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Declarative Management of Kubernetes Objects Using Kustomize

Kustomize is a standalone tool to customize Kubernetes objects through a kustomization file. Since 1.14, Kubectl also supports the management of Kubernetes objects using a kustomization file. To view Resources found in a directory containing a kustomization file, run the following command:

kubectl kustomize <kustomization_directory>

To apply those Resources, run kubectl apply with --kustomize or -k flag:

kubectl apply -k <kustomization_directory>

Overview of Kustomize

Kustomize is a tool for customizing Kubernetes configurations. It has the following features to manage application configuration files:

Generating Resources

ConfigMap and Secret hold config or sensitive data that are used by other Kubernetes objects, such as Pods. The source of truth of ConfigMap or Secret are usually from somewhere else, such as a .properties file or a ssh key file. Kustomize has secretGenerator and configMapGenerator, which generate Secret and ConfigMap from files or literals.

configMapGenerator

To generate a ConfigMap from a file, add an entry to files list in configMapGenerator. Here is an example of generating a ConfigMap with a data item from a file content.

# Create a application.properties file
cat <<EOF >application.properties
FOO=Bar
EOF

cat <<EOF >./kustomization.yaml
configMapGenerator:
- name: example-configmap-1
  files:
  - application.properties
EOF

The generated ConfigMap can be checked by the following command:

kubectl kustomize ./

The generated ConfigMap is

apiVersion: v1
data:
  application.properties: |
    FOO=Bar
kind: ConfigMap
metadata:
  name: example-configmap-1-8mbdf7882g

ConfigMap can also be generated from literal key-value pairs. To generate a ConfigMap from a literal key-value pair, add an entry to literals list in configMapGenerator. Here is an example of generating a ConfigMap with a data item from a key-value pair.

cat <<EOF >./kustomization.yaml
configMapGenerator:
- name: example-configmap-2
  literals:
  - FOO=Bar
EOF

The generated ConfigMap can be checked by the following command:

kubectl kustomize ./

The generated ConfigMap is

apiVersion: v1
data:
  FOO: Bar
kind: ConfigMap
metadata:
  name: example-configmap-2-g2hdhfc6tk

secretGenerator

Secret can also be generated from files or literal key-value pairs. To generate a Secret from a file, add an entry to files list in secretGenerator. Here is an example of generating a Secret with a data item from a file.

# Create a password.txt file
cat <<EOF >./password.txt
username=admin
password=secret
EOF

cat <<EOF >./kustomization.yaml
secretGenerator:
- name: example-secret-1
  files:
  - password.txt
EOF

The generated Secret is as follows:

apiVersion: v1
data:
  password.txt: dXNlcm5hbWU9YWRtaW4KcGFzc3dvcmQ9c2VjcmV0Cg==
kind: Secret
metadata:
  name: example-secret-1-t2kt65hgtb
type: Opaque

To generate a Secret from a literal key-value pair, add an entry to literals list in secretGenerator. Here is an example of generating a Secret with a data item from a key-value pair.

cat <<EOF >./kustomization.yaml
secretGenerator:
- name: example-secret-2
  literals:
  - username=admin
  - password=secert 
EOF

The generated Secret is as follows:

apiVersion: v1
data:
  password: c2VjZXJ0
  username: YWRtaW4=
kind: Secret
metadata:
  name: example-secret-2-t52t6g96d8
type: Opaque

generatorOptions

The generated ConfigMaps and Secrets have a suffix appended by hashing the contents. This ensures that a new ConfigMap or Secret is generated when the content is changed. To disable the behavior of appending a suffix, one can use generatorOptions. Besides that, it is also possible to specify cross-cutting options for generated ConfigMaps and Secrets.

cat <<EOF >./kustomization.yaml
configMapGenerator:
- name: example-configmap-3
  literals:
  - FOO=Bar
generatorOptions:
  disableNameSuffixHash: true
  labels:
    type: generated
  annotations:
    note: generated
EOF

Runkubectl kustomize ./ to view the generated ConfigMap:

apiVersion: v1
data:
  FOO: Bar
kind: ConfigMap
metadata:
  annotations:
    note: generated
  labels:
    type: generated
  name: example-configmap-3

Setting cross-cutting fields

It is quite common to set cross-cutting fields for all Kubernetes resources in a project. Some use cases for setting cross-cutting fields:

Here is an example:

# Create a deployment.yaml
cat <<EOF >./deployment.yaml
apiVersion: apps/v1
kind: Deployment
metadata:
  name: nginx-deployment
  labels:
    app: nginx
spec:
  selector:
    matchLabels:
      app: nginx
  template:
    metadata:
      labels:
        app: nginx
    spec:
      containers:
      - name: nginx
        image: nginx
EOF

cat <<EOF >./kustomization.yaml
namespace: my-namespace
namePrefix: dev-
nameSuffix: "-001"
commonLabels:
  app: bingo
commonAnnotations:
  oncallPager: 800-555-1212
resources:
- deployment.yaml
EOF

Run kubectl kustomize ./ to view those fields are all set in the Deployment Resource:

apiVersion: apps/v1
kind: Deployment
metadata:
  annotations:
    oncallPager: 800-555-1212
  labels:
    app: bingo
  name: dev-nginx-deployment-001
  namespace: my-namespace
spec:
  selector:
    matchLabels:
      app: bingo
  template:
    metadata:
      annotations:
        oncallPager: 800-555-1212
      labels:
        app: bingo
    spec:
      containers:
      - image: nginx
        name: nginx

Composing and Customizing Resources

It is common to compose a set of Resources in a project and manage them inside the same file or directory. Kustomize offers composing Resources from different files and applying patches or other customization to them.

Composing

Kustomize supports composition of different resources. The resources field, in the kustomization.yaml file, defines the list of resources to include in a configuration. Set the path to a resource’s configuration file in the resources list. Here is an example for an nginx application with a Deployment and a Service.

# Create a deployment.yaml file
cat <<EOF > deployment.yaml
apiVersion: apps/v1
kind: Deployment
metadata:
  name: my-nginx
spec:
  selector:
    matchLabels:
      run: my-nginx
  replicas: 2
  template:
    metadata:
      labels:
        run: my-nginx
    spec:
      containers:
      - name: my-nginx
        image: nginx
        ports:
        - containerPort: 80
EOF

# Create a service.yaml file
cat <<EOF > service.yaml
apiVersion: v1
kind: Service
metadata:
  name: my-nginx
  labels:
    run: my-nginx
spec:
  ports:
  - port: 80
    protocol: TCP
  selector:
    run: my-nginx
EOF

# Create a kustomization.yaml composing them
cat <<EOF >./kustomization.yaml
resources:
- deployment.yaml
- service.yaml
EOF

The Resources from kubectl kustomize ./ contains both the Deployment and the Service objects.

Customizing

On top of Resources, one can apply different customizations by applying patches. Kustomize supports different patching mechanisms through patchesStrategicMerge and patchesJson6902. patchesStrategicMerge is a list of file paths. Each file should be resolved to a strategic merge patch. The names inside the patches must match Resource names that are already loaded. Small patches that do one thing are recommended. For example, create one patch for increasing the deployment replica number and another patch for setting the memory limit.

# Create a deployment.yaml file
cat <<EOF > deployment.yaml
apiVersion: apps/v1
kind: Deployment
metadata:
  name: my-nginx
spec:
  selector:
    matchLabels:
      run: my-nginx
  replicas: 2
  template:
    metadata:
      labels:
        run: my-nginx
    spec:
      containers:
      - name: my-nginx
        image: nginx
        ports:
        - containerPort: 80
EOF

# Create a patch increase_replicas.yaml
cat <<EOF > increase_replicas.yaml
apiVersion: apps/v1
kind: Deployment
metadata:
  name: my-nginx
spec:
  replicas: 3
EOF

# Create another patch set_memory.yaml
cat <<EOF > set_memory.yaml
apiVersion: apps/v1
kind: Deployment
metadata:
  name: my-nginx
spec:
  template:
    spec:
      containers:
      - name: my-nginx
        resources:
        limits:
          memory: 512Mi
EOF

cat <<EOF >./kustomization.yaml
resources:
- deployment.yaml
patchesStrategicMerge:
- increase_replicas.yaml
- set_memory.yaml
EOF

Run kubectl kustomize ./ to view the Deployment:

apiVersion: apps/v1
kind: Deployment
metadata:
  name: my-nginx
spec:
  replicas: 3
  selector:
    matchLabels:
      run: my-nginx
  template:
    metadata:
      labels:
        run: my-nginx
    spec:
      containers:
      - image: nginx
        limits:
          memory: 512Mi
        name: my-nginx
        ports:
        - containerPort: 80

Not all Resources or fields support strategic merge patches. To support modifying arbitrary fields in arbitrary Resources, Kustomize offers applying JSON patch through patchesJson6902. To find the correct Resource for a Json patch, the group, version, kind and name of that Resource need to be specified in kustomization.yaml. For example, increasing the replica number of a Deployment object can also be done through patchesJson6902.

# Create a deployment.yaml file
cat <<EOF > deployment.yaml
apiVersion: apps/v1
kind: Deployment
metadata:
  name: my-nginx
spec:
  selector:
    matchLabels:
      run: my-nginx
  replicas: 2
  template:
    metadata:
      labels:
        run: my-nginx
    spec:
      containers:
      - name: my-nginx
        image: nginx
        ports:
        - containerPort: 80
EOF

# Create a json patch
cat <<EOF > patch.yaml
- op: replace
  path: /spec/replicas
  value: 3 
EOF

# Create a kustomization.yaml
cat <<EOF >./kustomization.yaml
resources:
- deployment.yaml

patchesJson6902:
- target:
    group: apps
    version: v1
    kind: Deployment
    name: my-nginx
  path: patch.yaml  
EOF

Run kubectl kustomize ./ to see the replicas field is updated:

apiVersion: apps/v1
kind: Deployment
metadata:
  name: my-nginx
spec:
  replicas: 3
  selector:
    matchLabels:
      run: my-nginx
  template:
    metadata:
      labels:
        run: my-nginx
    spec:
      containers:
      - image: nginx
        name: my-nginx
        ports:
        - containerPort: 80

In addition to patches, Kustomize also offers customizing container images or injecting field values from other objects into containers without creating patches. For example, you can change the image used inside containers by specifying the new image in images field in kustomization.yaml.

cat <<EOF > deployment.yaml
apiVersion: apps/v1
kind: Deployment
metadata:
  name: my-nginx
spec:
  selector:
    matchLabels:
      run: my-nginx
  replicas: 2
  template:
    metadata:
      labels:
        run: my-nginx
    spec:
      containers:
      - name: my-nginx
        image: nginx
        ports:
        - containerPort: 80
EOF

cat <<EOF >./kustomization.yaml
resources:
- deployment.yaml
images:
- name: nginx
  newName: my.image.registry/nginx
  newTag: 1.4.0
EOF

Run kubectl kustomize ./ to see that the image being used is updated:

apiVersion: apps/v1
kind: Deployment
metadata:
  name: my-nginx
spec:
  replicas: 2
  selector:
    matchLabels:
      run: my-nginx
  template:
    metadata:
      labels:
        run: my-nginx
    spec:
      containers:
      - image: my.image.registry/nginx:1.4.0
        name: my-nginx
        ports:
        - containerPort: 80

Sometimes, the application running in a Pod may need to use configuration values from other objects. For example, a Pod from a Deployment object need to read the corresponding Service name from Env or as a command argument. Since the Service name may change as namePrefix or nameSuffix is added in the kustomization.yaml file. It is not recommended to hard code the Service name in the command argument. For this usage, Kustomize can inject the Service name into containers through vars.

# Create a deployment.yaml file
cat <<EOF > deployment.yaml
apiVersion: apps/v1
kind: Deployment
metadata:
  name: my-nginx
spec:
  selector:
    matchLabels:
      run: my-nginx
  replicas: 2
  template:
    metadata:
      labels:
        run: my-nginx
    spec:
      containers:
      - name: my-nginx
        image: nginx
        command: ["start", "--host", "\$(MY_SERVICE_NAME)"]
EOF

# Create a service.yaml file
cat <<EOF > service.yaml
apiVersion: v1
kind: Service
metadata:
  name: my-nginx
  labels:
    run: my-nginx
spec:
  ports:
  - port: 80
    protocol: TCP
  selector:
    run: my-nginx
EOF

cat <<EOF >./kustomization.yaml
namePrefix: dev-
nameSuffix: "-001"

resources:
- deployment.yaml
- service.yaml

vars:
- name: MY_SERVICE_NAME
  objref:
    kind: Service
    name: my-nginx
    apiVersion: v1
EOF

Run kubectl kustomize ./ to see that the Service name injected into containers is dev-my-nginx-001:

apiVersion: apps/v1
kind: Deployment
metadata:
  name: dev-my-nginx-001
spec:
  replicas: 2
  selector:
    matchLabels:
      run: my-nginx
  template:
    metadata:
      labels:
        run: my-nginx
    spec:
      containers:
      - command:
        - start
        - --host
        - dev-my-nginx-001
        image: nginx
        name: my-nginx

Bases and Overlays

Kustomize has the concepts of bases and overlays. A base is a directory with a kustomization.yaml, which contains a set of resources and associated customization. A base could be either a local directory or a directory from a remote repo, as long as a kustomization.yaml is present inside. An overlay is a directory with a kustomization.yaml that refers to other kustomization directories as its bases. A base has no knowledge of an overlay and can be used in multiple overlays. An overlay may have multiple bases and it composes all resources from bases and may also have customization on top of them.

Here is an example of a base.

# Create a directory to hold the base
mkdir base
# Create a base/deployment.yaml
cat <<EOF > base/deployment.yaml
apiVersion: apps/v1
kind: Deployment
metadata:
  name: my-nginx
spec:
  selector:
    matchLabels:
      run: my-nginx
  replicas: 2
  template:
    metadata:
      labels:
        run: my-nginx
    spec:
      containers:
      - name: my-nginx
        image: nginx
EOF

# Create a base/service.yaml file
cat <<EOF > base/service.yaml
apiVersion: v1
kind: Service
metadata:
  name: my-nginx
  labels:
    run: my-nginx
spec:
  ports:
  - port: 80
    protocol: TCP
  selector:
    run: my-nginx
EOF
# Create a base/kustomization.yaml
cat <<EOF > base/kustomization.yaml
resources:
- deployment.yaml
- service.yaml

This base can be used in multiple overlays. You can add different namePrefix or other cross-cutting fields in different overlays. Here are two overlays using the same base.

mkdir dev
cat <<EOF > dev/kustomization.yaml
bases:
- ../base
namePrefix: dev-
EOF

mkdir prod
cat <<EOF > prod/kustomization.yaml
bases:
- ../base
namePrefix: prod-
EOF

How to apply/view/delete objects using Kustomize

Use --kustomize or -k in kubectl commands to recognize Resources managed by kustomization.yaml. Note that -k should point to a kustomization directory, such as

kubectl apply -k <kustomization directory>/

Given the following kustomization.yaml,

# Create a deployment.yaml file
cat <<EOF > deployment.yaml
apiVersion: apps/v1
kind: Deployment
metadata:
  name: my-nginx
spec:
  selector:
    matchLabels:
      run: my-nginx
  replicas: 2
  template:
    metadata:
      labels:
        run: my-nginx
    spec:
      containers:
      - name: my-nginx
        image: nginx
        ports:
        - containerPort: 80
EOF

# Create a kustomization.yaml
cat <<EOF >./kustomization.yaml
namePrefix: dev-
commonLabels:
  app: my-nginx
resources:
- deployment.yaml
EOF

Running the following command will apply the Deployment object dev-my-nginx:

> kubectl apply -k ./
deployment.apps/dev-my-nginx created

Running the following command will get he Deployment object dev-my-nginx:

kubectl get -k ./

or

kubectl describe -k ./

Running the following command will delete the Deployment object dev-my-nginx:

> kubectl delete -k ./
deployment.apps "dev-my-nginx" deleted

Kustomize Feature List

Here is a list of all the features in Kustomize.

FieldTypeExplanation
namespacestringadd namespace to all resources
namePrefixstringvalue of this field is prepended to the names of all resources
nameSuffixstringvalue of this field is appended to the names of all resources
commonlabelsmap[string]stringlabels to add to all resources and selectors
commonAnnotationsmap[string]stringannotations to add to all resources
resources[]stringeach entry in this list must resolve to an existing resource configuration file
configmapGenerator[]ConfigMapArgsEach entry in this list generates a ConfigMap
secretGenerator[]SecretArgsEach entry in this list generates a Secret
generatorOptionsGeneratorOptionsModify behaviors of all ConfigMap and Secret generatos
bases[]stringEach entry in this list should resolve to a directory containing a kustomization.yaml file
patchesStrategicMerge[]stringEach entry in this list should resolve a strategic merge patch of a Kubernetes object
patchesJson6902[]Json6902Each entry in this list should resolve to a Kubernetes object and a Json Patch
vars[]VarEach entry is to capture text from one resource’s field
images[]ImageEach entry is to modify the name, tags and/or digest for one image without creating patches
configurations[]stringEach entry in this list should resolve to a file containing Kustomize transformer configurations
crds[]stringEach entry in this list should resolve to an OpenAPI definition file for Kubernetes types

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