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A Beginner's Guide to Kubernetes, Part 4: Controllers

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A Beginner's Guide to Kubernetes

Note
This is the fourth guide in the Beginner’s Guide to Kubernetes series that explains the major parts and concepts of Kubernetes.

A Controller is a control loop that continuously watches the Kubernetes API and tries to manage the desired state of certain aspects of the cluster. There are a number of controllers. Below is a short reference of the most popular controllers you might interact with.

In this guide you will learn about Deployments, ReplicaSets, and Jobs.

Deployments

A Deployment has the ability to keep a defined number of replica Pods up and running. A Deployment can also update those Pods to resemble the desired state by means of rolling updates. For example, if you wanted to update a container image to a newer version, you would create a Deployment, and the controller would update the container images one by one until the desired state is achieved. This ensures that there is no downtime when updating or altering your Pods.

Below is an example of a Deployment:

my-apache-deployment.yaml 
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apiVersion: apps/v1
kind: Deployment
metadata:
  name: apache-deployment
  labels:
    app: web
spec:
  replicas: 5
  selector:
    matchLabels:
      app: web
  template:
    metadata:
      labels:
        app: web
    spec:
      containers:
      - name: apache-container
        image: httpd:2.4.35

As you will see, the only noticeable difference between a Deployment’s manifest and that of a ReplicaSet the kind. In this example we have chosen to initially install Apache 2.4.35. If you wanted to update that image to Apache 2.4.38, you would issue the following command:

kubectl --record deployment.apps/apache-deployment set image deployment.v1.apps/apache-deployment apache-container=httpd:2.4.38

You’ll see a confirmation that the images have been updated:

deployment.apps/apache-deployment image updated

To see for yourself that the images have updated, you can grab the Pod name from the get pods list:

kubectl get pods

NAME                                 READY   STATUS    RESTARTS   AGE
apache-deployment-574c8c4874-8zwgl   1/1     Running   0          8m36s
apache-deployment-574c8c4874-9pr5j   1/1     Running   0          8m36s
apache-deployment-574c8c4874-fbs46   1/1     Running   0          8m34s
apache-deployment-574c8c4874-nn7dl   1/1     Running   0          8m36s
apache-deployment-574c8c4874-pndgp   1/1     Running   0          8m33s

Issue the describe command to view all of the available details of the Pod:

kubectl describe pod apache-deployment-574c8c4874-pndgp

You’ll see a long list of details, of which the container image is included:

....

Containers:
  apache-container:
    Container ID:   docker://d7a65e7993ab5bae284f07f59c3ed422222100833b2769ff8ee14f9f384b7b94
    Image:          httpd:2.4.38

....

For more information on Deployments, visit the Kubernetes Deployments API documentation

ReplicaSets

Note
Kubernetes now recommends the use of Deployments instead of ReplicaSets. Deployments provide declarative updates to Pods, among other features, that allow you to define your application in the spec section. In this way, ReplicaSets have essentially become deprecated.

As has been mentioned, Kubernetes allows an application to scale horizontally. A ReplicaSet is one of the controllers responsible for keeping a given number of replica Pods running. If one Pod goes down in a ReplicaSet, another will be created to replace it. In this way, Kubernetes is self-healing. However, for most use cases it is recommended to use a Deployment instead of a ReplicaSet.

Below is an example of a ReplicaSet:

my-apache-replicaset.yaml 
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apiVersion: apps/v1
kind: ReplicaSet
metadata:
  name: apache-replicaset
  labels:
    app: web
spec:
  replicas: 5
  selector:
    matchLabels:
      app: web
  template:
    metadata:
      labels:
        app: web
    spec:
      containers:
      - name: apache-container
        image: httpd

There are three main things to note in this ReplicaSet. The first is the apiVersion, which is apps/v1. This differs from the previous examples, which were all apiVersion: v1, because ReplicaSets do not exist in the v1 core. They instead reside in the apps group of v1. The second and third things to note are the replicas field and the selector field. The replicas field defines how many replica Pods you want to be running at any given time. The selector field defines which Pods, matched by their label, will be controlled by the ReplicaSet.

To view your ReplicaSets, issue the get replicasets command:

kubectl get replicasets

You should see output like the following:

NAME                DESIRED   CURRENT   READY   AGE
apache-replicaset   5         5         0       5s

This output shows that of the five desired replicas, there are 5 currently active, but zero of those replicas are available. This is because the Pods are still booting up. If you issue the command again, you will see that all five have become ready:

NAME                DESIRED   CURRENT   READY   AGE
apache-replicaset   5         5         5       86s

You can view the Pods the ReplicaSet created by issuing the get pods command:

kubectl get pods

You should see output like the following:

NAME                      READY   STATUS    RESTARTS   AGE
apache-replicaset-5rsx2   1/1     Running   0          31s
apache-replicaset-8n52c   1/1     Running   0          31s
apache-replicaset-jcgn8   1/1     Running   0          31s
apache-replicaset-sj422   1/1     Running   0          31s
apache-replicaset-z8g76   1/1     Running   0          31s

To delete a ReplicaSet, issue the delete replicaset command:

kubectl delete replicaset apache-replicaset

If you issue the get pods command, you will see that the Pods the ReplicaSet created are in the process of terminating:

NAME                      READY   STATUS        RESTARTS   AGE
apache-replicaset-bm2pn   0/1     Terminating   0          3m54s

In the above example, four of the Pods have already terminated, and one is in the process of terminating.

For more information on ReplicaSets, view the Kubernetes ReplicaSets API documentation.

Jobs

A Job is a controller that manages a Pod that is created for a single, or set, of tasks. This is handy if you need to create a Pod that performs a single function, or calculates a value. The deletion of the Job will delete the Pod.

Below is an example of a Job that simply prints “Hello World!” and ends:

my-job.yaml 
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apiVersion: batch/v1
kind: Job
metadata:
  name: hello-world
spec:
  template:
    metadata:
      name: hello-world
    spec:
      containers:
      - name: output
        image: debian
        command:
         - "bin/bash"
         - "-c"
         - "echo 'Hello World!'"
      restartPolicy: Never

To create the Job, issue the create command:

kubectl create -f my-job.yaml

To see if the job has run, or is running, issue the get jobs command:

kubectl get jobs

You should see output like the following:

NAME          COMPLETIONS   DURATION   AGE
hello-world   1/1           9s         8m23s

To get the Pod of the Job, issue the get pods command:

kubectl get pods

You should see an output like the following:

NAME                               READY   STATUS             RESTARTS   AGE
hello-world-4jzdm                  0/1     Completed          0          9m44s

You can use the name of the Pod to inspect its output by consulting the log file for the Pod:

kubectl get logs hello-world-4jzdm

To delete the Job, and its Pod, issue the delete command:

kubectl delete job hello-world

Next Steps

There are other controllers not listed in this guide that you may find useful. Visit the official Kubernetes documentation for more information.

To continue in the Beginner’s Guide to Kubernetes series, visit part 5:

More Information

You may wish to consult the following resources for additional information on this topic. While these are provided in the hope that they will be useful, please note that we cannot vouch for the accuracy or timeliness of externally hosted materials.

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This guide is published under a CC BY-ND 4.0 license.