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

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.