CDK snapshot

Create a snapshot friendly version of a CDK (both v1 and v2) Cloud Assembly that can be commited as part of the source code. The goal is to have better control over structural stack changes before it is merged.

This can help automate the process of keeping CDK up-to-date while manually approving structural stack changes. It also helps reviewing code changes since the affected CloudFormation stack changes will be reviewable as well.

Usage

npm install --save-dev @henrist/cdk-snapshot rimraf

Add run-script to package.json:

{
  "scripts": {
    "cdk-snapshot": "rimraf cdk.out __snapshots__ && IS_SNAPSHOT=true npm run cdk -- synth && cdk-snapshot"
  }
}

Setting IS_SNAPSHOT means we can add conditionals in the code to stub some parts of the templates during snapshot mode. It is not needed for normal use.

Updated snapshot can now be created by running the script:

npm run cdk-snapshot

By default the cdk-snapshot script will take what it finds in cdk.out directory and transform it to a snapshot friendly version that it stores in __snapshots__.

Asserting snapshot is up-to-date

In your CI/CD-pipeline you can add:

npm run cdk-snapshot
git status
git diff --exit-code

This will cause the build to fail if the snapshot is not up-to-date.

In depth

A snapshot friendly version means an opinionated transformation of the Cloud Assembly so that "innocent" changes does not always invalidate the snapshot, to keep the maintenance burdon down.

The following is performed:

• The cdk.out file is removed. This only contains the version of the Cloud Manifest, which we think is not neccessary to keep track of.
• The tree.json file is removed. We think this does not contribute to the goal.
• The asset directories is removed. Asset changes are not kept track of, and should generally not invalidate the snapshot.
• The manifest.json file is modified to remove the Cloud Assemby version, runtime version informasjon (CDK library versions), trace output and asset details.
• From every template *.template.json the asset details is removed.

Collapsing changes in PR by default

To make GitHub not show preview of changes to snapshots by default in a PR, you can add this to the .gitattributes file:

# See https://help.github.com/en/github/administering-a-repository/customizing-how-changed-files-appear-on-github
**/__snapshots__/** linguist-generated=true