Clarify cluster diagram as one possible reference architecture (#47164)
* Clarify cluster diagram as one possible reference architecture * Update content/en/docs/concepts/architecture/_index.md Co-authored-by: Tim Bannister <tim@scalefactory.com> * Update content/en/docs/concepts/architecture/_index.md Co-authored-by: Tim Bannister <tim@scalefactory.com> * Update content/en/docs/concepts/architecture/_index.md Co-authored-by: Tim Bannister <tim@scalefactory.com> * Improve organization of cluster architecture content * Architecture concepts: fix grammar and improve clarity Co-authored-by: Charles Uneze <charlesniklaus@gmail.com> * Architecture concepts and overview pages refactor Improves clarity and consistency with existing documentation Co-authored-by: Tim Bannister <tim@scalefactory.com> * Improve organization of cluster architecture content * Improve organization of cluster architecture content Co-authored-by: Tim Bannister <tim@scalefactory.com> --------- Co-authored-by: Tim Bannister <tim@scalefactory.com> Co-authored-by: Charles Uneze <charlesniklaus@gmail.com>pull/47661/head
Robbie Cronin
GitHub
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9d0fbc5e30
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af0e08c038
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@ -5,4 +5,163 @@ description: >
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The architectural concepts behind Kubernetes.
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---
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{{< figure src="/images/docs/kubernetes-cluster-architecture.svg" alt="Components of Kubernetes" caption="Kubernetes cluster architecture" class="diagram-large" >}}
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A Kubernetes cluster consists of a control plane plus a set of worker machines, called nodes, that run containerized applications. Every cluster needs at least one worker node in order to run Pods.
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The worker node(s) host the Pods that are the components of the application workload. The control plane manages the worker nodes and the Pods in the cluster. In production environments, the control plane usually runs across multiple computers and a cluster usually runs multiple nodes, providing fault-tolerance and high availability.
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This document outlines the various components you need to have for a complete and working Kubernetes cluster.
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{{< figure src="/images/docs/kubernetes-cluster-architecture.svg" alt="The control plane (kube-apiserver, etcd, kube-controller-manager, kube-scheduler) and several nodes. Each node is running a kubelet and kube-proxy."
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title="Kubernetes cluster components"
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caption="**Note:** This diagram presents an example reference architecture for a Kubernetes cluster. The actual distribution of components can vary based on specific cluster setups and requirements." class="diagram-large" >}}
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## Control plane components
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The control plane's components make global decisions about the cluster (for example, scheduling), as well as detecting and responding to cluster events (for example, starting up a new {{< glossary_tooltip text="pod" term_id="pod">}} when a Deployment's `{{< glossary_tooltip text="replicas" term_id="replica" >}}` field is unsatisfied).
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Control plane components can be run on any machine in the cluster. However, for simplicity, setup scripts typically start all control plane components on the same machine, and do not run user containers on this machine. See [Creating Highly Available clusters with kubeadm](/docs/setup/production-environment/tools/kubeadm/high-availability/) for an example control plane setup that runs across multiple machines.
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### kube-apiserver
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{{< glossary_definition term_id="kube-apiserver" length="all" >}}
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### etcd
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{{< glossary_definition term_id="etcd" length="all" >}}
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### kube-scheduler
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{{< glossary_definition term_id="kube-scheduler" length="all" >}}
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### kube-controller-manager
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{{< glossary_definition term_id="kube-controller-manager" length="all" >}}
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There are many different types of controllers. Some examples of them are:
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- Node controller: Responsible for noticing and responding when nodes go down.
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- Job controller: Watches for Job objects that represent one-off tasks, then creates Pods to run those tasks to completion.
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- EndpointSlice controller: Populates EndpointSlice objects (to provide a link between Services and Pods).
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- ServiceAccount controller: Create default ServiceAccounts for new namespaces.
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The above is not an exhaustive list.
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### cloud-controller-manager
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{{< glossary_definition term_id="cloud-controller-manager" length="short" >}}
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The cloud-controller-manager only runs controllers that are specific to your cloud provider. If you are running Kubernetes on your own premises, or in a learning environment inside your own PC, the cluster does not have a cloud controller manager.
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As with the kube-controller-manager, the cloud-controller-manager combines several logically independent control loops into a single binary that you run as a single process. You can scale horizontally (run more than one copy) to improve performance or to help tolerate failures.
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The following controllers can have cloud provider dependencies:
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- Node controller: For checking the cloud provider to determine if a node has been deleted in the cloud after it stops responding
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- Route controller: For setting up routes in the underlying cloud infrastructure
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- Service controller: For creating, updating and deleting cloud provider load balancers
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## Node components
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Node components run on every node, maintaining running pods and providing the Kubernetes runtime environment.
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### kubelet
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{{< glossary_definition term_id="kubelet" length="all" >}}
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### kube-proxy (optional) {#kube-proxy}
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{{< glossary_definition term_id="kube-proxy" length="all" >}}
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If you use a [network plugin](#network-plugins) that implements packet forwarding for Services
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by itself, and providing equivalent behavior to kube-proxy, then you do not need to run
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kube-proxy on the nodes in your cluster.
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### Container runtime
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{{< glossary_definition term_id="container-runtime" length="all" >}}
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## Addons
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Addons use Kubernetes resources ({{< glossary_tooltip term_id="daemonset" >}}, {{< glossary_tooltip term_id="deployment" >}}, etc) to implement cluster features. Because these are providing cluster-level features, namespaced resources for addons belong within the `kube-system` namespace.
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Selected addons are described below; for an extended list of available addons, please see [Addons](/docs/concepts/cluster-administration/addons/).
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### DNS
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While the other addons are not strictly required, all Kubernetes clusters should have [cluster DNS](/docs/concepts/services-networking/dns-pod-service/), as many examples rely on it.
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Cluster DNS is a DNS server, in addition to the other DNS server(s) in your environment, which serves DNS records for Kubernetes services.
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Containers started by Kubernetes automatically include this DNS server in their DNS searches.
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### Web UI (Dashboard)
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[Dashboard](/docs/tasks/access-application-cluster/web-ui-dashboard/) is a general purpose, web-based UI for Kubernetes clusters. It allows users to manage and troubleshoot applications running in the cluster, as well as the cluster itself.
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### Container resource monitoring
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[Container Resource Monitoring](/docs/tasks/debug/debug-cluster/resource-usage-monitoring/) records generic time-series metrics about containers in a central database, and provides a UI for browsing that data.
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### Cluster-level Logging
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A [cluster-level logging](/docs/concepts/cluster-administration/logging/) mechanism is responsible for saving container logs to a central log store with a search/browsing interface.
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### Network plugins
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[Network plugins](/docs/concepts/extend-kubernetes/compute-storage-net/network-plugins) are software components that implement the container network interface (CNI) specification. They are responsible for allocating IP addresses to pods and enabling them to communicate with each other within the cluster.
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## Architecture variations
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While the core components of Kubernetes remain consistent, the way they are deployed and managed can vary. Understanding these variations is crucial for designing and maintaining Kubernetes clusters that meet specific operational needs.
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### Control plane deployment options
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The control plane components can be deployed in several ways:
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Traditional deployment
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: Control plane components run directly on dedicated machines or VMs, often managed as systemd services.
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Static Pods
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: Control plane components are deployed as static Pods, managed by the kubelet on specific nodes. This is a common approach used by tools like kubeadm.
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Self-hosted
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: The control plane runs as Pods within the Kubernetes cluster itself, managed by Deployments and StatefulSets or other Kubernetes primitives.
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Managed Kubernetes services
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: Cloud providers often abstract away the control plane, managing its components as part of their service offering.
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### Workload placement considerations
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The placement of workloads, including the control plane components, can vary based on cluster size, performance requirements, and operational policies:
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- In smaller or development clusters, control plane components and user workloads might run on the same nodes.
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- Larger production clusters often dedicate specific nodes to control plane components, separating them from user workloads.
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- Some organizations run critical add-ons or monitoring tools on control plane nodes.
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### Cluster management tools
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Tools like kubeadm, kops, and Kubespray offer different approaches to deploying and managing clusters, each with its own method of component layout and management.
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The flexibility of Kubernetes architecture allows organizations to tailor their clusters to specific needs, balancing factors such as operational complexity, performance, and management overhead.
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### Customization and extensibility
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Kubernetes architecture allows for significant customization:
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- Custom schedulers can be deployed to work alongside the default Kubernetes scheduler or to replace it entirely.
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- API servers can be extended with CustomResourceDefinitions and API Aggregation.
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- Cloud providers can integrate deeply with Kubernetes using the cloud-controller-manager.
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The flexibility of Kubernetes architecture allows organizations to tailor their clusters to specific needs, balancing factors such as operational complexity, performance, and management overhead.
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## {{% heading "whatsnext" %}}
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Learn more about the following:
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- [Nodes](/docs/concepts/architecture/nodes/) and [their communication](/docs/concepts/architecture/control-plane-node-communication/)
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with the control plane.
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- Kubernetes [controllers](/docs/concepts/architecture/controller/).
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- [kube-scheduler](/docs/concepts/scheduling-eviction/kube-scheduler/) which is the default scheduler for Kubernetes.
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- Etcd's official [documentation](https://etcd.io/docs/).
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- Several [container runtimes](/docs/setup/production-environment/container-runtimes/) in Kubernetes.
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- Integrating with cloud providers using [cloud-controller-manager](/docs/concepts/architecture/cloud-controller/).
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- [kubectl](/docs/reference/generated/kubectl/kubectl-commands) commands.
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@ -4,8 +4,7 @@ reviewers:
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title: Kubernetes Components
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content_type: concept
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description: >
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A Kubernetes cluster consists of the components that are a part of the control
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plane and a set of machines called nodes.
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An overview of the key components that make up a Kubernetes cluster.
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weight: 30
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card:
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title: Components of a cluster
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@ -14,140 +13,73 @@ card:
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---
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<!-- overview -->
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When you deploy Kubernetes, you get a cluster.
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{{< glossary_definition term_id="cluster" length="all" prepend="A Kubernetes cluster consists of">}}
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This document outlines the various components you need to have for
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a complete and working Kubernetes cluster.
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This page provides a high-level overview of the essential components that make up a Kubernetes cluster.
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{{< figure src="/images/docs/components-of-kubernetes.svg" alt="Components of Kubernetes" caption="The components of a Kubernetes cluster" class="diagram-large" clicktozoom="true" >}}
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<!-- body -->
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## Control Plane Components
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The control plane's components make global decisions about the cluster (for example, scheduling),
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as well as detecting and responding to cluster events (for example, starting up a new
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{{< glossary_tooltip text="pod" term_id="pod">}} when a Deployment's
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`{{< glossary_tooltip text="replicas" term_id="replica" >}}` field is unsatisfied).
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## Core Components
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Control plane components can be run on any machine in the cluster. However,
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for simplicity, setup scripts typically start all control plane components on
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the same machine, and do not run user containers on this machine. See
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[Creating Highly Available clusters with kubeadm](/docs/setup/production-environment/tools/kubeadm/high-availability/)
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for an example control plane setup that runs across multiple machines.
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A Kubernetes cluster consists of a control plane and one or more worker nodes. Here's a brief overview of the main components:
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### kube-apiserver
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### Control Plane Components
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{{< glossary_definition term_id="kube-apiserver" length="all" >}}
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Manage the overall state of the cluster:
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### etcd
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[kube-apiserver](/docs/concepts/architecture/#kube-apiserver)
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: The core component server that exposes the Kubernetes HTTP API
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{{< glossary_definition term_id="etcd" length="all" >}}
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[etcd](/docs/concepts/architecture/#etcd)
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: Consistent and highly-available key value store for all API server data
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### kube-scheduler
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[kube-scheduler](/docs/concepts/architecture/#kube-scheduler)
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: Looks for Pods not yet bound to a node, and assigns each Pod to a suitable node.
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{{< glossary_definition term_id="kube-scheduler" length="all" >}}
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[kube-controller-manager](/docs/concepts/architecture/#kube-controller-manager)
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: Runs {{< glossary_tooltip text="controllers" term_id="controller" >}} to implement Kubernetes API behavior.
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### kube-controller-manager
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[cloud-controller-manager](/docs/concepts/architecture/#cloud-controller-manager) (optional)
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: Integrates with underlying cloud provider(s)
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{{< glossary_definition term_id="kube-controller-manager" length="all" >}}
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### Node Components
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There are many different types of controllers. Some examples of them are:
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Run on every node, maintaining running pods and providing the Kubernetes runtime environment:
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* Node controller: Responsible for noticing and responding when nodes go down.
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* Job controller: Watches for Job objects that represent one-off tasks, then creates
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Pods to run those tasks to completion.
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* EndpointSlice controller: Populates EndpointSlice objects (to provide a link between Services and Pods).
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* ServiceAccount controller: Create default ServiceAccounts for new namespaces.
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[kubelet](/docs/concepts/architecture/#kubelet)
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: Ensures that Pods are running, including their containers.
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The above is not an exhaustive list.
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[kube-proxy](/docs/concepts/architecture/#kube-proxy) (optional)
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: Maintains network rules on nodes to implement {{< glossary_tooltip text="Services" term_id="service" >}}
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### cloud-controller-manager
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[Container runtime](/docs/concepts/architecture/#container-runtime)
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: Software responsible for running containers. Read [Container Runtimes](/docs/setup/production-environment/container-runtimes/) to learn more.
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{{< glossary_definition term_id="cloud-controller-manager" length="short" >}}
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The cloud-controller-manager only runs controllers that are specific to your cloud provider.
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If you are running Kubernetes on your own premises, or in a learning environment inside your
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own PC, the cluster does not have a cloud controller manager.
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{{% thirdparty-content single="true" %}}
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As with the kube-controller-manager, the cloud-controller-manager combines several logically
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independent control loops into a single binary that you run as a single process. You can
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scale horizontally (run more than one copy) to improve performance or to help tolerate failures.
|
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|
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The following controllers can have cloud provider dependencies:
|
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|
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* Node controller: For checking the cloud provider to determine if a node has been deleted in the cloud after it stops responding
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* Route controller: For setting up routes in the underlying cloud infrastructure
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* Service controller: For creating, updating and deleting cloud provider load balancers
|
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## Node Components
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Node components run on every node, maintaining running pods and providing the Kubernetes runtime environment.
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### kubelet
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{{< glossary_definition term_id="kubelet" length="all" >}}
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### kube-proxy
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{{< glossary_definition term_id="kube-proxy" length="all" >}}
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### Container runtime
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{{< glossary_definition term_id="container-runtime" length="all" >}}
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Your cluster may require additional software on each node; for example, you might also
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run [systemd](https://systemd.io/) on a Linux node to supervise local components.
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## Addons
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Addons use Kubernetes resources ({{< glossary_tooltip term_id="daemonset" >}},
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{{< glossary_tooltip term_id="deployment" >}}, etc)
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to implement cluster features. Because these are providing cluster-level features, namespaced resources
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for addons belong within the `kube-system` namespace.
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Addons extend the functionality of Kubernetes. A few important examples include:
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Selected addons are described below; for an extended list of available addons, please
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see [Addons](/docs/concepts/cluster-administration/addons/).
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[DNS](/docs/concepts/architecture/#dns)
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: For cluster-wide DNS resolution
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### DNS
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[Web UI](/docs/concepts/architecture/#web-ui-dashboard) (Dashboard)
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: For cluster management via a web interface
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While the other addons are not strictly required, all Kubernetes clusters should have
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[cluster DNS](/docs/concepts/services-networking/dns-pod-service/), as many examples rely on it.
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[Container Resource Monitoring](/docs/concepts/architecture/#container-resource-monitoring)
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: For collecting and storing container metrics
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Cluster DNS is a DNS server, in addition to the other DNS server(s) in your environment,
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which serves DNS records for Kubernetes services.
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[Cluster-level Logging](/docs/concepts/architecture/#cluster-level-logging)
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: For saving container logs to a central log store
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Containers started by Kubernetes automatically include this DNS server in their DNS searches.
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## Flexibility in Architecture
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### Web UI (Dashboard)
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Kubernetes allows for flexibility in how these components are deployed and managed. The architecture can be adapted to various needs, from small development environments to large-scale production deployments.
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[Dashboard](/docs/tasks/access-application-cluster/web-ui-dashboard/) is a general purpose,
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web-based UI for Kubernetes clusters. It allows users to manage and troubleshoot applications
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running in the cluster, as well as the cluster itself.
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### Container Resource Monitoring
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[Container Resource Monitoring](/docs/tasks/debug/debug-cluster/resource-usage-monitoring/)
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records generic time-series metrics
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about containers in a central database, and provides a UI for browsing that data.
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|
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### Cluster-level Logging
|
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|
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A [cluster-level logging](/docs/concepts/cluster-administration/logging/) mechanism is responsible for
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saving container logs to a central log store with search/browsing interface.
|
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|
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### Network Plugins
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[Network plugins](/docs/concepts/extend-kubernetes/compute-storage-net/network-plugins) are software
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components that implement the container network interface (CNI) specification. They are responsible for
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allocating IP addresses to pods and enabling them to communicate with each other within the cluster.
|
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|
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## {{% heading "whatsnext" %}}
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Learn more about the following:
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* [Nodes](/docs/concepts/architecture/nodes/) and [their communication](/docs/concepts/architecture/control-plane-node-communication/)
|
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with the control plane.
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* Kubernetes [controllers](/docs/concepts/architecture/controller/).
|
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* [kube-scheduler](/docs/concepts/scheduling-eviction/kube-scheduler/) which is the default scheduler for Kubernetes.
|
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* Etcd's official [documentation](https://etcd.io/docs/).
|
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* Several [container runtimes](/docs/setup/production-environment/container-runtimes/) in Kubernetes.
|
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* Integrating with cloud providers using [cloud-controller-manager](/docs/concepts/architecture/cloud-controller/).
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* [kubectl](/docs/reference/generated/kubectl/kubectl-commands) commands.
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For more detailed information about each component and various ways to configure your cluster architecture, see the [Cluster Architecture](/docs/concepts/architecture/) page.
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|
|
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