Understanding deployments

Deployment allows you to make declarative changes to pods and ReplicaSets. You can provide a desired state for the deployment, and the deployment controller will incrementally change the actual state to the desired state.

Deployments can be used to create new ReplicaSets or to replace existing deployments with new deployments. When a new version is ready to go live in production, the deployment can easily handle the upgrade with no downtime by using predefined rules. The following diagram shows an example of a deployment:

Figure 1.8 – A deployment

The following is an example of a deployment. It creates a ReplicaSet to bring up three nginx pods:

apiVersion: apps/v1
kind: Deployment
metadata:
  name: nginx-sample-deployment
  labels:
    app: nginx
spec:
  replicas: 3
  selector:
    matchLabels:
      app: nginx
  template:
    metadata:
      labels:
        app: nginx
    spec:
      containers:
      - name: nginx
        image: nginx:1:21
        ports:
        - containerPort: 80

In the preceding example, the following occurred:

• A deployment called nginx-sample-deployment is created, as indicated by the metadata.name field.
• The image for this deployment is set by the Spec.containers.image field (nginx:latest).
• The deployment creates three replicated pods, as indicated by the replicas field.

The most commonly used kubectl commands concerning deployment are as follows:

• The apply command creates the pod:

  kubectl apply -f FILENAME.

For example, the kubectl apply -f ./nginx-deployment.yaml command will create a new deployment from the nginx-deployment.yaml YAML file.

• The get deployments command checks the status of the deployment:

  kubectl get deployments 

This will produce the following output:

NAME               READY   UP-TO-DATE   AVAILABLE   AGE
nginx-sample-deployment   3/3     0            0           1s

The following fields are displayed:

• NAME indicates the names of the deployments in the namespace.
• READY shows how many replicas of the application are available.
• UP-TO-DATE shows the number of replicas that have been updated to achieve the desired state.
• AVAILABLE shows the number of available replicas.

• AGE indicates the length of time the application has been running.
• The describe deployments command indicates the details of the deployment:

  kubectl describe deployments

• The delete command removes the deployment that was made by the apply command:

  kubectl delete -f FILENAME.

With that, we have learned that deployments are used to define the life cycle of an application, including which container images to use, how many pods you should have, and how they should be updated. In the next section, we will look at StatefulSets and DaemonSets.