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Kustomize

Kustomize is similar to Helm, both are good for bundling kubernetes elements such as services, deployments, etc...
Helm can act as a package manager for kubernetes/oc as well as apt or yum can do for Debian, RedHat.
The main difference is Helm uses Templates whereas Kustomize uses Overlays. Kustomize is also developed by the Kubernetes teams and it is built in recent version of oc and kubectl. You can build a project using this command:

oc kustomize build FOLDER

However some features are missing with the built-in tool, here is a list of the commands available with kustomize:

  • build Build a kustomization target from a directory or URL
  • cfg Commands for reading and writing configuration
  • completion Generate shell completion script
  • create Create a new kustomization in the current directory
  • edit Edits a kustomization file
  • fn Commands for running functions against configuration
  • help Help about any command
  • localize [Alpha] Creates localized copy of target kustomization root at destination
  • version Prints the kustomize version

You can install the tool separately. The command to build with kustomize is:

kustomize build FOLDER

A build won't apply, it will only output to stdout.
If you want to apply your kustomize build, you can use this command: 

kustomize build FOLDER | oc apply -f -

Here is a table that compares both solutions:

Helm Kustomize
Pros - Template functions are powerful
- Helm is a package manager, like apt or yum does, but for kubernetes
- Large amount of existing charts already out that can boost productivity		 - Native in from kubectl v1.14
- Uses of plain YAML
- Not a templating system but a yaml patching system
Cons - More abstraction layers
- Less readable templates
- Require an external dependency
- Folder structure		 - The strength of Helm is to be used as a package manager
- Does not follow the DRY principle

When using Kustomize?

It can be harsh to use Helm in a way that your applications will contains more curly brackets than nouns in your YAML files. Kustomize allows you to work with a bunch of YAML files. It can be a good alternative by using overlays instead of templates.

What are overlays?

Overlays are a kustomization (kustomization.yaml) that can depend on another kustomization. They can include new resource manifests, or patches for existing ones.

Example

Let's see an example on how kustomize works. We'll take this repo: https://github.com/lvarin/kustomize-openshift

If we look at the directory, this is what we have:

├── README.md
├── base
│   ├── deployment.yaml
│   ├── kustomization.yaml
│   ├── pvc.yaml
│   ├── route.yaml
│   └── service.yaml
└── overlays
    └── production
        ├── db.yaml
        ├── deployment.yaml
        ├── kustomization.yaml
        ├── pvc.yaml
        └── route.yaml

4 directories, 11 files

We have a base and an overlays folder. Inside the overlays folder, we can find another folder called production. To start using kustomize, you need to create a kustomization.yaml file. Use this command to create a kustomization file (optional): 

kustomize create

Here is the content of the kustomization file inside the base folder:

apiVersion: kustomize.config.k8s.io/v1beta1
kind: Kustomization
metadata:
  name: arbitrarky

resources:
- pvc.yaml
- deployment.yaml
- service.yaml
- route.yaml

You will notice a resources key, with different yaml files as values. A resource is a root relative path to a YAML or JSON file describing a k8s API object.
And now, let's have a look to the content of the kustomization.yaml file inside overlays/production:

resources:
- ../../base

patchesStrategicMerge:
- pvc.yaml
- route.yaml

configMapGenerator:
- name: dbparams
  files:
  - db.yaml

Basically, if you run the command oc kustomize base or kustomize build base, you will have the output of pvc.yaml, deployment.yaml, service.yaml and route.yaml.

Now, if you run the same command as above but with overlays/production instead of base, you will have the same output but with some new stuff, like a configMap and modifications in pvc.yaml and route.yaml:

+apiVersion: v1
+data:
+  db.yaml: |
+    user: pepe
+    password: pardo
+kind: ConfigMap
+metadata:
+  name: dbparams-btb7dgb89t

apiVersion: v1
kind: PersistentVolumeClaim
metadata:
  name: html
spec:
  accessModes:
  - ReadWriteOnce
  resources:
    requests:
-      storage: 500Mi
status: {}


apiVersion: v1
kind: PersistentVolumeClaim
metadata:
  name: html
spec:
  resources:
    requests:
+      storage: 1Gi


apiVersion: route.openshift.io/v1
kind: Route
metadata:
  name: nginx
spec:
-  host: example.rahtiapp.fi
  tls:
    insecureEdgeTerminationPolicy: Redirect
    termination: edge
  to:
    kind: Service
    weight: 100
    name: nginx
status:
  ingress: []


apiVersion: route.openshift.io/v1
kind: Route
metadata:
  name: nginx
spec:
+  host: t-test-kustomize.2.rahtiapp.fi
What does this mean?
You can see by applying overlays you'll patch your files without editing the originals. The only thing to do is to add different values on what you want to be changed and apply the overlays.
With overlays you can have several files ordered into folders. For example, if you need to modify some values inside a yaml file for a production environment, you can easily do it by using overlays without affecting your main files. You can also create another folder nightly for beta testing and put there different values.

To apply an overlay, use this command:

oc apply -k overlays/production

It also possible to delete everything created by an overlay using this command:

oc delete -k overlays/production