Hello Kubernauts! Welcome to the "Kubernetes in a nutshell" blog series :-)

This is the first part that will cover native Kubernetes primitives for managing stateless applications. One of the most common use cases for Kubernetes is to orchestrate and operate stateless services. In Kubernetes, you need a Pod (or a group of Pods in most cases) to represent a service or application - but there is more to it! We will go beyond a basic Pod and get explore other high level components namely ReplicaSets and Deployments.

As always, the code is available on GitHub

You will need a Kubernetes cluster to begin with. This could be a simple, single-node local cluster using minikube, Docker for Mac etc. or a managed Kubernetes service from Azure (AKS), Google, AWS etc. To access your Kubernetes cluster, you will need kubectl, which is pretty easy to install.

e.g. to install kubectl for Mac, all you need is

curl -LO https://storage.googleapis.com/kubernetes-release/release/$(curl -s https://storage.googleapis.com/kubernetes-release/release/stable.txt)/bin/darwin/amd64/kubectl && \
chmod +x ./kubectl && \
sudo mv ./kubectl /usr/local/bin/kubectl

In case you already have the Azure CLI installed, all you need to do is az acs kubernetes install-cli.

If you are interested in learning Kubernetes and Containers using Azure, simply create a free account and get going! A good starting point is to use the quickstarts, tutorials and code samples in the documentation to familiarize yourself with the service. I also highly recommend checking out the 50 days Kubernetes Learning Path. Advanced users might want to refer to Kubernetes best practices or the watch some of the videos for demos, top features and technical sessions.

Let's start off by understanding the concept of a Pod.

Pod

A Pod is the smallest possible abstraction in Kubernetes and it can have one or more containers running within it. These containers share resources (storage, volume) and can communicate with each other over localhost.

Create a simple Pod using the YAML file below.

Pod is just a Kubernetes resource or object. The YAML file is something that describes its desired state along with some basic information - it is also referred to as a manifest, spec (shorthand for specification) or definition.

As a part of the Pod spec, we convey our intent to run nginx in Kubernetes and use the spec.containers.image to point to its container image on DockerHub.

Use the kubectl apply command to submit the Pod information to Kubernetes.

To keep things simple, the YAML file is being referenced directly from the GitHub repo, but you can also download the file to your local machine and use it in the same way.

$ kubectl apply -f https://raw.githubusercontent.com/abhirockzz/kubernetes-in-a-nutshell/master/stateless-apps/kin-stateless-pod.yaml

pod/kin-stateless-1 created

$ kubectl get pods

NAME                               READY   STATUS    RESTARTS   AGE
kin-stateless-1                    1/1     Running   0          10s

This should work as expected. Now, let’s delete the Pod and see what happens. For this, we will need to use kubectl delete pod <pod_name>

$ kubectl delete pod kin-stateless-1
pod "kin-stateless-1" deleted

$ kubectl get pods
No resources found.

For serious applications, you have to take care of the following aspects:

• High availability and resiliency — Ideally, your application should be robust enough to self-heal and remain available in face of failure e.g. Pod deletion due to node failure, etc.
• Scalability — What if a single instance of your app (Pod) does not suffice? Wouldn’t you want to run replicas/multiple instances?

Once you have multiple application instances running across the cluster, you will need to think about:

• Scale — Can you count on the underlying platform to handle horizontal scaling automatically?
• Accessing your application — How do clients (internal or external) reach your application and how is the traffic regulated across multiple instances (Pods)?
• Upgrades — How can you handle application updates in a non-disruptive manner i.e. without downtime?

Enough about problems. Let’s look into some possible solutions!

Pod Controllers

Although it is possible to create Pods directly, it makes sense to use higher-level components that Kubernetes provides on top of Pods in order to solve the above mentioned problems. In simple words, these components (also called Controllers) can create and manage a group of Pods.

The following controllers work in the context of Pods and stateless apps:

• ReplicaSet
• Deployment
• ReplicationController

There are other Pod controllers like StatefulSet, Job, DaemonSet etc. but they are not relevant to stateless apps, hence not discussed here

ReplicaSet

A ReplicaSet can be used to ensure that a fixed number of replicas/instances of your application (Pod) are always available. It identifies the group of Pods that it needs to manage with the help of (user-defined) selector and orchestrates them (creates or deletes) to maintain the desired instance count.

Here is what a common ReplicaSet spec looks like

Let's create the ReplicaSet

$ kubectl apply -f  https://raw.githubusercontent.com/abhirockzz/kubernetes-in-a-nutshell/master/stateless-apps/kin-stateless-replicaset.yaml

replicaset.apps/kin-stateless-rs created

$ kubectl get replicasets

NAME               DESIRED   CURRENT   READY   AGE
kin-stateless-rs   2         2         2       1m11s

$ kubectl get pods --selector=app=kin-stateless-rs

NAME                     READY   STATUS    RESTARTS   AGE
kin-stateless-rs-zn4p2   1/1     Running   0          13s
kin-stateless-rs-zxp5d   1/1     Running   0          13s

Our ReplicaSet object (named kin-stateless-rs) was created along with two Pods (notice that the names of the Pods contain a random alphanumeric string e.g. zn4p2)

This was as per what we had supplied in the YAML (spec):

• spec.replicas was set to two
• selector.matchLabels was set to app: kin-stateless-rs and matched the .spec.template.metadata.labels field in the Pod specification.

Labels are simple key-value pairs which can be added to objects (such as a Pod in this case).

We used --selector in the kubectl get command to filter the Pods based on their labels which in this case was app=kin-stateless-rs.

Try deleting one of the Pods (just like you did in the previous case)

Please note that the Pod name will be different in your case, so make sure you use the right one.

$ kubectl delete pod kin-stateless-rs-zxp5d

pod "kin-stateless-rs-zxp5d" deleted

$ kubectl get pods -l=app=kin-stateless-rs

NAME                     READY   STATUS    RESTARTS   AGE
kin-stateless-rs-nghgk   1/1     Running   0          9s
kin-stateless-rs-zn4p2   1/1     Running   0          5m

We still have two Pods! This is because a new Pod (highlighted) was created to satisfy the replica count (two) of the ReplicaSet.

To scale your application horizontally, all you need to do is update the spec.replicas field in the manifest file and submit it again.

As an exercise, try scaling it up to five replicas and then going back to three.

So far so good! But this does not solve all the problems. One of them is handling application updates — specifically, in a way that does not require downtime. Kubernetes provides another component which works on top of ReplicaSets to handle this and more.

Deployment

A Deployment is an abstraction which manages a ReplicaSet — recall from the previous section, that a ReplicaSet manages a group of Pods. In addition to elastic scalability, Deployments provide other useful features that allow you to manage updates, rollback to a previous state, pause and resume the deployment process, etc. Let’s explore these.

A Kubernetes Deployment borrows the following features from its underlying ReplicaSet:

• Resiliency — If a Pod crashes, it is automatically restarted, thanks to the ReplicaSet. The only exception is when you set the restartPolicy in the Pod specification to Never.
• Scaling — This is also taken care of by the underlying ReplicaSet object.

This what a typical Deployment spec looks like

Create the Deployment and see which Kubernetes objects get created

$ kubectl apply -f  https://raw.githubusercontent.com/abhirockzz/kubernetes-in-a-nutshell/master/stateless-apps/kin-stateless-deployment.yaml
deployment.apps/kin-stateless-depl created

$ kubectl get deployment kin-stateless-dp
NAME               READY   UP-TO-DATE   AVAILABLE   AGE
kin-stateless-dp   2/2     2            2           10

$ kubectl get replicasets
NAME                         DESIRED   CURRENT   READY   AGE
kin-stateless-dp-8f9b4d456   2         2         2       12

$ kubectl get pods -l=app=kin-stateless-dp
NAME                               READY   STATUS    RESTARTS   AGE
kin-stateless-dp-8f9b4d456-csskb   1/1     Running   0          14s
kin-stateless-dp-8f9b4d456-hhrj7   1/1     Running   0          14s

Deployment (kin-stateless-dp) got created along with the ReplicaSet and (two) Pods as specified in the spec.replicas field. Great! Now, let’s peek into the Pod to see which nginx version we’re using — please note that the Pod name will be different in your case, so make sure you use the right one

$ kubectl exec kin-stateless-dp-8f9b4d456-csskb -- nginx -v
nginx version: nginx/1.17.3

This is because the latest tag of the nginx image was picked up from DockerHub which happens to be v1.17.3 at the time of writing.

What's kubectl exec? In simple words, it allows you to execute a command in specific container within a Pod. In this case, our Pod has a single container, so we don't need to specify one

Update a Deployment

You can trigger an update to an existing Deployment by modifying the template section of the Pod spec — a common example being updating to a newer version (label) of a container image. You can specify it using spec.strategy.type of the Deployment manifest and valid options are - Rolling update and Recreate.

Rolling update

Rolling updates ensure that you don’t incur application downtime during the update process — this is because the update happens one Pod at a time. There is a point in time where both the previous and current versions of the application co-exist. The old Pods are deleted once the update is complete, but there will a phase where the total number of Pods in your Deployment will be more than the specified replicas count.

It is possible to further tune this behavior using the maxSurge and maxUnavailable settings.

• spec.strategy.rollingUpdate.maxSurge — maximum no. of Pods which can be created in addition to the specified replica count
• spec.strategy.rollingUpdate.maxUnavailable — defines the maximum no. of Pods which are not available

Recreate

This is quite straightforward — the old set of Pods are deleted before the new versions are rolled out. You could have achieved the same results using ReplicaSets by first deleting the old one and then creating a new one with the updated spec (e.g. new docker image etc.)

Let's try and update the application by specifying an explicit Docker image tag — in this case, we'll use 1.16.0. This means that once we update our app, this version should reflect when we introspect our Pod.

Download the Deployment manifest above, update it to change spec.containers.image from nginx to nginx:1.16.0 and submit it to the cluster - this will trigger an update

$ kubectl apply -f deployment.yaml
deployment.apps/kin-stateless-dp configured

$ kubectl get pods -l=app=kin-stateless-dp
NAME                                READY   STATUS    RESTARTS   AGE
kin-stateless-dp-5b66475bd4-gvt4z   1/1     Running   0          49s
kin-stateless-dp-5b66475bd4-tvfgl   1/1     Running   0          61s

You should now see a new set of Pods (notice the names). To confirm the update:

$ kubectl exec kin-stateless-dp-5b66475bd4-gvt4z -- nginx -v
nginx version: nginx/1.16.0

Please note that the Pod name will be different in your case, so make sure you use the right one

Rollback

If things don't go as expected with the current Deployment, you can revert back to the previous version in case the new one is not working as expected. This is possible since Kubernetes stores the rollout history of a Deployment in the form of revisions.

To check the history for the Deployment:

$ kubectl rollout history deployment/kin-stateless-dp

deployment.extensions/kin-stateless-dp

REVISION  CHANGE-CAUSE
1         <none>
2         <none>

Notice that there are two revisions, with 2 being the latest one. We can roll back to the previous one using kubectl rollout undo

$ kubectl rollout undo deployment kin-stateless-dp
deployment.extensions/kin-stateless-dp rolled back

$ kubectl get pods -l=app=kin-stateless-dp
NAME                                READY   STATUS        RESTARTS   AGE
kin-stateless-dp-5b66475bd4-gvt4z   0/1     Terminating   0          10m
kin-stateless-dp-5b66475bd4-tvfgl   1/1     Terminating   0          10m
kin-stateless-dp-8f9b4d456-d4v97    1/1     Running       0          14s
kin-stateless-dp-8f9b4d456-mq7sb    1/1     Running       0          7s

Notice the intermediate state where Kubernetes was busy terminating the Pods of the old Deployment while making sure that new Pods are created in response to the rollback request.

If you check the nginx version again, you will see that the app has indeed been rolled back to 1.17.3.

$ kubectl exec kin-stateless-dp-8f9b4d456-d4v97 -- nginx -v
nginx version: nginx/1.17.3

Pause and Resume

It is also possible to pause a Deployment rollout and resume it back after applying changes to it (during the paused state).

ReplicationController

A ReplicationController is similar to a Deployment or ReplicaSet. However, it is not a recommended approach for stateless app orchestration since a Deployment offers a richer set of capabilities (as described in the previous section). You can read more about them in the Kubernetes documentation.

References

Check out Kubernetes documentation for the API details of the resources we discussed in this post i.e. Pod, ReplicaSet and Deployment

• Pod API reference
• ReplicaSet API reference
• Deployment API reference

Stay tuned for more in the next part of the series!

I really hope you enjoyed and learned something from this article! Please like and follow if you did. Happy to get feedback via @abhi_tweeter or just drop a comment.