Advanced Helm Chart Customization

Prerequisites

Before you begin, check the following prerequisites:

1. Download the Istio release.
2. Perform any necessary platform-specific setup.
3. Check the Requirements for Pods and Services.
4. Usage of helm for Istio installation.
5. Helm version that supports post rendering. (>= 3.1)
6. kubectl or kustomize.

Advanced Helm Chart Customization

Istio’s helm chart tries to incorporate most of the attributes needed by users for their specific requirements. However, it does not contain every possible Kubernetes value you may want to tweak. While it is not practical to have such a mechanism in place, in this document we will demonstrate a method which would allow you to do some advanced helm chart customization without the need to directly modify Istio’s helm chart.

Using Helm with kustomize to post-render Istio charts

Using the Helm post-renderer capability, you can tweak the installation manifests to meet your requirements easily. Post-rendering gives the flexibility to manipulate, configure, and/or validate rendered manifests before they are installed by Helm. This enables users with advanced configuration needs to use tools like Kustomize to apply configuration changes without the need for any additional support from the original chart maintainers.

Adding a value to an already existing chart

In this example, we will add a sysctl value to Istio’s ingress-gateway deployment. We are going to:

1. Create a sysctl deployment customization patch template.
2. Apply the patch using Helm post-rendering.
3. Verify that the sysctl patch was correctly applied to the pods.

Create the Kustomization

First, we create a sysctl patch file, adding a securityContext to the ingress-gateway pod with the additional attribute:

$ cat > sysctl-ingress-gw-customization.yaml <<EOF
apiVersion: apps/v1
kind: Deployment
metadata:
  name: istio-ingress
  namespace: istio-ingress
spec:
  template:
    spec:
      securityContext:
          sysctls:
          - name: net.netfilter.nf_conntrack_tcp_timeout_close_wait
            value: "10"
EOF

The below shell script helps to bridge the gap between Helm post-renderer and Kustomize, as the former works with stdin/stdout and the latter works with files.

$ cat > kustomize.sh <<EOF
#!/bin/sh
cat > base.yaml
exec kubectl kustomize # you can also use "kustomize build ." if you have it installed.
EOF
$ chmod +x ./kustomize.sh

Finally, let us create the kustomization yaml file, which is the input for kustomize with the set of resources and associated customization details.

$ cat > kustomization.yaml <<EOF
resources:
- base.yaml
patchesStrategicMerge:
- sysctl-ingress-gw-customization.yaml
EOF

Apply the Kustomization

Now that the Kustomization file is ready, let us use Helm to make sure this gets applied properly.

Add the Helm repository for Istio

$ helm repo add istio https://istio-release.storage.googleapis.com/charts
$ helm repo update

Render and Verify using Helm Template

We can use Helm post-renderer to validate rendered manifests before they are installed by Helm

$ helm template istio-ingress istio/gateway --namespace istio-ingress --post-renderer ./kustomize.sh | grep -B 2 -A 1 netfilter.nf_conntrack_tcp_timeout_close_wait

In the output, check for the newly added sysctl attribute for ingress-gateway pod:

securityContext:
  sysctls:
  - name: net.netfilter.nf_conntrack_tcp_timeout_close_wait
    value: "10"

Apply the patch using Helm Post-Renderer

Use the below command to install an Istio ingress-gateway, applying our customization using Helm post-renderer:

$ kubectl create ns istio-ingress
$ helm upgrade -i istio-ingress istio/gateway --namespace istio-ingress --wait --post-renderer ./kustomize.sh

Verify the Kustomization

Examine the ingress-gateway deployment, you will see the newly manipulated sysctl value:

$ kubectl -n istio-ingress get deployment istio-ingress -o yaml

apiVersion: apps/v1
kind: Deployment
metadata:
  …
  name: istio-ingress
  namespace: istio-ingress
spec:
  template:
    metadata:
      …
    spec:
      securityContext:
        sysctls:
        - name: net.netfilter.nf_conntrack_tcp_timeout_close_wait
          value: "10"

Additional Information

For further detailed information about the concepts and techniques described in this document, please refer to:

1. IstioOperator - Customize Installation
2. Advanced Helm Techniques
3. Kustomize