Increase testability of Reconcilers in Lifecycle-Manager

[Tomasz-Smelcerz-SAP]

Description

Currently the majority of the code involved in the reconciliation loops cannot be unit tested.
We need to find out a way to make them testable.
The outcome of this issue should be:

• An idea what to do to make our reconcilers unit-testable. It should be applicable to all reconcilers, so that we don't end up with three different approaches for three different reconcilers.
• A proof of concept: refactoring the Purge Reconciler.

We should be pragmatic: Because reconciliation is coupled to the controller-runtime library and, ultimately, to the K8s environment, it's unlikely that we can unit-test everything. Probably some small amount code used for object lookup, object updates etc. can't be easily unit tested. This is fine - this will be tested by the integration and E2E suites.
What we should aim for is to have the business logic split into many independent functions/methods. Every function should be unit-testable. All behaviors that cannot be tested directly, like client-go functions, should be extracted out as explicit dependencies.

Reasons

Lack of unit tests for the reconciliation means we have to rely on integration and E2E tests. The problem is that these tests are verbose, long-running and slow - in comparison with unit tests.
We also cannot test all required scenarios and corner cases in this way because our test suites would take too much time to execute.
This is causing, among other things, the following problems:

• we don't have the "support" provided by proper unit tests: clearly documenting expected behaviors vs incorrect ones.
• all sorts of "code smells" are now visible: tight coupling, low cohesion, blurry responsibilities etc.

All of the above make extending and debugging reconciler's code very hard, especially in the complex cases like the Manifest Reconciler.

Acceptance Criteria

• Find out a systematic way to refactor the reconciliation code so that it can be unit tested.
• Verify the approach by refactoring the Purge Reconciler

Feature Testing

No response

Testing approach

No response

Attachments

No response

[Tomasz-Smelcerz-SAP]

For the systematic way to refactor the code, see:

• the presentation: https://github.com/Tomasz-Smelcerz-SAP/presentations
• the POC branch with first function refactored (POC code): https://github.com/kyma-project/lifecycle-manager/compare/main...Tomasz-Smelcerz-SAP:lifecycle-manager:feat/purge-reconciler-testability?expand=1