Getting started with Kubernetes
Pods
A pod is the lowest unit of an application in Kubernetes. Now, before we move on, we need to get one thing straight — and that is a pod is not equal to a container in the Docker world. A pod can be made up of multiple containers. If you have come from a pure Docker background, this can be hard to wrap your head around. If a pod can have more than one container, how does it work? There are some limits we need to be aware of. A pod has the following:
• A single IP address
• Share localhost
• A shared IPC space
• A shared network port range
• Shared volumes
The containers in a pod talk to each other via local host, whereas pod-to-pod communication is done via services.
ReplicaSets
Now in the pod section, we discovered that pods are mortal, and if they die, that is the end of them. What if you want to have three versions of the same pod running for availability?
Enter the replication controller.
The main responsibility for the replication controller is to prevent against failure, and it sits above the pod resource type and controls it. Let’s look at an example. I want to deploy 4 of my pod x, this time I would create a replica set. A replica set has a defined number of pods that needs to be running, in this case 4. If one of the pods fails or dies, the replication controller will start a new pod for me and again, I will have 4 of pod x running. So, this functionality looks after the issue we mentioned earlier about pods being mortal.
Services
If we want to have connectivity to our pods, we will need to create a service. In Kubernetes, a service is a network abstraction over a set of pods. This allows for the traffic to be load balanced for failures. A service allows Kubernetes to set a single DNS record for the pods. As we mentioned earlier, each pod has a separate IP address.
Deployments
The deployment resource type sits above a replica set and can manipulate them. Why would we want to manipulate a replica set? Replica sets are all or nothing. If you need to do an upgrade, you need to replace the replica set. This action will cause downtime to your application.
One of the main benefits of Kubernetes is high availability. Deployments give us the functionality to do upgrades without downtime. As you do in a replica set, you specify the number of pods you would like to run. Once you trigger an update a deployment will do a rolling upgrade on the pods, all while making sure the upgrade is successful on the pod before moving to the next one.
Remember: Deployments control replica sets and replica sets control pods; this means that when you use a deployment resource type, you can’t forget that you still need a service to access it.
Getting started with Kubernetes and deploying a cluster locally
Installing kubectl
Kubernetes’ command-line tool, kubectl, is used to manage a cluster and applications running inside it. Here’s how to install it on Windows, Linux, and Mac:
Windows
Linux
Mac
Verifying your setup
Installing Minikube
Minikube supports several virtualization technologies. We’ll use VirtualBox since it is the only virtualization supported in all operating systems. Please keep in mind that for VirtualBox or HyperV to work, virtualization must be enabled in the BIOS. Most laptops should have it enabled by default.
Windows
Finally, you will not get a command if you are a Windows user. Instead, download the latest release from the minikube-windows-amd64.exe file, rename it to minikube.exe, and add it to your path.
Linux
Mac
Creating a local cluster on Minikube
Minikube makes creating a cluster as easy as it can get. All you need to do is to execute a single command. Minikube will start a virtual machine locally and deploy the necessary Kubernetes components into it. The VM will get configured with Docker and Kubernetes via a single binary called localkube.
rkt is an application container engine developed for modern production cloud-native environments.
The localkube library includes all the components necessary for running Kubernetes. For now, the important thing is that localkube provides everything we need to run a Kubernetes cluster locally.
Remember that this is a single-node cluster running locally on our machine only. With that said, it is still the easiest way to “play” with Kubernetes locally and get to know the tool.
Creating a pod through declarative syntax
First create a local cluster using Minikube like the one shown here:
Let’s take a look at a simple Pod by accessing the db.yml file from this git repository. Now let’s create the Pod defined in the db.yml file.
Let’s take a look at the Pods in the cluster.
The output is as follows, and you can see that our pod is up and running.
Creating services through declarative syntax
Creating the service:
We created the Service and retrieved its information from the API server. The output of the latter command is as follows.
Now that the Service is running, you can double-check that it is working as expected by trying to access MongoDB UI.
Deploying a new release
Again start with creating a cluster:
Let’s create the Deployment.
The output of the latter command is as follows.
Wrapping up
This wraps up our intro to Docker, and Kubernetes.
Both technologies are here to stay and are becoming some of the most in-demand technologies on the market. Mastering it now will help you produce better software, level up your career, and stand out from the crowd.
