Deploying a First (Stateless) Application

Introduction to Kubernetes

Frank Heilmann

Platform Architect and Freelance Instructor

More on "kubectl"

kubectl: main command to interact with Kubernetes objects
Objects are, e.g., pod, service, etc.

Typical usage patterns:
- kubectl create -f <Manifest.yml>: create new objects, with -f for "filename"
- kubectl apply -f <Manifest.yml>: create new objects & change the state of objects
- kubectl get <object>: overview about objects deployed on Kubernetes
- kubectl describe <object>: detailed information about an object
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Detailed help available via command line option --help

More on Manifests

apiVersion: apps/v1
kind: Deployment
metadata:
  name: nginx-deployment
  labels:
    app: nginx
spec:
  replicas: 5
  selector:
    matchLabels:
      app: nginx
  template:
    metadata:
      labels:
        app: nginx
    spec:
      containers:
      - name: nginx
        image: nginx:1.25.4
        ports:
        - containerPort: 80

Remember: Manifests are declarative
Typically YAML, but also in JSON format
Two important sections:
- metadata: essential information about the object or resource
- spec: defines the specifications, or desired state, of the object or resource
Sections can be quite deep, depending on the resource to be deployed

Stateless Applications

Stateless apps:
- General concept
- Not specific to Kubernetes
- Do not save an internal state, or context of processed data
When interrupted, a new replica of the stateless app is recreated and starts operating.

Examples:
- The database frontend querying a database backend
- A search app querying a full text index
- A data stream app that converts temperature readings from an IoT sensor from °F to °C

Kubernetes Deployments

"Stateless applications" translate to "Kubernetes Deployments"
A sample Manifest consists of:
- apiVersion and kind
- metadata and spec
spec defines number of replicas, a selector, and a template
More on selector later
template describes details for the creation the pods in the Deployment

apiVersion: apps/v1
kind: Deployment
metadata:
  name: <deployment name>
  labels:
    app: <a label for the application>
spec:
  replicas: <number of initial replicas>
  selector:
    matchLabels:
      app: <matches the label above>
  template:
    metadata:
      labels:
        app: <label to be given to each pod>
    spec:
      containers:
      - name: <container name>
        image: <the image to be used>
        ports:
        - containerPort: <ports for networking>

Deploying to a Kubernetes Cluster

kubectl apply -f <manifest.yml> for creating pods and applying changes.
Kubernetes Control Plane will schedule the Deployment on Nodes.
- Then, Pods created is triggered on the Nodes.
Pods get a unique, but random (unpredictable) identifier, each Pod is "as good as any other"

Let's practice!

Introduction to Kubernetes