Overview of Containers and Kubernetes

A container is a lightweight virtual machine, running the Linux operating system. A container usually runs one application that is started from an image. Files that are created and modified are usually not persistent. However, persistent storage is available. Containers are a key component of cloud computing environments.

Kubernetes is a portable, extensible, open-source platform for managing containerized workloads and services, that facilitates both declarative configuration and automation. Kubernetes has the ability to manage the resources of multiple hosts (called Nodes) in a cluster, and to run containers as required across these nodes. It can automatically spawn containers to react to various failures. Kubernetes also manages the networking among the containers and to the outside world. Kubernetes is portable across many cloud and on-premises environments.

Key Kubernetes concepts include:

• Pod: One or more containers that share an IP address. For more information on Pods, see:

  https://kubernetes.io/docs/concepts/workloads/pods/pod/

• Deployment: A named collection of n identical Pods (where n is the number of Pods). Kubernetes ensures that n identical Pods are running. For more information on Deployments, see:

  https://kubernetes.io/docs/concepts/workloads/controllers/deployment/

• PersistentVolume: Storage that can be mounted to a Pod and is persistent beyond the lifetime of Pod. For more information on Persistent Volumes, see:

  https://kubernetes.io/docs/concepts/storage/persistent-volumes/

• StatefulSet: Similar to a Deployment, but each Pod has an associated PersistentVolume. For more information on StatefulSets, see:

  https://kubernetes.io/docs/concepts/workloads/controllers/statefulset/

• Service: A network endpoint for a Deployment or StatefulSet. It defines the set of addresses and ports that should be exposed to applications in the Kubernetes cluster. For more information on a Service, see:

  https://kubernetes.io/docs/concepts/services-networking/service/

Kubernetes provides the facilities for the provisioning of Pods and other Kubernetes resources that are required to deploy applications. Once deployed, the objects must be monitored and managed.

Kubernetes does some monitoring and managing of applications, but not all. It does handle problems at the Pod level automatically. For example, if a container fails, Kubernetes restarts it automatically. If an entire Node fails, Kubernetes starts replacement Pods on the other Nodes. However, Kubernetes has no knowledge about problems inside a container. This is not problematic for stateless applications, but for databases (which are stateful), Kubernetes needs help managing what is inside the containers.

This help comes in the form of:

• Custom Resource Definition

• Kubernetes Operator

Custom Resource Definition

A Custom Resource Definition (commonly known as a CRD) extends the Kubernetes' object model. It adds a new object type to the Kubernetes cluster, similar to the Pod, the StatefulSet, and the Service object types that it natively supports.

Kubernetes Operator

A Kubernetes Operator (also called Operator) is the brains behind a CRD. An Operator is an application that performs the functions of a human computer operator. It starts, stops, monitors, and manages other applications.

An Operator runs in one or more Pods, one active and the others idle. The active Operator performs the work. The remaining Operators are idle and remain idle until the active Operator fails. The active Operator manages all objects of a particular type and when combined with a CRD enables you to add custom facilities to Kubernetes.