Run a Stateless Application Using a Deployment

This page shows how to run an application using a Kubernetes Deployment object.

Objectives

• Create an nginx deployment.
• Use kubectl to list information about the deployment.
• Update the deployment.

Before you begin

You need to have a Kubernetes cluster, and the kubectl command-line tool must be configured to communicate with your cluster. It is recommended to run this tutorial on a cluster with at least two nodes that are not acting as control plane hosts. If you do not already have a cluster, you can create one by using minikube or you can use one of these Kubernetes playgrounds:

• Killercoda
• Play with Kubernetes

Creating and exploring an nginx deployment

You can run an application by creating a Kubernetes Deployment object, and you can describe a Deployment in a YAML file. For example, this YAML file describes a Deployment that runs the nginx:1.14.2 Docker image:

application/deployment.yaml

apiVersion: apps/v1
kind: Deployment
metadata:
  name: nginx-deployment
spec:
  selector:
    matchLabels:
      app: nginx
  replicas: 2 # tells deployment to run 2 pods matching the template
  template:
    metadata:
      labels:
        app: nginx
    spec:
      containers:
      - name: nginx
        image: nginx:1.14.2
        ports:
        - containerPort: 80

1. Create a Deployment based on the YAML file:

   kubectl apply -f https://k8s.io/examples/application/deployment.yaml
   

2. Display information about the Deployment:

   kubectl describe deployment nginx-deployment
   

   The output is similar to this:

   Name:     nginx-deployment
   Namespace:    default
   CreationTimestamp:  Tue, 30 Aug 2016 18:11:37 -0700
   Labels:     app=nginx
   Annotations:    deployment.kubernetes.io/revision=1
   Selector:   app=nginx
   Replicas:   2 desired | 2 updated | 2 total | 2 available | 0 unavailable
   StrategyType:   RollingUpdate
   MinReadySeconds:  0
   RollingUpdateStrategy:  1 max unavailable, 1 max surge
   Pod Template:
     Labels:       app=nginx
     Containers:
       nginx:
       Image:              nginx:1.14.2
       Port:               80/TCP
       Environment:        <none>
       Mounts:             <none>
     Volumes:              <none>
   Conditions:
     Type          Status  Reason
     ----          ------  ------
     Available     True    MinimumReplicasAvailable
     Progressing   True    NewReplicaSetAvailable
   OldReplicaSets:   <none>
   NewReplicaSet:    nginx-deployment-1771418926 (2/2 replicas created)
   No events.
   

3. List the Pods created by the deployment:

   kubectl get pods -l app=nginx
   

   The output is similar to this:

   NAME                                READY     STATUS    RESTARTS   AGE
   nginx-deployment-1771418926-7o5ns   1/1       Running   0          16h
   nginx-deployment-1771418926-r18az   1/1       Running   0          16h
   

4. Display information about a Pod:

   kubectl describe pod <pod-name>
   

   where <pod-name> is the name of one of your Pods.

Updating the deployment

You can update the deployment by applying a new YAML file. This YAML file specifies that the deployment should be updated to use nginx 1.16.1.

application/deployment-update.yaml

apiVersion: apps/v1
kind: Deployment
metadata:
  name: nginx-deployment
spec:
  selector:
    matchLabels:
      app: nginx
  replicas: 2
  template:
    metadata:
      labels:
        app: nginx
    spec:
      containers:
      - name: nginx
        image: nginx:1.16.1 # Update the version of nginx from 1.14.2 to 1.16.1
        ports:
        - containerPort: 80

1. Apply the new YAML file:

   kubectl apply -f https://k8s.io/examples/application/deployment-update.yaml
   

2. Watch the deployment create pods with new names and delete the old pods:

   kubectl get pods -l app=nginx
   

Scaling the application by increasing the replica count

You can increase the number of Pods in your Deployment by applying a new YAML file. This YAML file sets replicas to 4, which specifies that the Deployment should have four Pods:

application/deployment-scale.yaml

apiVersion: apps/v1
kind: Deployment
metadata:
  name: nginx-deployment
spec:
  selector:
    matchLabels:
      app: nginx
  replicas: 4 # Update the replicas from 2 to 4
  template:
    metadata:
      labels:
        app: nginx
    spec:
      containers:
      - name: nginx
        image: nginx:1.16.1
        ports:
        - containerPort: 80

1. Apply the new YAML file:

   kubectl apply -f https://k8s.io/examples/application/deployment-scale.yaml
   

2. Verify that the Deployment has four Pods:

   kubectl get pods -l app=nginx
   

   The output is similar to this:

   NAME                               READY     STATUS    RESTARTS   AGE
   nginx-deployment-148880595-4zdqq   1/1       Running   0          25s
   nginx-deployment-148880595-6zgi1   1/1       Running   0          25s
   nginx-deployment-148880595-fxcez   1/1       Running   0          2m
   nginx-deployment-148880595-rwovn   1/1       Running   0          2m
   

Deleting a deployment

Delete the deployment by name:

kubectl delete deployment nginx-deployment

ReplicationControllers -- the Old Way

The preferred way to create a replicated application is to use a Deployment, which in turn uses a ReplicaSet. Before the Deployment and ReplicaSet were added to Kubernetes, replicated applications were configured using a ReplicationController.

What's next

• Learn more about Deployment objects.