How to Add Logic to Kustomize File - kubernetes

Question

I am trying to deploy a K8s application using Kustomize. Up to now I have done simple implementations where we have a few of the K8s files such as ingress.yaml with something like the following:
apiVersion: extensions/v1beta1
kind: Ingress
metadata:
name: ingressname
namespace: namespace
labels:
app: appname
spec:
tls:
- hosts:
- $(variable1)
secretName: $(variable2)-tls
Under my overlays directory for each environment, I then have another kustomize.yaml which gives the values in a configmap:
apiVersion: kustomize.config.k8s.io/v1beta1
kind: Kustomization
resources:
- path
configMapGenerator:
- behavior: merge
literals:
- variable1=foo
- variable2=bar
name: configmapname
images:
- name: imagename
newName: registryurl
This works well, but now I need to do something more complicated. Say for example I have multiple ingress. Instead of creating multiple base ingress yaml files, how can I have one base yaml file that creates every ingress based on the values in my overlay file? Is that possible?

Answer 1

Kustomize isn't a templating system and doesn't do variable substitution. It can perform a variety of YAML patching tricks, so one option you have is to start with a base manifest like this:
apiVersion: extensions/v1beta1
kind: Ingress
metadata:
name: ingressname
spec:
tls:
- hosts: []
secretName:
And then patch it in your kustomization.yaml files:
apiVersion: kustomize.config.k8s.io/v1beta1
kind: Kustomization
resources:
- ../base
patches:
- target:
kind: Ingress
name: ingressname
patch: |
- op: replace
path: /spec/tls
value:
- hosts:
- host1.example.com
secretName: host1-tls
What I've shown here works well if you have an application consisting of a single Ingress and you want to produce multiple variants (maybe one per cluster, or per namespace, or something). That is, you have:
A Deployment
A Service
An Ingress
(etc.)
Then you would have one directory for each variant of the app, giving you a layout something like:
.
├── base
│   ├── deployment.yaml
│   ├── ingress.yaml
│   ├── kustomization.yaml
│   └── service.yaml
└── overlays
├── variant1
│   └── kustomization.yaml
└── variant2
└── kustomization.yaml
If your application has multiple Ingress resources, and you want to apply the same patch to all of them, Kustomize can do that. If you were to modify the patch in your kustomization.yaml so that it looks like this instead:
apiVersion: kustomize.config.k8s.io/v1beta1
kind: Kustomization
resources:
- ../base
patches:
- target:
kind: Ingress
name: ".*"
patch: |
- op: replace
path: /spec/tls
value:
- hosts:
- host1.example.com
secretName: host1-tls
This would apply the same patch to all matching Ingress resources (which is "all of them", in this case, because we used .* as our match expression).