Motivation

For a very long time, the LocalStack CloudFormation engine has been frustrating to use. In particular it did not correctly the resource update process. This lead to the common advice of "restart LocalStack" if you want to make a change to your stack.

LocalStack excels at providing a local first experience, particularly for devops and IaC with its ability to emulate the cloud locally. However with larger stacks or regular changes being required, it was a slow and frustrating experience needing to regularly restart LocalStack because CloudFormation resources did not update.

This PR finishes a long journey implementing the internals (the "engine") of CloudFormation with a focus on correctness, parity with AWS and modelling which resources are and are not updated.

The new engine models the changes to a stack as a whole, rather than attempting to work out which resources should and should not be updated as an operation that requires two static and poorly defined templates. We strongly type the concepts of a pair of stacks and build an internal representation that can be traversed, or "visited".

We primarily achieve this with

• the ChangeSetModel which is a visitor over a pair of templates, and constructs
• the UpdateModel our internal representation (a graph) of the templates, where each node in the graph has a ChangeType representing if it is
  • Unchanged
  • Created
  • Modified
  • Deleted
• a class heirarchy of visitors (see diagram below) that operate on this UpdateModel and through depth first search perform one of a few operations:
  • transforming the target template
  • performing template level validations
  • supporting describing the change set
  • deploying the target template

flowchart TD
    ChangeSetModelVisitor --> ChangeSetModelPreproc
    ChangeSetModelPreproc --> ChangeSetModelTransformer
    ChangeSetModelPreproc --> ChangeSetModelValidator
    ChangeSetModelPreproc --> ChangeSetModelDescriber
    ChangeSetModelPreproc --> ChangeSetModelExecutor

Where the previous engine would order resources based on their dependencies (topological sort) and hope that the resources deployed correctly, this engine visits the update model in a depth first order and therefore fully resolves resource dependencies (such as intrinsic function calls) before deploying parent resources.

This new engine passes all of the current (now legacy) engine's tests, and passes over 100 parity tests that the legacy engine does not.

PROVIDER_OVERRIDE_CLOUDFORMATION=engine-legacy localstack start

A huge thanks to @MEPalma , @pinzon and @dominikschubert for collaborating on this work 🎉

Changes

• Switch the default engine version to the previous engine-v2
• Update CI to continue to run the CloudFormation test suite against the legacy engine in the specific Test CloudFormation Engine legacy step
• Remove the temporary test skip reporting from the CloudFormation test directory