Kubernetes (commonly stylized as "K8s") is an open-source system for automating deployment, scaling and management of containerized applications that was originally designed by Google and now maintained by the Cloud Native Computing Foundation. It aims to provide a "platform for automating deployment, scaling, and operations of application containers across clusters of hosts". It works with a range of container tools, including Docker.
Video Kubernetes
History
Kubernetes (??????????, Greek for "helmsman" or "pilot") was founded by Joe Beda, Brendan Burns and Craig McLuckie, was quickly joined by other Google engineers including Brian Grant and Tim Hockin, and was first announced by Google in mid-2014. Its development and design are heavily influenced by Google's Borg system, and many of the top contributors to the project previously worked on Borg. The original codename for Kubernetes within Google was Project Seven, a reference to Star Trek character Seven of Nine that is a 'friendlier' Borg. The seven spokes on the wheel of the Kubernetes logo is a nod to that codename.
Kubernetes v1.0 was released on July 21, 2015. Along with the Kubernetes v1.0 release, Google partnered with the Linux Foundation to form the Cloud Native Computing Foundation (CNCF) and offered Kubernetes as a seed technology.
Rancher Labs includes a Kubernetes distribution in its Rancher container management platform. Kubernetes is also being used by Pivotal for its PKS product, Red Hat for its OpenShift product, CoreOS for its Tectonic product, Mirantis for its Mirantis Cloud Platform, and IBM for its IBM Cloud Container Service and IBM Cloud Private product. Oracle joined the Cloud Native Computing Foundation as a platinum member on September 13, 2017. Oracle open sourced a Kubernetes installer for Oracle Cloud Infrastructure and released Kubernetes on Oracle Linux.
Maps Kubernetes
Design
Kubernetes defines a set of building blocks ("primitives") collectively, which together provide mechanisms for deploying, maintaining, and scaling applications. The components which make up Kubernetes are designed to be loosely coupled and extensible so that it can meet a wide variety of different workloads. The extensibility is provided in large part by the Kubernetes API, which is used by internal components as well as extensions and containers running on Kubernetes.
Pods
The basic scheduling unit in Kubernetes is called a "pod". It adds a higher level of abstraction by grouping containerized components. A pod consists of one or more containers that are guaranteed to be co-located on the host machine and can share resources. Each pod in Kubernetes is assigned a unique IP address within the cluster, which allows applications to use ports without the risk of conflict. A pod can define a volume, such as a local disk directory or a network disk, and expose it to the containers in the pod. Pods can be managed manually through the Kubernetes API, or their management can be delegated to a controller.
Labels and selectors
Kubernetes enables clients (users or internal components) to attach key-value pairs called "labels" to any API object in the system, such as pods and nodes. Correspondingly, "label selectors" are queries against labels that resolve to matching objects.
Labels and selectors are the primary grouping mechanism in Kubernetes, and are used to determine the components to which an operation applies.
For example, if the Pods of an application have labels for a system tier ("front-end", "back-end", for example) and a release_track ("canary", "production", for example), then an operation on all of the "back-end" and "canary" nodes could use a label selector such as the following:
tier=back-end AND release_track=canary
Controllers
A controller is a reconciliation loop that drives actual cluster state toward the desired cluster state. It does this by managing a set of pods. One kind of controller is a "Replication Controller," which handles replication and scaling by running a specified number of copies of a pod across the cluster. It also handles creating replacement pods if the underlying node fails. Other controllers that are part of the core Kubernetes system include a "DaemonSet Controller" for running exactly one pod on every machine (or some subset of machines), and a "Job Controller" for running pods that run to completion, e.g. as part of a batch job. The set of pods that a controller manages is determined by label selectors that are part of the controller's definition.
Services
A Kubernetes service is a set of pods that work together, such as one tier of a multi-tier application. The set of pods that constitute a service are defined by a label selector. Kubernetes provides service discovery and request routing by assigning a stable IP address and DNS name to the service, and load balances traffic in a round-robin manner to network connections of that IP address among the pods matching the selector (even as failures cause the pods to move from machine to machine). By default a service is exposed inside a cluster (e.g. back end pods might be grouped into a service, with requests from the front-end pods load-balanced among them), but a service can also be exposed outside a cluster (e.g. for clients to reach frontend pods)
Architecture
Kubernetes follows the master-slave architecture. The components of Kubernetes can be divided into those that manage an individual node and those that are part of the control plane.
Kubernetes control plane
The Kubernetes Master is the main controlling unit of the cluster that manages its workload and directs communication across the system. The Kubernetes control plane consists of various components, each its own process, that can run both on a single master node or on multiple masters supporting high-availability clusters. The various components of Kubernetes control plane are as follows:
etcd
etcd is a persistent, lightweight, distributed, key-value data store developed by CoreOS that reliably stores the configuration data of the cluster, representing the overall state of the cluster at any given point of time. Other components watch for changes to this store to bring themselves into the desired state.
API server
The API server is a key component and serves the Kubernetes API using JSON over HTTP, which provides both the internal and external interface to Kubernetes. The API server processes and validates REST requests and updates state of the API objects in etcd, thereby allowing clients to configure workloads and containers across Worker nodes.
Scheduler
The scheduler is the pluggable component that selects which node an unscheduled pod (the basic entity managed by the scheduler) should run on based on resource availability. Scheduler tracks resource utilization on each node to ensure that workload is not scheduled in excess of the available resources. For this purpose, the scheduler must know the resource requirements, resource availability and a variety of other user-provided constraints and policy directives such as quality-of-service, affinity/anti-affinity requirements, data locality and so on. In essence, the scheduler's role is to match resource "supply" to workload "demand".
Controller manager
The controller manager is the process that the core Kubernetes controllers like DaemonSet Controller and Replication Controller run in. The controllers communicate with the API server to create, update and delete the resources they manage (pods, service endpoints, etc.)
Kubernetes node
The Node, also known as Worker or Minion, is a machine where containers (workloads) are deployed. Every node in the cluster must run a container runtime such as Docker, as well as the below-mentioned components, for communication with master for network configuration of these containers.
Kubelet
Kubelet is responsible for the running state of each node, ensuring that all containers on the node are healthy. It takes care of starting, stopping, and maintaining application containers organized into pods as directed by the control plane.
Kubelet monitors the state of a pod and if not in the desired state, the pod will be redeployed to the same node. The node status is relayed every few seconds via heartbeat messages to the master. Once the master detects a node failure, the Replication Controller observes this state change and launches pods on other healthy nodes.
Container
A container resides inside a Pod. The container is the lowest level of a micro-service which holds the running application, the libraries and their dependencies. Containers can be exposed to the world through an external IP address.
Kube-proxy
The Kube-proxy is an implementation of a network proxy and a load balancer, and it supports the service abstraction along with other networking operation. It is responsible for routing traffic to the appropriate container based on IP and port number of the incoming request.
cAdvisor
cAdvisor is an agent that monitors and gathers resource usage and performance metrics such as CPU, memory, file and network usage of containers on each node.
References
External links
- Official website
- kubernetes on GitHub
Source of the article : Wikipedia
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