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authorBrian Grant <bgrant0607@users.noreply.github.com>2017-02-21 11:03:52 -0800
committerGitHub <noreply@github.com>2017-02-21 11:03:52 -0800
commitd1160c0c58e45ca980fa0953d3504dcbf7e3f3c9 (patch)
treeb05a7942ab8f654fb38d511af3e6a899f842baef
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-# Kubernetes Design and Architecture
+# Kubernetes Design Documents and Proposals
-## Overview
+This directory contains Kubernetes design documents and accepted design proposals.
-Kubernetes is production-grade, open-source infrastructure for the deployment, scaling,
-management, and composition of application containers across clusters of hosts, inspired
-by [previous work at Google](https://research.google.com/pubs/pub44843.html). Kubernetes
-is more than just a “container orchestrator”. It aims to eliminate the burden of orchestrating
-physical/virtual compute, network, and storage infrastructure, and enable application operators
-and developers to focus entirely on container-centric primitives for self-service operation.
-Kubernetes also provides a stable, portable foundation (a platform) for building customized
-workflows and higher-level automation.
+For a design overview, please see [the architecture document](architecture.md).
-Kubernetes is primarily targeted at applications composed of multiple containers. It therefore
-groups containers using *pods* and *labels* into tightly coupled and loosely coupled formations
-for easy management and discovery.
+Note that a number of these documents are historical and may be out of date or unimplemented.
-## Scope
+TODO: Add the current status to each document and clearly indicate which are up to date.
-Kubernetes is a [platform for deploying and managing containers]
-(https://kubernetes.io/docs/whatisk8s/). Kubernetes provides a container runtime, container
-orchestration, container-centric infrastructure orchestration, self-healing mechanisms such as health checking and re-scheduling, and service discovery and load balancing.
-
-Kubernetes aspires to be an extensible, pluggable, building-block OSS
-platform and toolkit. Therefore, architecturally, we want Kubernetes to be built
-as a collection of pluggable components and layers, with the ability to use
-alternative schedulers, controllers, storage systems, and distribution
-mechanisms, and we're evolving its current code in that direction. Furthermore,
-we want others to be able to extend Kubernetes functionality, such as with
-higher-level PaaS functionality or multi-cluster layers, without modification of
-core Kubernetes source. Therefore, its API isn't just (or even necessarily
-mainly) targeted at end users, but at tool and extension developers. Its APIs
-are intended to serve as the foundation for an open ecosystem of tools,
-automation systems, and higher-level API layers. Consequently, there are no
-"internal" inter-component APIs. All APIs are visible and available, including
-the APIs used by the scheduler, the node controller, the replication-controller
-manager, Kubelet's API, etc. There's no glass to break -- in order to handle
-more complex use cases, one can just access the lower-level APIs in a fully
-transparent, composable manner.
-
-## Goals
-
-The project is committed to the following (aspirational) [design ideals](principles.md):
-* _Portable_. Kubernetes runs everywhere -- public cloud, private cloud, bare metal, laptop --
- with consistent behavior so that applications and tools are portable throughout the ecosystem
- as well as between development and production environments.
-* _General-purpose_. Kubernetes should run all major categories of workloads to enable you to run
- all of your workloads on a single infrastructure, stateless and stateful, microservices and
- monoliths, services and batch, greenfield and legacy.
-* _Meet users partway_. Kubernetes doesn’t just cater to purely greenfield cloud-native
- applications, nor does it meet all users where they are. It focuses on deployment and management
- of microservices and cloud-native applications, but provides some mechanisms to facilitate
- migration of monolithic and legacy applications.
-* _Flexible_. Kubernetes functionality can be consumed a la carte and (in most cases) Kubernetes
- does not prevent you from using your own solutions in lieu of built-in functionality.
-* _Extensible_. Kubernetes enables you to integrate it into your environment and to add the
- additional capabilities you need, by exposing the same interfaces used by built-in
- functionality.
-* _Automatable_. Kubernetes aims to dramatically reduce the burden of manual operations. It
- supports both declarative control by specifying users’ desired intent via its API, as well as
- imperative control to support higher-level orchestration and automation. The declarative
- approach is key to the system’s self-healing and autonomic capabilities.
-* _Advance the state of the art_. While Kubernetes intends to support non-cloud-native
- applications, it also aspires to advance the cloud-native and DevOps state of the art, such as
- in the [participation of applications in their own management]
- (http://blog.kubernetes.io/2016/09/cloud-native-application-interfaces.html). However, in doing
- so, we strive not to force applications to lock themselves into Kubernetes APIs, which is, for
- example, why we prefer configuration over convention in the [downward API]
- (https://kubernetes.io/docs/user-guide/downward-api/). Additionally, Kubernetes is not bound by
- the lowest common denominator of systems upon which it depends, such as container runtimes and
- cloud providers. An example where we pushed the envelope of what was achievable was in its [IP
- per Pod networking model](https://kubernetes.io/docs/admin/networking/#kubernetes-model).
-
-## Architecture
-
-A running Kubernetes cluster contains node agents (kubelet) and a cluster control plane (AKA
-*master*), with cluster state backed by a distributed storage system
-([etcd](https://github.com/coreos/etcd)).
-
-### Cluster control plane (AKA *master*)
-
-The Kubernetes [control plane](https://en.wikipedia.org/wiki/Control_plane) is split
-into a set of components, which can all run on a single *master* node, or can be replicated
-in order to support high-availability clusters, or can even be run on Kubernetes itself (AKA
-[self-hosted](self-hosted-kubernetes.md#what-is-self-hosted)).
-
-Kubernetes provides a REST API supporting primarily CRUD operations on (mostly) persistent resources as the nucleus of its control plane. Kubernetes’s API provides IaaS-like
-container-centric primitives such as [Pods](https://kubernetes.io/docs/user-guide/pods/),
-[Services](https://kubernetes.io/docs/user-guide/services/), and [Ingress]
-(https://kubernetes.io/docs/user-guide/ingress/), and also lifecycle APIs to support orchestration
-(self-healing, scaling, updates, termination) of common types of workloads, such as [ReplicaSet]
-(https://kubernetes.io/docs/user-guide/replicasets/) (simple fungible/stateless app manager),
-[Deployment](https://kubernetes.io/docs/user-guide/deployments/) (orchestrates updates of
-stateless apps), [Job](https://kubernetes.io/docs/user-guide/jobs/) (batch), [CronJob]
-(https://kubernetes.io/docs/user-guide/cron-jobs/) (cron), [DaemonSet]
-(https://kubernetes.io/docs/admin/daemons/) (cluster services), and [StatefulSet]
-(https://kubernetes.io/docs/concepts/abstractions/controllers/statefulsets/) (stateful apps).
-We deliberately decoupled service naming/discovery and load balancing from application
-implementation, since the latter is diverse and open-ended.
-
-Both user clients and components containing asynchronous controllers interact with the same API resources, which serve as coordination points, common intermediate representation, and shared state. Most resources contain metadata, including [labels](https://kubernetes.io/docs/user-guide/labels/) and [annotations](https://kubernetes.io/docs/user-guide/annotations/), fully elaborated desired state (spec), including default values, and observed state (status).
-
-Controllers work continuously to drive the actual state towards the desired state, while reporting back the currently observed state for users and for other controllers.
-
-While the controllers are [level-based]
-(http://gengnosis.blogspot.com/2007/01/level-triggered-and-edge-triggered.html) to maximize fault
-tolerance, they typically `watch` for changes to relevant resources in order to minimize reaction
-latency and redundant work. This enables decentralized and decoupled
-[choreography-like](https://en.wikipedia.org/wiki/Service_choreography) coordination without a
-message bus.
-
-#### API Server
-
-The [API server](https://kubernetes.io/docs/admin/kube-apiserver/) serves up the
-[Kubernetes API](https://kubernetes.io/docs/api/). It is intended to be a relatively simple
-server, with most/all business logic implemented in separate components or in plug-ins. It mainly
-processes REST operations, validates them, and updates the corresponding objects in `etcd` (and
-perhaps eventually other stores). Note that, for a number of reasons, Kubernetes deliberately does
-not support atomic transactions across multiple resources.
-
-Kubernetes cannot function without this basic API machinery, which includes:
-* REST semantics, watch, durability and consistency guarantees, API versioning, defaulting, and
- validation
-* Built-in admission-control semantics, synchronous admission-control hooks, and asynchronous
- resource initialization
-* API registration and discovery
-
-Additionally, the API server acts as the gateway to the cluster. By definition, the API server
-must be accessible by clients from outside the cluster, whereas the nodes, and certainly
-containers, may not be. Clients authenticate the API server and also use it as a bastion and
-proxy/tunnel to nodes and pods (and services).
-
-#### Cluster state store
-
-All persistent cluster state is stored in an instance of `etcd`. This provides a way to store
-configuration data reliably. With `watch` support, coordinating components can be notified very
-quickly of changes.
-
-
-#### Controller-Manager Server
-
-Most other cluster-level functions are currently performed by a separate process, called the
-[Controller Manager](https://kubernetes.io/docs/admin/kube-controller-manager/). It performs
-both lifecycle functions (e.g., namespace creation and lifecycle, event garbage collection,
-terminated-pod garbage collection, cascading-deletion garbage collection, node garbage collection)
-and API business logic (e.g., scaling of pods controlled by a [ReplicaSet]
-(https://kubernetes.io/docs/user-guide/replicasets/)).
-
-The application management and composition layer, providing self-healing, scaling, application lifecycle management, service discovery, routing, and service binding and provisioning.
-
-These functions may eventually be split into separate components to make them more easily
-extended or replaced.
-
-#### Scheduler
-
-
-Kubernetes enables users to ask a cluster to run a set of containers. The scheduler
-component automatically chooses hosts to run those containers on.
-
-The scheduler watches for unscheduled pods and binds them to nodes via the `/binding` pod
-subresource API, according to the availability of the requested resources, quality of service
-requirements, affinity and anti-affinity specifications, and other constraints.
-
-Kubernetes supports user-provided schedulers and multiple concurrent cluster schedulers,
-using the shared-state approach pioneered by [Omega]
-(https://research.google.com/pubs/pub41684.html). In addition to the disadvantages of
-pessimistic concurrency described by the Omega paper, [two-level scheduling models]
-(http://mesos.berkeley.edu/mesos_tech_report.pdf) that hide information from the upper-level
-schedulers need to implement all of the same features in the lower-level scheduler as required by
-all upper-layer schedulers in order to ensure that their scheduling requests can be satisfied by
-available desired resources.
-
-
-### The Kubernetes Node
-
-The Kubernetes node has the services necessary to run application containers and
-be managed from the master systems.
-
-Each node runs a container runtime (like docker, rkt, or Hyper). The container
-runtime is responsible for downloading images and running containers.
-
-#### Kubelet
-
-
-The most important and most prominent controller in Kubernetes is the Kubelet, which is the
-primary implementer of the Pod and Node APIs that drive the container execution layer. Without
-these APIs, Kubernetes would just be a CRUD-oriented REST application framework backed by a
-key-value store (and perhaps the API machinery will eventually be spun out as an independent
-project).
-
-Kubernetes executes isolated application containers as its default, native mode of execution, as
-opposed to processes and traditional operating-system packages. Not only are application
-containers isolated from each other, but they are also isolated from the hosts on which they
-execute, which is critical to decoupling management of individual applications from each other and
-from management of the underlying cluster physical/virtual infrastructure.
-
-Kubernetes provides [Pods](https://kubernetes.io/docs/user-guide/pods/) that can host multiple
-containers and storage volumes as its fundamental execution primitive in order to facilitate
-packaging a single application per container, decoupling deployment-time concerns from build-time
-concerns, and migration from physical/virtual machines. The Pod primitive is key to glean the
-[primary benefits](https://kubernetes.io/docs/whatisk8s/#why-containers) of deployment on modern
-cloud platforms, such as Kubernetes.
-
-#### Container runtime
-
-TODO
-
-#### `kube-proxy`
-
-The [service](https://kubernetes.io/docs/user-guide/services/) abstraction provides a way to
-group pods under a common access policy (e.g., load-balanced). The implementation of this creates
-A virtual IP which clients can access and which is transparently proxied to the pods in a Service.
-Each node runs a [kube-proxy](https://kubernetes.io/docs/admin/kube-proxy/) process which programs
-`iptables` rules to trap access to service IPs and redirect them to the correct backends. This provides a highly-available load-balancing solution with low performance overhead by balancing
-client traffic from a node on that same node.
-
-Service endpoints are found primarily via [DNS](https://kubernetes.io/docs/admin/dns/).
-
-### Add-ons and other dependencies
-
-A number of components, called [*add-ons*]
-(https://github.com/kubernetes/kubernetes/tree/master/cluster/addons) typically run on Kubernetes
-itself:
-* [DNS](https://github.com/kubernetes/kubernetes/tree/master/cluster/addons/dns)
-* [Ingress controller](https://github.com/kubernetes/ingress/tree/master/controllers)
-* [Heapster](https://github.com/kubernetes/heapster/) (resource monitoring)
-* [Dashboard](https://github.com/kubernetes/dashboard/) (GUI)
-
-### Federation
-
-A single Kubernetes cluster may span multiple availability zones.
-
-However, for the highest availability, we recommend using [cluster federation](federation.md).
+TODO: Document the [proposal process](../devel/faster_reviews.md#1-dont-build-a-cathedral-in-one-pr).
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