Resource Proliferation Helm Library Chart

This is a Helm library chart containing partial templates designed to enable sets of template files within a chart to be repeated with varying effective contexts. This can be useful when needing to support multiple similar replicates of a set of resources alongside common resources; it can also simply be used where the multiple resource replicates share much of their configuration, replacing the use of multiple aliased instances of a chart in an umbrella chart.

As an example, this chart could be used to specify once the templated Deployment, ConfigMaps HorizontalPodAutoscalers and Secrets for a web service, then replicate those resources for a whole set of similarly configured web services. These multiple similar web service Deployments could be joined, in the same chart, by a single shared Ingress for exposing all services. This is the example available here.

Of course, the replicated resources can be renderred alongside any number and kind of other resources, such as a Deployment of, for example, a Controller, or a logging DaemonSet, etc. Furthermore, these web service Deployments could be joined in the same chart by any number of other sets of replicated resources.

Behaviour and Intent

The intent of this library chart is to support the specification of repeating resources in a manner that feels like native Helm templating:

• The top context available to a replicated template is a normal Helm context;
• The expected values are the same both for the replicated and "base"/"singleton" templates;
• Given the above, conventional partial templates defined in the base templates should work as expected (mostly, see below).

Chart naming within replicated templates

Where templates are replicated multiple times, the notion of the chart's "name" (following the convention of <chart>.name and <chart>.fullname partial templates, as well as nameOverride and fullnameOverride, established by the canonical example chart) becomes more complicated: It is often used as the base part of the name for the majority of a chart's resources, so that the specific resource for that chart is identifiable from the same kind of resource defined in another chart in the same release. However, with multiple replicates of the same resources would need separate "names"; these would most naturally be specific to their specific context.

To this end, whenever a new context is created, the "name" according to the root context is recorded as a new value called baseNameOverride; this is either the nameOverride in the root context, or the name of the chart itself. Furthemore, the nameOverride value is set in each created context based on the first applicable rule in the following list:

1. If the nameOverride is set in the more specific context, use that exactly.
2. If the nameOverride is set in the more general context, use that value suffixed with the name of the specific context, separated by a hyphen.
3. Otherwise, use the baseNameOverride of the specific context, suffixed with its specific name, separated by a hyphen.

This will ensure that the resources in each replica of a set replicated templates will have unique names when using the canonical <chart>.name and <chart>.fullname partial templates. It is advised that a <chart>.basefullname partial template is also created, for referring to resources from the base templates directory from within a replicated template.

Another new value called proliferationStack is also made available, to support more complex naming conventions than expressed by the above rules. This makes the full stack of nested proliferation contexts that have contributed to the current context available, in the form of a list of objects with attributes for the group and instance within that group. For example, one "specific context" produced from the given "input values" would look like this:

input values:

wheels: 2
wings: 0
vehicles:
  plane:
    nameOverride: mechanicalBird
    wings: 2
    models:
      biplane:
        wings: 4
        wheels: 3
      fighter:
        wheels: 3
      carrier:
        wheels: 12
  cars:
    wheels: 4
    models:
      morrissMinor:
        wheels: 3
      vwGolf: {}

specific context:

wheels: 3
wings: 2
proliferationStack:
- group: vehicles
  instance: plane
  nameOverride: mechanicalBird
- group: models
  instance: fighter

Installation

This chart is not currently available in a Helm chart museum or repository. However, it can be installed as a Git submodule directly into a parent chart's charts directory. Alternatively, it could be simply cloned onto the same machine as a parent chart, then referenced as a local dependency. Finally, this repository can be referenced as a Helm repository using Helm-Git.

Partial Templates

There are three partial templates made available by this library chart. Their details are shown in the table below.

Name	Description	Signature
proliferation.renderTemplatesFromDirectory	Given a directory relative to the (parent) chart root, this partial template will first establish all partials from any files from that directory, then render all the resources from the template files in that directory whose names do not start with an underscore. The context used to render these templates is intended to be a top-level Helm-style context, though that isn't strictly necessary when using this partial template directly. On the other hand, this template is not expected to be used directly in most cases.	list <directory> <context>
proliferation.renderValuesForAllKinds	This partial may be given a values key located at the root of the values tree that holds the mapping of replicate instance names to the corresponding instance-specific values. Alternatively, this argument may be omitted to use the default of kinds. The one required argument is a top-level Helm-style context. This partial will take the given context, remove the given key (e.g. kinds), then construct a new context for each entry in the map under that given key. In this new context, .Values are those of the given context merged with those of the given entry, with the entry's taking precedence. Furthermore, the nameOverride value in the new context is either based on the combination of the entry's unique key and the .Chart.Name, or an extension of the given context's nameOverride. The output is the list of contexts renderred to YAML, under the single key instanceValues of a map, ready for conversion back to data using fromYaml. If the key specified in the arguments (or kinds, if not otherwise specified) is not present in the given context then a the original context is returned as the sole item in the list of contexts.	<context> | list <context> [<replicate values key>]
proliferation.renderTemplatesFromDirectoryForAllKinds	This partial may be given a directory (relative to the parent chart root) from which to source template files, and a values key located at the root of the values tree that holds the mapping of replicate instance names to the corresponding instance-specific values. Alternatively, both of these arguments may be omitted to use the defaults of instance-templates and kinds. The one required argument is a top-level Helm-style context. This partial will render the values for all kinds specified in the context, then use each set of values in turn to render the templates from the given directory. When no kinds are specified in the given context then a single set of the resources is rendered, using the given context unchanged. This makes it equivalent to the partial renderTemplatesFromDirectory.	<context> | list <context> [<template directory> <replicate values key>]