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---
title: Extensible Templating Processing for clusterctl
authors:
* "@wfernandes"
reviewers:
* "@timothysc"
* "@ncdc"
* "@fabriziopandini"
* "@vincepri"


creation-date: 2020-04-27
last-updated:  2020-05-27
status: implementable
see-also:
* https://github.com/kubernetes-sigs/cluster-api/issues/2339
replaces:
* N/A
superseded-by:
* N/A
---

Extensible Template Processing for clusterctl

Table of Contents

Glossary

Refer to the Cluster API Book Glossary.

SimpleYamlProcessor - This is the processor that will handle the existing variable substitution method of templating. This will be the default mode of operation.

Some templating tools that can be used to manage your templates.

Summary

Clusterctl handles the lifecycle of Cluster API management and workload clusters. It automates fetching the YAML files defining provider components and the cluster templates used to define workload clusters. However, as more providers contribute to CAPI and the variety of workload cluster templates increase, it becomes harder for operators to manage all the cluster template flavor permutations. Operators and providers are now leaning towards yaml templating tools to properly manage the variety of cluster templates, and to simplify the end user experience by limiting the amount of boilerplate YAML that operators need to supply.

This proposal outlines the need to create an extensible templating mechanism within clusterctl so that it can be used effectively with other templating tools and continue to provide a seamless user experience.

Motivation

To support a Cluster API that is cloud agnostic, operators need to deploy workloads in a multi-cloud environment. Operators and providers may need to create and manage multiple cluster templates across a matrix of options and configurations. To manage the explosion of yaml manifests, operators may want to explore templating tools to minimize duplication and assert type correctness.

Users looking to deploy workload clusters may be overwhelmed by the number of variables needed to configure a workload cluster. Some of these options may be circumvented via defaults provided by the operators as part of the templating tool.

This proposal aims to extend clusterctl’s current templating mechanism of simple variable substitution to support other templating formats natively, thus benefiting the operators to deploy workload clusters with minimal configuration.

Goals

  • Clusterctl will not be changing the way it gets values for yaml variables. That is, it will get values from environment variables and the config file.
  • To provide an interface such that other templating solutions may be supported.

Non-Goals

  • To disrupt upstream templating solutions such as kustomize.
  • To replace Clusterctl default mode of processing templates, aka SimpleYamlProcessor.

Future Work

  • To automatically detect template files for the appropriate template engine.
  • To use the configuration file to determine which provider should use preferred templating mechanisms.

Proposal

This section outlines the high level work that needs to be done in order to accomplish the task of supporting other templating tools.

User Stories

  • As a clusterctl user, I would like to preserve the current behavior of using existing workload cluster templates with simple variable substitution from the clusterctl config and environment variables.
  • As an infrastructure provider, I would like to provide templated yaml with defaults and have that be consumed by clusterctl config cluster to generate cluster manifests.
  • As a developer, I would like to easily provide a way to extend clusterctl with my preferred templating solution.

Implementation Details/Notes/Constraints

Currently the clusterctl command fetches yaml for the provider components (yaml defining the management cluster components) and the workload cluster templates.

UX for using Extensible Templating Tools

For the first iteration, we won't be making any user facing changes to the clusterctl CLI. Therefore all the clusterctl commands will hold true with the current expected behavior. The changes will be exposed via the clusterctl library.

Interface and Library Changes

Figure 1

Package Method Calls for `clusterctl config cluster`

The TemplateClient will contain a field adhering to the interface - YamlProcessor.

Figure 2

Figure 3

The YamlProcessor will provide three behaviors.

  1. ArtifactName will return the name of the template artifacts that need to be retrieved from the source.

    • For the default SimpleYamlProcessor, it will be cluster-template.yaml. If the flavor option is provided, it will be cluster-template-<flavor>.yaml.
    • For other YamlProcessors, it could be something that follows a separate convention such as cluster-template-<template-engine-type>.<compression-format>. This should include all the artifacts required by the template processor to produce the final YAML template.

    Once we retrieve the artifact blob of bytes we can then perform the required two functions.

  2. GetVariables is responsible for parsing the template artifact blob of bytes and providing a list of variables that the template requires.

  3. Process is responsible for parsing the artifact blob of bytes and will return the final manifest with values retrieved from the VariablesGetter.

The yaml specific processor can be passed in via the clusterctl library.

Notes

  • Another method of defining a yaml processor instead of using its library would be to shell out to the templating engine binary/executable. The advantage of this is that the templating engine is not a dependency within the clusterctl codebase. The disadvantage is that the user needs to ensure the binary is installed separately which disrupts the UX of clusterctl. If each infrastructure provider provides different template styles, the user would need to install multiple templating tool binaries.

Risks and Mitigations

  • Support path for supporting extended templating tools within clusterctl. That is, if there are templating issues, who owns them and how do we address them. This is for each templating engine that is included in clusterctl. However since if we decide to put extended templating libraries within a contrib folder, it will be the responsibility of the contributor and not the project maintainers to support the templating libraries so the issue of support should be solved with this contract.

  • Each provider may want to implement their own templating standard. This could affect the UX of clusterctl when automatically pulling workload cluster templates from various providers.

  • Define a format or convention for retrieving multiple template files. Currently, clusterctl supports template retrieval from a ConfigMap in a cluster, URL, Github release artifacts and local file system. We default to retrieving provider cluster templates from github release assets. In order to avoid running into github quota limits on retrieving asset files we can pull a compressed file with all the template contents.

    As mentioned above in the Constraints and Implementation section a suggestion for convention would be cluster-template-<template-engine-type>.<compression-format>. Compression format could be tar, gzip, zip, etc.

    We will be leaning towards tar.gz compression format unless any objections.

Constraints

  • Currently, clusterctl relies on the conformance of file name conventions such as infrastructure-components.yaml and cluster-template-<flavor>.yaml. Other templating tools might require other conventions to be defined and followed to allow the same "day 1" experience.
  • Some templating tools will require multiple files to be defined rather than a single yaml file. These artifacts will need to be "grouped" together to support current retrieval mechanisms. Currently, clusterctl config cluster retrieves templates from multiple sources such as ConfigMaps within a cluster, URL, Github Repository, Local Repository and even the overrides directory. To ensure compatibility, we’ll need to establish a compression format like tar.gz

Alternatives

  • An alternative to allowing clusterctl handle templates would be to just use another tool upstream to generate the various templates with variables defined. Then use clusterctl to consume that template and use the SimpleYamlProcessor to produce the template.

Upgrade Strategy

Any individual template engine libraries would be upgraded via go modules preferably using semver standards.

Additional Details

Currently, cluster-api-provider-azure is using kustomize to author their cluster template flavors. See issue 540 for more details. Maybe we can collect input from them to understand their strategy if they need to manage their template variations.

Implementation History

  • 04/22/2020: Proposed idea in an issue or community meeting
  • 04/27/2020: Compile a CAEP Google Doc following the CAEP template
  • 05/06/2020: First round of feedback from community
  • 05/11/2020: Open proposal PR
  • 05/13/2020: Present proposal at a community meeting