[Kubernetes] Declarative Kubernetes YAML

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Declarative Kubernetes YAML

• Kubectl apply
• Kubernetes Configuration YAML
• Building YAML Files
  • Building YAML spec
• Dry Runs and Diff’s
• Labels and Annotations

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Kubectl apply

kubectl apply -f filename.yaml

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Kubernetes Configuration YAML

• Each file contains one or more manifests
• Each manifest describe an API object(e.g. deployment, job, secret, etc.)
• Each manifest needs four parts(apiVersion, kind, metadata, spec)

Pod

apiVersion: v1
kind: Pod
metadata:
  name: nginx
spec:
  containers:
    - name: nginx
      image: nginx:1.17.3
      ports:
        - containerPort: 80

Deployment

apiVersion: apps/v1 # api version is different with above since it is related with the kind(in this case deployment)
kind: Deployment
metadata:
  name: nginx-deployment
spec:
  selector:
    matchLabels:
      app: nginx
  replicas: 2
  template:
    metadata:
      labels:
        app: nginx
    spec:
      containers:
        - name: nginx
          image: nginx:1.17.3
          ports:
            - containerPort: 80

Deployment & Service

apiVersion: v1
kind: Service
metadata:
  name: app-nginx-service
spec:
  type: NodePort
  ports:
    - port: 80
  selector:
    app: app-nginx
--- # way of telling k8s that the code above and below are entities
apiVersion: apps/v1
kind: Deployment
metadata:
  name: app-nginx-deployment
spec:
  replicas: 3
  selector: # how one object find the resources that it wants to talk to 
    matchLabels:
      app: app-nginx
  template:
    metadata:
      labels:
        app: app-nginx
    spec:
      containers:
        - name: nginx
          image: nginx:1.17.3
          ports:
            - containerPort: 80

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Building YAML Files

• kind: get a list of resources the cluster supports(kubectl api-resources)
  • List will get longer when third party tools gets included
  • Some Resource have multiple API’s
  • Find the documents of the corresponding APIVERSION
• apiVersion: get the api version the cluster supports(kubectl api-versions)
• kind & apiVersion together decides which resource to get and which api version will be used for the service
• metadata: only name is required
• spec: where all the action is at

Building YAML spec

# get all the keys each kind supports
kubectl explain services --recursive
kubectl explain deployment --recursive

kubectl explain services.spec # see only 'spec' service
kubectl explain services.spec.type # see only the 'type' key (of spec service)

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Dry Runs and Diff’s

• Supported after k8s 1.15

kubectl apply -f app.yml --dry-run=client
kubectl apply -f app.yml --dry-run=server

# show changes between the one running on the server and the current YAML file
# does not check errors
kubectl diff -f app.yml

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Labels and Annotations

• Labels goes under metadata in the YAML, which are optional
• List of key: value pair for identifying resources later by selecting, grouping, or filtering
• Common examples include tier: backend, app: api, env: prod
• Annotations are used often to store config data(proxy, ingress)

# filter a get command
kubectl get pods -l app=nginx

# apply only matching labels
kubectl apply -f myfile.yaml -l app=nginx

Label Selectors

• Telling Services and Deployments which pod are theirs
• Many resources use Label Selectors to “link” resources dependencies
  • Which is why Service, and Deployment kind YAML have these Label Selectors
• controls which pods go to which nodes

apiVersion: v1
kind: Service
metadata:
  name: app-nginx-service
spec:
  type: NodePort
  ports:
    - port: 80
  selector: 
    app: app-nginx # key, value of a label for pods that it needs to direct traffic to
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apiVersion: apps/v1
kind: Deployment
metadata:
  name: app-nginx-deployment
spec:
  replicas: 3
  selector: # how one object find the resources that it wants to talk to 
    matchLabels:
      app: app-nginx #  selecting pods based on app-nginx match
  template:
    metadata:
      labels:
        app: app-nginx
    spec:
      containers:
        - name: nginx
          image: nginx:1.17.3
          ports:
            - containerPort: 80

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