Deployment

Deployment is an higher level object in k8s that defines a desired state for a ReplicaSet.

It is used for stateless application.

Use Cases:

There are many use cases for Deployments, such as:

Easily scale an application up or down by changing the number of replicas.

Perform rolling updates to deploy a new software version.

Roll back to a previous software version.

There are two deployment strategies:

Recreate: Bring down all the running containers and then bring up the newer version (application downtime)

Rolling Update (default): Bring down a container and then bring up a new container one at a time (no application downtime)

Other

Provides the capability to upgrade the instances seamlessly using rolling updates, rollback to previous versions seamlessly, undo, pause and resume changes as required.

Abstraction over

Blueprint for stateless pods (application layer)

When a deployment is created, it automatically creates a which in turn creates pods. If we run k get all we can see the resources created by deployment.


apiVersion: apps/v1
kind: Deployment
metadata:
  name: httpd-frontend
  labels:
    name: frontend
spec:
  replicas: 3
  selector:
    matchLabels:
      name: frontend
  template:
    metadata:
      labels:
        name: frontend
    spec:
      containers:
      - name: httpd
        image: httpd:2.4-alpine

⛔

Update the kind from Replicaset to Deployment in a Replicaset config file.

Deployment Strategy

Rollout and Versioning

When you first create a deployment, it triggers a rollout which creates the first revision of the deployment. Every subsequent update to the deployment triggers a rollout which creates a new revision of the deployment. This keeps a track of the deployment and helps us rollback to a previous version of the deployment if required.

When we upgrade the version of a deployment, a new replica set is created under the hood where the new pods are spawned while bringing down pods from the old replica set one at a time, following a rolling update strategy. We can see the new and old replica sets by running k get replicasets

k rollout status deployment <deployment-name> - view the status of rollout for a deployment

k rollout history deployment <deployment-name> - view the history of rollouts for a deployment

k rollout history deployment <deployment-name> --revision=1 - view the status of rollouts for a deployment revision

Rollback

When we rollback to the previous version of a deployment, the pods in the current replica set are brought down one at a time while spawning pods in the previous replica set.

Rollback a deployment - k rollout undo deployment <deployment-name>

Rollback a deployment to a specific revision - k rollout undo deployment <deployment-name> --to-revision=1