Glossary

Kubernetes Object

• An entity in the Kubernetes system.
• The Kubernetes API uses these entities to represent the state of your cluster.
• A Kubernetes object is typically a “record of intent”.
• Once you create the object, the Kubernetes control plane works constantly to ensure that the item it represents actually exists.
• By creating an object, you’re effectively telling the Kubernetes system what you want that part of your cluster’s workload to look like; this is your cluster’s desired state.

Pod

• A Pod is the smallest and simplest Kubernetes object.
• A Pod represents a set of running containers on your cluster.
• A Pod is typically set up to run a single primary containers.
• A Pod can also run optional sidecar containers that add supplementary features like logging.
• Pods are commonly managed by a Deployment.

Node

• Also known as a worker node in Kubernetes.
• Also known as a worker machine in Kubernetes.
• Also known as a minion in early Kubernetes versions.
• A node may be a virtual-machine or physical machine, depending on the cluster.
• A node is managed by the control plane.
• A node has local daemons or services necessary to run Pods.
• The daemons on a node include kubelet, kube-proxy, and a containers runtime implementing the CRI such as Docker.

Control Plane

• In production environments, the control plane usually runs across multiple computers.
• The control plane manages the worker nodes and the Pods in the cluster.
• The container orchestration layer that exposes the API and interfaces to define, deploy, and manage the lifecycle of containers.
• This layer is composed by many different components, such as (but not restricted to):
  • etcd
  • API Server
  • Scheduler
  • Controller Manager
  • Cloud Controller Manager
• These components can be run as traditional operating system services (daemons) or as containers.
• The hosts running these components were historically called masters.

Cluster

• A set of nodes, that run containerized applications.
• Every cluster has at least one worker node.
  • The worker node host the Pods that are the components of the application workload.
• a cluster usually runs multiple nodes, providing fault-tolerance and high availability.

Kubectl

• A command line tool for communicating with a Kubernetes API server.
• You can use kubectl to create, inspect, update, and delete Kubernetes objects.
• An agent that runs on each node in the cluster. It makes sure that containers are running in a Pod.

Kubernetes API

• The application that serves Kubernetes functionality through a RESTful interface and stores the state of the cluster.
• Kubernetes resources and “records of intent” are all stored as API objects, and modified via RESTful calls to the API.
• Users can interact with the Kubernetes API directly, or via tools like kubectl.

API Server

• Also known as kube-apiserver
• The API server is a component of the Kubernetes control plane that exposes the Kubernetes API.
• The API server is the front end for the Kubernetes control plane.
• The main implementation of a Kubernetes API server is kube-apiserver.
• kube-apiserver is designed to scale horizontally—that is, it scales by deploying more instances.
• You can run several instances of kube-apiserver and balance traffic between those instances.

Minikube

• A tool for running Kubernetes locally.
• minikube runs a single-node cluster inside a virtual machine on your computer.
• You can use minikube to try Kubernetes in a testin genvironment.

Workload

• A workload is an application running on Kubernetes.
• Various core objects that represent different types or parts of a workload include the DaemonSet, Deployment, Job, ReplicaSet, and StatefulSet objects.
• For example, a workload that has a web server and a database might run the database in one StatefulSet and the web server in a Deployment.

Container

• A lightweight and portable executable image that contains software and all of its dependencies.
• Containers decouple applications from underlying host infrastructure to make deployment easier in different cloud or OS environments, and for easier scaling.

Image

• Stored instance of a container that holds a set of software needed to run an application.
• A way of packaging software that allows it to be stored in a container registry, pulled to a local system, and run as an application.
• Meta data is included in the image that can indicate what executable to run, who built it, and other information.

Virtual Machine

• In computing, a virtual machine is an emulation of a computer system.

Service

• An abstract way to expose an application running on a set of Pods as a network service.
• The set of Pods targeted by a Service is (usually) determined by a selector.
• If more Pods are added or removed, the set of Pods matching the selector will change.
• The Service makes sure that network traffic can be directed to the current set of Pods for the workload.