the legacy behavior or flip the logic for all these features and then reset the FEATURE_FLAGS map for the next cycle.

Versioning

All package.json files in this repo use a stable marker version of 0.0.0. This means that when you declare dependencies, you should always use 0.0.0. This makes it easier for us to bump a new version (the bump.sh script will just update the central version and create a CHANGELOG entry) and also reduces the chance of merge conflicts after a new version is released.

Additional scripts that take part in the versioning mechanism:

  • scripts/get-version.js can be used to obtain the actual version of the repo. You can use either from JavaScript code by require('./scripts/get-version') or from a shell script node -p "require('./scripts/get-version')".
  • scripts/get-version-marker.js returns 0.0.0 and used to DRY the version marker.
  • scripts/align-version.sh and scripts/align-version.js are used to align all package.json files in the repo to the official version. This script is invoked in CI builds and should not be used inside a development environment.

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 so -

    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",
    ],
  }]
}

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.

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