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Should Kubernetes controllers+CRDs managing resources in the external world (non-k8s APIs) try to fix drifts (such as out of band modifications made in the external API) periodically –or assume they're the only actor, and wait for the next modification to reconfigure? This makes scaling harder.

CR is the intended config, so I expect it to overwrite the reality –the question is whether it should try to course-correct periodically. There are some systems (like Prodspec/Annealing) that learn from the current state and bring it back to the configuration SoT, but they're few and rare.

My interpretation is that the CR is the intended state during reconciliation, not forever. If something/someone changes the actual state, they'll be surprised to find it asynchronously revert. I don't think the CR can realistically be a SoT most of the time.

For most folks that ship has sailed. KRM-based tools like Crossplane manage infra in many shops. That’s why probably as a platform team, the moment you give users access to CRs, take the access away to modify the resource another way.