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Secret Replicator

As explained when creating our LetsEncrypt Wildcard certificates, it can be problematic that Certificates can't be shared between namespaces. One simple solution to this problem is simply to "replicate" secrets from one "source" namespace into all other namespaces.

Ingredients

Kiwigrid's "Secret Replicator" is a simple controller which replicates secrets from one namespace to another.1

Preparation

Namespace

We need a namespace to deploy our HelmRelease and associated ConfigMaps into. Per the flux design, I create this in my flux repo at flux-system/namespaces/namespace-secret-replicator.yaml:

Example Namespace (click to expand)
apiVersion: v1
kind: Namespace
metadata:
  name: secret-replicator

HelmRepository

Next, we need to define a HelmRepository (a repository of helm charts), to which we'll refer when we create the HelmRelease. We only need to do this once per-repository. Per the flux design, I create this in my flux repo at flux-system/helmrepositories/helmrepository-kiwigrid.yaml:

Example HelmRepository (click to expand)
apiVersion: source.toolkit.fluxcd.io/v1beta1
kind: HelmRepository
metadata:
  name: kiwigrid
  namespace: flux-system
spec:
  interval: 15m
  url: https://kiwigrid.github.io

Kustomization

Now that the "global" elements of this deployment have been defined, we do some "flux-ception", and go one layer deeper, adding another Kustomization, telling flux to deploy any YAMLs found in the repo at /secret-replicator. I create this Kustomization in my flux repo at flux-system/kustomizations/kustomization-secret-replicator.yaml:

Example Kustomization (click to expand)
apiVersion: kustomize.toolkit.fluxcd.io/v1beta1
kind: Kustomization
metadata:
  name: secret-replicator
  namespace: flux-system
spec:
  interval: 15m
  path: ./secret-replicator
  prune: true # remove any elements later removed from the above path
  timeout: 2m # if not set, this defaults to interval duration, which is 1h
  sourceRef:
    kind: GitRepository
    name: flux-system
  validation: server
  healthChecks:
    - apiVersion: apps/v1
      kind: Deployment
      name: secret-replicator
      namespace: secret-replicator

ConfigMap

Now we're into the secret-replicator-specific YAMLs. First, we create a ConfigMap, containing the entire contents of the helm chart's values.yaml. Paste the values into a values.yaml key as illustrated below, indented 4 tabs (since they're "encapsulated" within the ConfigMap YAML). I create this in my flux repo at secret-replicator/configmap-secret-replicator-helm-chart-value-overrides.yaml:

Example ConfigMap (click to expand)
apiVersion: v1
kind: ConfigMap
metadata:
  name: secret-replicator-helm-chart-value-overrides
  namespace: secret-replicator
data:
  values.yaml: |-
    # Default values for secret-replicator.
    # This is a YAML-formatted file.
    # Declare variables to be passed into your templates.

    image:
    repository: kiwigrid/secret-replicator
    tag: 0.2.0
    pullPolicy: IfNotPresent
    ## Specify ImagePullSecrets for Pods
    ## ref: https://kubernetes.io/docs/concepts/containers/images/#specifying-imagepullsecrets-on-a-pod
    # pullSecrets: myregistrykey

    # csv list of secrets
    secretList: "letsencrypt-wildcard-cert"
    # secretList: "secret1,secret2

    ignoreNamespaces: "kube-system,kube-public"

    # If defined, allow secret-replicator to watch for secrets in _another_ namespace
    secretNamespace: letsencrypt-wildcard-cert"

    rbac:
    enabled: true

    resources: {}
    # limits:
    #   cpu: 50m
    #   memory: 20Mi
    # requests:
    #   cpu: 20m
    #   memory: 20Mi

    nodeSelector: {}

    tolerations: []

    affinity: {}

That's a lot of unnecessary text!

Why not just paste in the subset of values I want to change?

You know what's harder than working out which values from a 2000-line values.yaml to change?

Answer: Working out what values to change when the upstream helm chart has refactored or added options! By pasting in the entirety of the upstream chart, when it comes time to perform upgrades, you can just duplicate your ConfigMap YAML, paste the new values into one of the copies, and compare them side by side to ensure your original values/decisions persist in the new chart.

Note that the following values changed from default, above:

  • secretList: letsencrypt-wildcard-cert
  • secretNamespace: letsencrypt-wildcard-cert

HelmRelease

Lastly, having set the scene above, we define the HelmRelease which will actually deploy the secret-replicator controller into the cluster, with the config we defined above. I save this in my flux repo as secret-replicator/helmrelease-secret-replicator.yaml:

Example HelmRelease (click to expand)
apiVersion: helm.toolkit.fluxcd.io/v2beta1
kind: HelmRelease
metadata:
name: secret-replicator
namespace: secret-replicator
spec:
chart:
    spec:
    chart: secret-replicator
    version: 0.6.x
    sourceRef:
        kind: HelmRepository
        name: kiwigrid
        namespace: flux-system
interval: 15m
timeout: 5m
releaseName: secret-replicator
valuesFrom:
- kind: ConfigMap
    name: secret-replicator-helm-chart-value-overrides
    valuesKey: values.yaml # This is the default, but best to be explicit for clarity

Why not just put config in the HelmRelease?

While it's true that we could embed values directly into the HelmRelease YAML, this becomes unweildy with large helm charts. It's also simpler (less likely to result in error) if changes to HelmReleases, which affect deployment of the chart, are defined in separate files to changes in helm chart values, which affect operation of the chart.

Serving

Once you've committed your YAML files into your repo, you should soon see some pods appear in the secret-replicator namespace!

How do we know it worked?

Look for secrets across the whole cluster, by running kubectl get secrets -A | grep letsencrypt-wildcard-cert. What you should see is an identical secret in every namespace. Note that the Certificate only exists in the letsencrypt-wildcard-cert namespace, but the secret it generates is what gets replicated to every other namespace.

Troubleshooting

If your certificate is not created aren't created as you expect, then the best approach is to check the secret-replicator logs, by running kubectl logs -n secret-replicator -l app.kubernetes.io/name=secret-replicator.

Chef's notes 📓


  1. To my great New Zealandy confusion, "Kiwigrid GmbH" is a German company 🤷 

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    Last update: November 22, 2021
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