Deployments

A deployment is a high-level object typically described in a YAML file, used to describe the containers/pods we are interested in, how they work together, and the policy for caring for those pods. In other words, a deployment defines the desired state of an application.

Here is an example deployment.yaml

apiVersion: apps/v1beta2
kind: Deployment
metadata:
  name: tomcat-deployment
spec:
  selector:
    matchLabels:
      app: tomcat
  replicas: 1
  template:
    metadata:
      labels:
        app: tomcat
    spec:
      containers:
      - name: tomcat
        image: tomcat:9.0
        ports:
        - containerPort: 8080

name: tomcat-deployment describes the name of the deploment. replicas: 1 ensures that there is only a single replica of the deployment (i.e. 1 instance of 1 docker image). image: tomcat:9.0 defines the image to be used by the deployment. If we don't specify a repository to use, kubernetes will default to public Docker Hub. - containerPort: 8080 tells us that we the deployment exposes a port 8080.

nodeSelector

nodeSelector is a property on a deployment that uses labels and selectors to choose which nodes the master decides to run a given pod on.

To add a nodeSelector to a deployment, under spec in deployment.yaml, we add a nodeSelector field, with the labels that we want to select by:

spec:
    containers:
        ....
    nodeSelector:
        {label key}: {label value}

This tells kubernetes that when it is looking for nodes to deploy that deployment onto, it should look for a label on the node with the given key and value pair.

Applying the deployment

To apply the deployment specified in our deployment.yaml to our cluster, we run the command

kubectl apply -f {path to deployment.yaml}

Listing deployments

kubectl get deployments

Viewing status of deployment rollouts

kubectl rollout status deployment {deployment name}

Setting the image of a deployment

kubectl set image deployment/{deployment name} {container naem}={image name}:{image tag}

Viewing the history of a rollout, including previous versions

``` kubectl rollout history deployment/{deployment name}