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Cloud Snapshot Tests

  • Author(s): @eladb
  • Submission Date: 2024-03-24
  • Stage: Approved
  • Stage Date: 2024-04-03


We would like to be able to capture a snapshot of the synthesized output when compiling to cloud targets, and use it to protect against regressions in CI, without having to deploy to the cloud.

For example, let's say I am creating a Wing library with a resource that supports both sim and tf-aws. I am writing a bunch of tests for it.

I can run these tests in the simulator (wing test) and can also run them on AWS through wing test -t tf-aws.

As my code or its dependencies are updated, I'd like to continuously run all these tests to make sure my library is not broken. Running the tests against sim is easy (I can just run wing test in my CI build environment), but it's not realistic to run all these tests against the real cloud target, given it could take a very long time and could also be very expensive.

In some scenarios (such as the Wing SDK tests), users will prefer to setup a continuous integration workflow and run tests in the cloud, but for the most part running all my tests on the cloud for every commit is too expensive and requires a complicated setup (i.e. an AWS account), so I'd like to use a more lightweight approach...

NOTE: This is not (which is about using snapshots for assertions).


We propose to add a feature to wing test which will capture a snapshot of the synthesized output of every .test.w file and store them in the repository.

This is a similar approach used successfully and scalably in the AWS CDK. See docs for details. The main difference is that cdk-integ there was no way to add assertions to tests, so this is so much cooler.

When I run wing test -t tf-aws foo.test.w in my dev environment, if the test succeeds (e.g. deploys and all test cases pass), the test framework will create foo.test.w.tfaws.snap which will include a nicely formatted (markdown?) snapshot of all the synthesis output from this test.

In a future version we can add support for placing all snapshots in an adjacent subdirectory.

Now, when wing test -t tf-aws foo.test.w is executed in CI (CI=1), by default it won't actually go and deploy to the cloud. Instead, it will just synthesize the output and compare it to the committed .snap file. If the output is not the same, the test will fail with a nice diff indicating that there was an unexpected regression.

The --snapshots or -s switch can be used to control behavior:

  • --snapshots=auto - auto-detect based on CI flag (described above)
  • --snapshots=never - disables snapshots altogether
  • --snapshots=update - skips deployment and only update the snapshots.
  • --snapshots=deploy - forces a deployment even if CI=1.
  • --snapshots=assert - only asserts that the snapshots have not changed

Implementation Notes

  • We need to decide if we want the inflight JavaScript code to also be included or just trust the fact that if inflight code has changed, it should be reflected somehow in the asset names referenced from the IAC model.
  • Ideally this shouldn't be Terraform-specific. app.synth() should return something that can be used to capture the snapshots.
  • The output will need to be sanitized somehow so that it will be deterministic (e.g. paths should not be included).