What is a Kubernetes cluster?

A Kubernetes cluster is a set of nodes that run containerized applications. Containerizing applications packages an app with its dependences and some necessary services. They are more lightweight and flexible than virtual machines. In this way, Kubernetes clusters allow for applications to be more easily developed, moved and managed. 

Kubernetes clusters allow containers to run across multiple machines and environments: virtual, physical, cloud-based, and on-premises. Kubernetes containers are not restricted to a specific operating system, unlike virtual machines. Instead, they are able to share operating systems and run anywhere.

Kubernetes clusters are comprised of one master node and a number of worker nodes. These nodes can either be physical computers or virtual machines, depending on the cluster.

The master node controls the state of the cluster; for example, which applications are running and their corresponding container images. The master node is the origin for all task assignments. It coordinates processes such as:

• Scheduling and scaling applications
  
• Maintaining a cluster’s state
• Implementing updates

The worker nodes are the components that run these applications. Worker nodes perform tasks assigned by the master node. They can either be virtual machines or physical computers, all operating as part of one system.

There must be a minimum of one master node and one worker node for a Kubernetes cluster to be operational. For production and staging, the cluster is distributed across multiple worker nodes. For testing, the components can all run on the same physical or virtual node.

A namespace is a way for a Kubernetes user to organize many different clusters within just one physical cluster. Namespaces enable users to divide cluster resources within the physical cluster among different teams via resource quotas. For this reason, they are ideal in situations involving complex projects or multiple teams. 

A Kubernetes cluster contains six main components:

1. API server: Exposes a REST interface to all Kubernetes resources. Serves as the front end of the Kubernetes control plane.
   
2. Scheduler: Places containers according to resource requirements and metrics. Makes note of Pods with no assigned node, and selects nodes for them to run on.
3. Controller manager: Runs controller processes and reconciles the cluster’s actual state with its desired specifications. Manages controllers such as node controllers, endpoints controllers and replication controllers.
4. Kubelet: Ensures that containers are running in a Pod by interacting with the Docker engine , the default program for creating and managing containers. Takes a set of provided PodSpecs and ensures that their corresponding containers are fully operational.
5. Kube-proxy: Manages network connectivity and maintains network rules across nodes. Implements the Kubernetes Service concept across every node in a given cluster.
6. Etcd: Stores all cluster data. Consistent and highly available Kubernetes backing store. 

These six components can each run on Linux or as Docker containers. The master node runs the API server, scheduler and controller manager, and the worker nodes run the kubelet and kube-proxy.

To work with a Kubernetes cluster, you must first determine its desired state. The desired state of a Kubernetes cluster defines many operational elements, including:

• Applications and workloads that should be running
• Images that these applications will need to use
• Resources that should be provided for these apps
• Quantity of needed replicas

To define a desired state, JSON or YAML files (called manifests) are used to specify the application type and the number of replicas needed to run the system.

Developers use the Kubernetes API to define a cluster’s desired state. This developer interaction uses the command line interface (kubectl) or leverages the API to directly interact with the cluster to manually set the desired state. The master node will then communicate the desired state to the worker nodes via the API.

Kubernetes automatically manages clusters to align with their desired state through the Kubernetes control plane. Responsibilities of a Kubernetes control plane include scheduling cluster activity and registering and responding to cluster events.

The Kubernetes control plane runs continuous control loops to ensure that the cluster’s actual state matches its desired state. For example, if you deploy an application to run with five replicas, and one of them crashes, the Kubernetes control plane will register this crash and deploy an additional replica so that the desired state of five replicas is maintained.

Automation occurs via the Pod Lifecycle Event Generator, or PLEG. These automatic tasks can include:

• Starting and restarting containers
• Adjusting the number of replicas for an application
• Validating container images
• Launching and managing containers
• Implementing updates and rollbacks