Chapter 5. Exploring Kubernetes Storage Concepts

In order to power modern microservices and other stateless applications, Kubernetes operators need to have a way to manage stateful data storage on the cluster. While it's advantageous to maintain as much state as possible outside of the cluster in dedicated database clusters as a part of cloud-native service offerings, there's often a need to keep a statement of record or state cluster for stateless and ephemeral services. We'll explore what's considered a more difficult problem in the container orchestration and scheduling world: managing locality-specific, mutable data in a world that relies on declarative state, decoupling physical devices from logical objects, and immutable approaches to system updates. We'll explore strategies for setting up reliable, replicated storage for modern database engines.

In this chapter, we will discuss how to attach persistent volumes and create storage for stateful applications and data. We will walk through...

Technical requirements

You'll need to have a running Kubernetes cluster to go through these examples. Please start your cluster up on your cloud provider of choice, or a local Minikube instance.

The code for this repository can be found here: https://github.com/PacktPublishing/Getting-Started-with-Kubernetes-third-edition/tree/master/Code-files/Chapter05.

Persistent storage

So far, we only worked with workloads that we could start and stop at will, with no issue. However, real-world applications often carry state and record data that we prefer (even insist) not to lose. The transient nature of containers themselves can be a big challenge. If you recall our discussion of layered filesystems in Chapter 1, Introduction to Kubernetes, the top layer is writable. (It's also frosting, which is delicious.) However, when the container dies, the data goes with it. The same is true for crashed containers that Kubernetes restarts.

This is where volumes or disks come into play. Volumes exist outside the container and are coupled to the pod, which allows us to save our important data across containers outages. Further more, if we have a volume at the pod level, data can be shared between containers in the same application stack and within the same pod. A volume itself on Kubernetes is a directory, which the Pod provides to the containers running on it....

StatefulSets

The purpose of StatefulSets is to provide some consistency and predictability to application deployments with stateful data. Thus far, we have deployed applications to the cluster, defining loose requirements around required resources such as compute and storage. The cluster has scheduled our workload on any node that can meet these requirements. While we can use some of these constraints to deploy in a more predictable manner, it will be helpful if we had a construct built to help us provide this consistency.

 

Note

StatefulSets were set to GA in 1.6 as we went to press. There were previously beta in version 1.5 and were known as Pet Sets prior to that (alpha in 1.3 and 1.4). 

This is where StatefulSets come in. StatefulSets provide us first with numbered and reliable naming for both network access and storage claims. The pods themselves are named with the following convention, where N is from 0 to the number of replicas:

"Name of Set"-N

This means that a StatefulSet called db with...

Summary

In this chapter, we explored a variety of persistent storage options and how to implement them with our pods. We looked at PersistentVolumes and also PersistentVolumeClaim, which allow us to separate storage provisioning and application storage requests. Additionally, we looked at StorageClasses for provisioning groups of storage according to a specification.

We also explored the new StatefulSets abstraction and learned how we can deploy stateful applications in a consistent and ordered manner. In the next chapter, we will look at how to integrate Kubernetes with Continuous Integration and Delivery pipelines.

Questions

1. Name four kinds of volumes that Kubernetes supports
2. What's the parameter that you can use to enable a simple, semi-persistent temporary disk?
3. Name two backing technologies that make PersistentVolumes easy to implement with Cloud Service Providers (CSPs)
4. What's a good reason for creating different types of StorageClasses?
5. Name two phases in the PersistentVolume and PersistentVolumeClaim lifecycle
6. Which Kubernetes object is used to provide a stateful storage-based application?

Further reading

• If you'd like to know more about dynamic storage provisioning, please read this blog post: https://kubernetes.io/blog/2017/03/dynamic-provisioning-and-storage-classes-kubernetes/
• If you'd like to know more about the cutting edge of the Storage Special Interest Group (SIG), you can read about it here: https://github.com/kubernetes/community/tree/master/sig-storage