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kustomization of a helm chart

Helm charts aren't natively read by kustomize, but kustomize has a plugin system that allows one to access helm charts.

One pattern combining kustomize and helm is the last mile modification, where one uses an inflated chart as a base, then modifies it on the way to the cluster using kustomize.

The plugin used in the example below is coded to work only for charts found in the stable chart repo. The example arbitrarily uses minecraft, but should work for any chart.

The following example assumes you have helm on your $PATH.

Make a place to work:

DEMO_HOME=$(mktemp -d)
mkdir -p $DEMO_HOME/base
mkdir -p $DEMO_HOME/dev
mkdir -p $DEMO_HOME/prod

Use a remote chart

Define a kustomization representing your development variant (aka environment).

This could involve any number of kustomizations (see other examples), but in this case just add the name prefix dev- to all resources:

cat <<'EOF' >$DEMO_HOME/dev/kustomization.yaml
namePrefix:  dev-
resources:
- ../base
EOF

Likewise define a production variant, with a name prefix prod-:

cat <<'EOF' >$DEMO_HOME/prod/kustomization.yaml
namePrefix:  prod-
resources:
- ../base
EOF

These two variants refer to a common base.

Define this base:

cat <<'EOF' >$DEMO_HOME/base/kustomization.yaml
generators:
- chartInflator.yaml
EOF

The base refers to a generator configuration file called chartInflator.yaml.

This file lets one specify the name of a stable chart, and other things like a path to a values file, defaulting to the values.yaml that comes with the chart.

Create the config file chartInflator.yaml, specifying the arbitrarily chosen chart name minecraft:

cat <<'EOF' >$DEMO_HOME/base/chartInflator.yaml
apiVersion: someteam.example.com/v1
kind: ChartInflator
metadata:
  name: notImportantHere
chartName: minecraft
EOF

Because this particular YAML file is listed in the generators: stanza of a kustomization file, it is treated as the binding between a generator plugin - identified by the apiVersion and kind fields - and other fields that configure the plugin.

Download the plugin to your DEMO_HOME and make it executable:

plugin=plugin/someteam.example.com/v1/chartinflator/ChartInflator
curl -s --create-dirs -o \
"$DEMO_HOME/kustomize/$plugin" \
"https://raw.githubusercontent.com/\
kubernetes-sigs/kustomize/master/$plugin"

chmod a+x $DEMO_HOME/kustomize/$plugin

Check the directory layout:

tree $DEMO_HOME

Expect something like:

/tmp/whatever
├── base
│   ├── chartInflator.yaml
│   └── kustomization.yaml
├── dev
│   └── kustomization.yaml
├── kustomize
│   └── plugin
│       └── someteam.example.com
│           └── v1
│               └── chartinflator
│                  └── ChartInflator
└── prod
   └── kustomization.yaml

Define a helper function to run kustomize with the correct environment and flags for plugins:

function kustomizeIt {
  XDG_CONFIG_HOME=$DEMO_HOME \
  kustomize build --enable_alpha_plugins \
    $DEMO_HOME/$1
}

Finally, build the prod variant. Notice that all resource names now have the prod- prefix:

clear
kustomizeIt prod

Compare dev to prod:

diff <(kustomizeIt dev) <(kustomizeIt prod) | more

To see the unmodified but inflated chart, run kustomize on the base. Every invocation here is re-downloading and re-inflating the chart.

kustomizeIt base

Use a local chart

The example above fetches a new copy of the chart to a temporary directory with each kustomize build, because a local chart home isn't specified in the configuration.

To suppress fetching, specify a chart home explicitly, and just make sure the chart is already there.

To demo this so that it won't interfere with your existing helm environment, do this:

helmHome=$DEMO_HOME/dothelm
chartHome=$DEMO_HOME/base/charts

function doHelm {
  helm --home $helmHome $@
}

# Create helm config files in a new location.
# The init command is extremely chatty
doHelm init --client-only >& /dev/null

Now download a chart; again use minecraft (but you could use anything):

doHelm fetch --untar \
    --untardir $chartHome \
    stable/minecraft

The tree has more stuff now; helm config data and a complete copy of the chart:

tree $DEMO_HOME

Add a chartHome field to the generator config file so that it knows where to find the local chart:

echo "chartHome: $chartHome" >>$DEMO_HOME/base/chartInflator.yaml

Change the values file, to show that the results generated below are from the locally stored chart:

sed -i 's/CHANGEME!/SOMETHINGELSE/' $chartHome/minecraft/values.yaml
sed -i 's/LoadBalancer/NodePort/' $chartHome/minecraft/values.yaml

Finally, built it

kustomizeIt prod

and observe the change from LoadBalancer to NodePort, and the change in the encoded password.