The most common and recommended way to run code on Kubernetes is with a deployment, which is managed by a deployment controller. We will explore deployments in the next and further chapters, both specifying them directly and creating them implicitly with commands such as kubectl run.
A Pod by itself is interesting, but limited, specifically because it is intended to be ephemeral. If a Node were to die (or get powered down), all the Pods on that Node would stop running. ReplicaSets provide self-healing capabilities. The work within the cluster to recognize when a Pod is no longer available and will attempt to schedule another Pod, typically to bring a service back online, or otherwise continue doing work.
The deployment controller wraps around and extends the ReplicaSet controller, and is primarily responsible for rolling out software updates and managing the process of that rollout when you update your deployment resource with new versions of your software. The deployment controller includes metadata settings to know how many Pods to keep running so that you can enable a seamless rolling update of your software by adding new versions of a container, and stopping old versions when you request it.
Kubernetes resources can generally be represented as either a JSON or YAML data structure. Kubernetes is specifically built so that you can save these files, and when you want to run your software, you can use a command such askubectl deployand provide the definitions you've created previously, and it uses that to run your software. In our next chapter, we will start to show specific examples of these resources and build them up for our use.
As we get into the examples in the next, and future chapters, we will use YAML to describe our resources and request data through kubectl back in JSON format. All of these data structures are formally defined for each version of Kubernetes, along with the REST APIs that Kubernetes provides to manipulate them. The formal definitions of all Kubernetes resources are maintained with OpenAPI (also known as Swagger) in source code control, and can be viewed at https://github.com/kubernetes/kubernetes/tree/master/api/swagger-spec.
