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Kustomizer

Kustomizer is an experimental package manager for distributing Kubernetes configuration as OCI artifacts. It offers commands to publish, fetch, diff, customize, validate, apply and prune Kubernetes resources.

Concepts

OCI Artifacts

Kustomizer offers a way to distribute Kubernetes configuration using container registries. It can package Kubernetes manifests in an OCI image and store them in a container registry, right next to your applications' images.

Kustomizer comes with commands for managing OCI artifacts:

  • kustomizer push artifact oci://<image-url>:<tag> -k [-f] [-p]
  • kustomizer tag artifact oci://<image-url>:<tag> <new-tag>
  • kustomizer list artifacts oci://<repo-url> --semver <condition>
  • kustomizer pull artifact oci://<image-url>:<tag>
  • kustomizer inspect artifact oci://<image-url>:<tag>
  • kustomizer diff artifact <oci url> <oci url>

Kustomizer is compatible with Docker Hub, GHCR, ACR, ECR, GCR, Artifactory, self-hosted Docker Registry and others. For auth, it uses the credentials from ~/.docker/config.json.

Assuming you've automated your application's build & push workflow using Docker, you can extend the automation to do the same for your Kubernetes configuration that describes how your application gets deployed.

Sign & Verify Artifacts

Kustomizer can sign and verify artifacts using sigstore/cosign either with static keys, Cloud KMS or keyless signatures (when running kustomizer with GitHub Actions):

  • kustomizer push artifact --sign --cosign-key <private key>
  • kustomizer pull artifact --verify --cosign-key <public key>
  • kustomizer inspect artifact --verify --cosign-key <public key>

For an example on how to secure your Kubernetes supply chain with Kustomizer and Cosign please see this guide.

Resource Inventories

Kustomizer offers a way for grouping Kubernetes resources. It generates an inventory which keeps track of the set of resources applied together. The inventory is stored inside the cluster in a ConfigMap object and contains metadata such as the resources provenance and revision.

The Kustomizer garbage collector uses the inventory to keep track of the applied resources and prunes the Kubernetes objects that were previously applied but are missing from the current revision.

You specify an inventory name and namespace at apply time, and then you can use Kustomizer to list, diff, update, and delete inventories:

  • kustomizer apply inventory <name> [--artifact <oci url>] [-f] [-p] -k
  • kustomizer diff inventory <name> [-a] [-f] [-p] -k
  • kustomizer get inventories --namespace <namespace>
  • kustomizer inspect inventory <name> --namespace <namespace>
  • kustomizer delete inventory <name> --namespace <namespace>

When applying resources from OCI artifacts, Kustomizer saves the artifacts URL and the image SHA-2 digest in the inventory. For deterministic and repeatable apply operations, you could use digests instead of tags.

Encryption at rest

Kustomizer has builtin support for encrypting and decrypting Kubernetes configuration (packaged as OCI artifacts) using age asymmetric keys.

To securely distribute sensitive Kubernetes configuration to trusted users, you can encrypt the artifacts with their age public keys:

  • kustomizer push artifact <oci url> --age-recipients <public keys>

Users can access the artifacts by decrypting them with their age private keys:

  • kustomizer inspect artifact <oci url> --age-identities <private keys>
  • kustomizer pull artifact <oci url> --age-identities <private keys>
  • kustomizer apply inventory -a <oci url> --age-identities <private keys>
  • kustomizer diff inventory -a <oci url> --age-identities <private keys>

Comparison with other tools

vs flux

Kustomizer produces OCI artifacts which are compatible with Flux. For more details on how to continuously deploy applications to Kubernetes clusters with Flux and Kustomizer please see this guide.

Kustomizer is akin to Flux's kustomize-controller, and it shares the same reconcile engine that leverages Kubernetes server-side apply.

Kustomizer can be used as intermediary step when migrating from CI driven deployments to Flux and GitOps. If you're running kubectl apply in your CI pipelines, replacing kubectl with kustomizer, would smooth the transition to a continuous delivery system powered by Flux.

At times, Kustomizer servers as a testing bench for experimental features that are proposed to the Flux community. For example, Kustomizer is the project where features like staged-apply, garbage collection and diffing where first introduced.

vs kubectl

Kustomizer makes use of k8s.io/cli-runtime for loading kubeconfigs and enables users to configure access to Kubernetes clusters in the same way as with kubectl.

Compared to kubectl apply -f, kustomizer apply -f does things a little different:

  • Validates all resources with dry-run apply, and applies only the ones with changes.
  • Applies first custom resource definitions (CRDs) and namespaces, waits for them to register and only then applies the custom resources.
  • Waits for the applied resources to be fully reconciled (checks the ready status of replicasets, services, ingresses, and other custom resources).
  • Deletes stale objects like ConfigMaps and Secrets generated with Kustomize or other tools.

Warn

Note that if you use kubectl to edit an object managed by Kustomizer, all changes will be undone when Kustomizer reconciles an inventory containing that object. In addition, Kustomizer removes the last-applied-configuration annotation set by kubectl apply.

vs kustomize

Kustomizer uses the sigs.k8s.io/kustomize Go packages to patch Kubernetes manifests and is compatible with kustomize.config.k8s.io/v1beta1 overlays.

Compared to kustomize build, kustomizer build -k does things a little different:

  • Pulls resources from container registries.
  • Reorders the resources according to the provided configuration.
  • Allows kustomization.yaml to load files from outside their root directory.
  • Disallows the usage of Kustomize exec and container-based plugins.
  • Extra patches can be specified with kustomizer build -k ./overlay --patch ./patch1.yaml --patch ./patch2.yaml.

License

Kustomizer is Apache 2.0 licensed and accepts contributions via GitHub pull requests.