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Design

Shouldn't a design precede installation instructions?

In this case, I felt that an installation and a practical demonstration upfront, would help readers to understand the flux design, and make it simpler to then explain how to operate flux themselves! 💪

Flux is power and flexible enough to fit many use-cases. After some experience and dead-ends, I've worked out a way to deploy Flux with enough flexibility but structure to make it an almost-invisible part of how my cluster "just works" on an ongoing basis..

Diagram

Consider this entity relationship diagram:

    erDiagram
          repo-path-flux-system ||..|{ app-namespace : "contains yaml for"
          repo-path-flux-system ||..|{ app-kustomization : "contains yaml for"
          repo-path-flux-system ||..|{ helmrepositories : "contains yaml for"

          app-kustomization ||..|| repo-path-app : "points flux at"

          flux-system-kustomization ||..|| repo-path-flux-system : "points flux at"

          repo-path-app ||..|{ app-helmreleases: "contains yaml for"
          repo-path-app ||..|{ app-configmap: "contains yaml for"
          repo-path-app ||..|o app-sealed-secrets: "contains yaml for"

          app-configmap ||..|| app-helmreleases : configures
          helmrepositories ||..|| app-helmreleases : "host charts for"

          app-helmreleases ||..|{ app-containers : deploys
          app-containers }|..|o app-sealed-secrets : references

Explanation

And here's what it all means, starting from the top...

  1. The flux-system Kustomization tells flux to look in the repo in /flux-system, and apply any YAMLs it finds (with optional kustomize templating, if you're an uber-ninja!).
  2. Within /flux-system, we've defined (for convenience), 3 subfolders, containing YAML for:
    1. namespaces : Any other Namespaces we want to deploy for our apps
    2. helmrepositories : Any HelmRepositories we later want to pull helm charts from
    3. kustomizations : An Kustomizations we need to tell flux to import YAMLs from elsewhere in the repository
  3. In turn, each app's Kustomization (which we just defined above) tells flux to look in the repo in the /<app name> path, and apply any YAMLs it finds (with optional kustomize templating, if you're an uber-ninja!).
  4. Within the /<app name> path, we define at least the following:
    1. A HelmRelease for the app, telling flux which version of what chart to apply from which HelmRepository
    2. A ConfigMap for the HelmRelease, which contains all the custom (and default!) values for the chart
  5. Of course, we can also put any other YAML into the /<app name> path in the repo, which may include additional ConfigMaps, SealedSecrets (for safely storing secrets in a repo), Ingresses, etc.

That seems overly complex!

"Why not just stick all the YAML into one folder and let flux reconcile it all-at-once?"

Several reasons:

  • We need to be able to deploy multiple copies of the same helm chart into different namespaces. Imagine if you wanted to deploy a "postgres" helm chart into a namespace for KeyCloak, plus another one for NextCloud. Putting each HelmRelease resource into its own namespace allows us to do this, while sourcing them all from a common HelmRepository
  • As your cluster grows in complexity, you end up with dependency issues, and sometimes you need one chart deployed first, in order to create CRDs which are depended upon by a second chart (like Prometheus' ServiceMonitor). Isolating apps to a kustomization-per-app means you can implement dependencies and health checks to allow a complex cluster design without chicken vs egg problems!

Got it?

Good! I describe how to put this design into action on the next page...

Chef's notes 📓


  1. ERDs are fancy diagrams for nERDs which represent cardinality between entities scribbled using the foot of a crow 🐓 

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    Last update: November 22, 2021
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