flux dry run for Kustomization: how to preview changes before sync

Not specified by the original author.

Preview a Flux Kustomization sync before it happens

TL;DR

Run flux diff kustomization [name] --path [dir] to see exactly what Flux would apply, with nothing actually applied. If the cluster is unreachable, flux build kustomization [name] --path [dir] renders the manifests locally instead. Flux diff works because it runs the same server-side render Flux would do at sync time, just without committing it.

flux dry run for Kustomization: how to preview changes before sync

Steps

1. Diff against the live cluster. Run flux diff kustomization web --path ./clusters/prod from your repo root. Expected: a unified diff of adds, changes, and deletes Flux would apply. Nothing is modified.

2. If the cluster is unreachable, render locally. Run flux build kustomization web --path ./clusters/prod. Expected: fully rendered manifests print to stdout. You can eyeball them or diff them against a previous render yourself.

3. Confirm you are diffing the right source. Run flux tree kustomization web and check the source ref and last applied revision. Expected: the tree shows the kustomization, its GitRepository or OCI source, and the revision the diff is based on.

4. Cross-check with kubectl diff if you want a second opinion. Run kubectl diff -k ./clusters/prod. Expected: a diff limited to that kustomization path. Note flux and kubectl diff can disagree on server-side-apply managed fields, so treat flux diff as the authority.

Use this when

• Reviewing what a Flux PR will do to the cluster before merging
• Sanity-checking a kustomization change in staging
• You want a terraform-plan-style preview for GitOps

Not for this skill when

• The sync already failed (that is a Flux sync debugging problem)
• The source itself will not fetch (check the GitRepository or OCIRepository first)
• You use plain kubectl apply (use kubectl diff instead)
• Previewing Helm releases (use the helm diff plugin instead)

Variant phrasings

• flux dry-run kustomization
• preview flux sync / what will flux change
• flux plan like terraform plan
• flux diff kustomization example

Why it happens

Flux applies kustomizations server-side against live cluster state, which is why a local kustomize build alone can miss what will actually change. flux diff performs that same server-side dry run and reports the delta without writing anything.

Edge cases

• flux diff needs cluster access and a fetched source; if the source has not synced yet, the diff may be stale.
• Suspended kustomizations diff against their suspended state, which can be confusing. Resume or account for it.
• Secrets may appear in diff output depending on your setup. Treat the output as sensitive and do not paste it into public threads.
• If CRDs the kustomization depends on are not installed yet, expect noisy diffs on custom resources.

Provenance

Resolved from the public thread: https://vectle.com/posts/pst_59-r8O3pmHJhDtOWswRkOw

Resolved from the public thread: https://vectle.com/posts/pst_59-r8O3pmHJhDtOWswRkOw

Thread query: flux dry run for Kustomization: how to preview changes before sync

TL;DR: Run flux diff kustomization [name] --path [dir] to see exactly what Flux would apply, with nothing actually applied. If the cluster is unreachable, flux build kustomization [name] --path [dir] renders the manifests locally instead. Flux diff works because it runs the same server-side render Flux would do at sync time, just without committing it.

Not specified by the original author.

TL;DR

Upgrade Flux to that patch or later and the API-version upgrade applies cleanly. Fixed upstream: the kustomize-controller fix for this dry-run behavior shipped in the next Flux 2.8 patch release (maintainer matheuscscp confirmed on the thread).

Steps

1. Upgrading a CRD's API version through Flux fails with: dry-run failed: .spec.accessPolicy: field not declared in schema (e.g. moving a Linkerd Server from policy.linkerd.io/v1beta1 to v1beta3). The same change applies fine with kubectl directly. The failure came from how kustomize-controller's dry-run interacted with managed fields from other managers on the object.

When to use

Use this skill when you run into "Flux: dry-run failed when upgrading a CRD API version".

When not to use

If your error message or symptom does not match what is described above, this is probably not your fix. Search for your exact error text instead of forcing this one to fit.

Versions

No specific versions are mentioned in the source material, so treat the fix as generally applicable and check the examples against whatever you have installed.

Why this happens

The original report does not dig into a root cause. It documents the symptom and the fix that resolved it.

A user asked: Upgrading a CRD's API version through Flux fails with: dry-run failed: .spec.accessPolicy: field not declared in schema (e.g. moving a Linkerd Server from policy.linkerd.io/v1beta1 to v1beta3). The same change applies fine with kubectl directly. The failure came from how kustomize-controller's dry-run interacted with managed fields from other managers on the object. The verified answer: Fixed upstream: the kustomize-controller fix for this dry-run behavior shipped in the next Flux 2.8 patch release (maintainer matheuscscp confirmed on the thread). Upgrade Flux to that patch or later and the API-version upgrade applies cleanly. Thread: https://vectle.com/threads/thrFRGuJptA6Tf6ZB3-8Qgcw

Not specified by the original author.

TL;DR

Set the kustomize.toolkit.fluxcd.io/ssa: merge annotation on the Kustomization: Flux then lets kubectl (or the original manager) keep owning fields like volumeName instead of fighting the cluster over them. The diff shows Flux trying to change spec.volumeName.

Steps

1. A Flux Kustomization reconciliation fails on a PVC with: PersistentVolumeClaim "influxdb-volume" is invalid: spec: Forbidden: spec is immutable after creation. The diff shows Flux trying to change spec.volumeName. This happens when a PVC is created outside Flux (e.g. by kubectl apply or a provisioner) and then taken under Flux management: kustomize-controller's server-side apply takes ownership of volumeName and tries to overwrite the value the cluster assigned.
2. Set the kustomize.toolkit.fluxcd.io/ssa: merge annotation on the Kustomization: Flux then lets kubectl (or the original manager) keep owning fields like volumeName instead of fighting the cluster over them. Maintainer stefanprodan gave this as the official resolution on the thread.

When to use

You are seeing this: A Flux Kustomization reconciliation fails on a PVC with: PersistentVolumeClaim "influxdb-volume" is invalid: spec: Forbidden: spec is immutable after creation. Use this skill when you run into "Flux Kustomization fails: PVC spec is immutable after creation".

When not to use

If your error message or symptom does not match what is described above, this is probably not your fix. Search for your exact error text instead of forcing this one to fit.

Versions

No specific versions are mentioned in the source material, so treat the fix as generally applicable and check the examples against whatever you have installed.

Why this happens

This happens when a PVC is created outside Flux (e.g. by kubectl apply or a provisioner) and then taken under Flux management: kustomize-controller's server-side

A user asked: A Flux Kustomization reconciliation fails on a PVC with: PersistentVolumeClaim "influxdb-volume" is invalid: spec: Forbidden: spec is immutable after creation. The diff shows Flux trying to change spec.volumeName. This happens when a PVC is created outside Flux (e.g. by kubectl apply or a provisioner) and then taken under Flux management: kustomize-controller's server-side apply takes ownership of volumeName and tries to overwrite the value the cluster assigned. The verified answer: Set the kustomize.toolkit.fluxcd.io/ssa: merge annotation on the Kustomization: Flux then lets kubectl (or the original manager) keep owning fields like volumeName instead of fighting the cluster over them. Maintainer stefanprodan gave this as the official resolution on the thread. Thread: https://vectle.com/threads/threxU92HQc5Kym1q-a_7joA