This glossary is intended to be a comprehensive, standardized list of Kubernetes terminology. It includes technical terms that are specific to Kubernetes, as well as more general terms that provide useful context.
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Click on the [+] indicators below to get a longer explanation for any particular term.
- Admission ControllerLINK
A piece of code that intercepts requests to the Kubernetes API server prior to persistence of the object.[+]
Admission controllers are configurable for the Kubernetes API server and may be "validating", "mutating", or both. Any admission controller may reject the request. Mutating controllers may modify the objects they admit; validating controllers may not.
- API GroupLINK
A set of related paths in Kubernetes API.[+]
You can enable or disable each API group by changing the configuration of your API server. You can also disable or enable paths to specific resources. API group makes it easier to extend the Kubernetes API. The API group is specified in a REST path and in the
apiVersionfield of a serialized object.
- Read API Group for more information.
- API serverLINKAlso 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.
- App ContainerLINK[+]
An init container lets you separate initialization details that are important for the overall workload, and that don't need to keep running once the application container has started. If a pod doesn't have any init containers configured, all the containers in that pod are app containers.
- Application ArchitectLINK
A person responsible for the high-level design of an application.[+]
An architect ensures that an app's implementation allows it to interact with its surrounding components in a scalable, maintainable way. Surrounding components include databases, logging infrastructure, and other microservices.
A person who can review and approve Kubernetes code contributions.[+]
While code review is focused on code quality and correctness, approval is focused on the holistic acceptance of a contribution. Holistic acceptance includes backwards/forwards compatibility, adhering to API and flag conventions, subtle performance and correctness issues, interactions with other parts of the system, and others. Approver status is scoped to a part of the codebase. Approvers were previously referred to as maintainers.
CIDR (Classless Inter-Domain Routing) is a notation for describing blocks of IP addresses and is used heavily in various networking configurations.[+]
- Cloud Controller ManagerLINK
A Kubernetes control plane component that embeds cloud-specific control logic. The cloud controller manager lets you link your cluster into your cloud provider's API, and separates out the components that interact with that cloud platform from components that just interact with your cluster.[+]
By decoupling the interoperability logic between Kubernetes and the underlying cloud infrastructure, the cloud-controller-manager component enables cloud providers to release features at a different pace compared to the main Kubernetes project.
- Cloud Native Computing Foundation (CNCF)LINK
The Cloud Native Computing Foundation (CNCF) builds sustainable ecosystems and fosters a community around projects that orchestrate containers as part of a microservices architecture.
Kubernetes is a CNCF project.[+]
The CNCF is a sub-foundation of the Linux Foundation. Its mission is to make cloud native computing ubiquitous.
- Cloud ProviderLINKAlso known as:Cloud Service Provider
A business or other organization that offers a cloud computing platform.[+]
Cloud providers, sometimes called Cloud Service Providers (CSPs), offer cloud computing platforms or services.
Many cloud providers offer managed infrastructure (also called Infrastructure as a Service or IaaS). With managed infrastructure the cloud provider is responsible for servers, storage, and networking while you manage layers on top of that such as running a Kubernetes cluster.
You can also find Kubernetes as a managed service; sometimes called Platform as a Service, or PaaS. With managed Kubernetes, your cloud provider is responsible for the Kubernetes control plane as well as the nodes and the infrastructure they rely on: networking, storage, and possibly other elements such as load balancers.
A set of worker machines, called nodes, that run containerized applications. Every cluster has at least one worker node.[+]
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.
- Cluster OperationsLINK
The work involved in managing a Kubernetes cluster: managing day-to-day operations, and co-ordinating upgrades.[+]
Examples of cluster operations work include: deploying new Nodes to scale the cluster; performing software upgrades; implementing security controls; adding or removing storage; configuring cluster networking; managing cluster-wide observability; and responding to events.
- Cluster OperatorLINK
A person who configures, controls, and monitors clusters.[+]
Their primary responsibility is keeping a cluster up and running, which may involve periodic maintenance activities or upgrades.Note: Cluster operators are different from the Operator pattern that extends the Kubernetes API.
A ConfigMap allows you to decouple environment-specific configuration from your container images, so that your applications are easily portable.
- Container Environment VariablesLINK
Container environment variables are name=value pairs that provide useful information into containers running in a pod[+]
Container environment variables provide information that is required by the running containerized applications along with information about important resources to the containers. For example, file system details, information about the container itself, and other cluster resources such as service endpoints.
- Container Lifecycle HooksLINK
The lifecycle hooks expose events in the Container management lifecycle and let the user run code when the events occur.[+]
Two hooks are exposed to Containers: PostStart which executes immediately after a container is created and PreStop which is blocking and is called immediately before a container is terminated.
- Container Storage Interface (CSI)LINK
The Container Storage Interface (CSI) defines a standard interface to expose storage systems to containers.[+]
CSI allows vendors to create custom storage plugins for Kubernetes without adding them to the Kubernetes repository (out-of-tree plugins). To use a CSI driver from a storage provider, you must first deploy it to your cluster. You will then be able to create a Storage Class that uses that CSI driver.
- Control PlaneLINK
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):
These components can be run as traditional operating system services (daemons) or as containers. The hosts running these components were historically called masters.
In Kubernetes, controllers are control loops that watch the state of your cluster, then make or request changes where needed. Each controller tries to move the current cluster state closer to the desired state.[+]
Some controllers also run inside the control plane, providing control loops that are core to Kubernetes' operations. For example: the deployment controller, the daemonset controller, the namespace controller, and the persistent volume controller (and others) all run within the kube-controller-manager.
A tool that lets you use OCI container runtimes with Kubernetes CRI.[+]
Deploying CRI-O allows Kubernetes to use any OCI-compliant runtime as the container runtime for running Pods, and to fetch OCI container images from remote registries.
- Device PluginLINK[+]
Device plugins advertise resources to the kubelet, so that workload Pods can access hardware features that relate to the Node where that Pod is running. You can deploy a device plugin as a DaemonSet, or install the device plugin software directly on each target Node.
See Device Plugins for more information.
If you, as cluster operator, destroy a Pod that belongs to an application, Kubernetes terms that a voluntary disruption. If a Pod goes offline because of a Node failure, or an outage affecting a wider failure zone, Kubernetes terms that an involuntary disruption.
See Disruptions for more information.
Docker (specifically, Docker Engine) is a software technology providing operating-system-level virtualization also known as containers.[+]
Docker uses the resource isolation features of the Linux kernel such as cgroups and kernel namespaces, and a union-capable file system such as OverlayFS and others to allow independent containers to run within a single Linux instance, avoiding the overhead of starting and maintaining virtual machines (VMs).
- Downstream (disambiguation)LINK
May refer to: code in the Kubernetes ecosystem that depends upon the core Kubernetes codebase or a forked repo.[+]
- In the Kubernetes Community: Conversations often use downstream to mean the ecosystem, code, or third-party tools that rely on the core Kubernetes codebase. For example, a new feature in Kubernetes may be adopted by applications downstream to improve their functionality.
- In GitHub or git: The convention is to refer to a forked repo as downstream, whereas the source repo is considered upstream.
- Dynamic Volume ProvisioningLINK
Allows users to request automatic creation of storage Volumes.[+]
Dynamic provisioning eliminates the need for cluster administrators to pre-provision storage. Instead, it automatically provisions storage by user request. Dynamic volume provisioning is based on an API object, StorageClass, referring to a Volume Plugin that provisions a Volume and the set of parameters to pass to the Volume Plugin.
- Ephemeral ContainerLINK[+]
If you want to investigate a Pod that's running with problems, you can add an ephemeral container to that Pod and carry out diagnostics. Ephemeral containers have no resource or scheduling guarantees, and you should not use them to run any part of the workload itself.
Extensions are software components that extend and deeply integrate with Kubernetes to support new types of hardware.[+]
Most cluster administrators will use a hosted or distribution instance of Kubernetes. As a result, most Kubernetes users will need to install extensions and fewer will need to author new ones.
FlexVolume is an interface for creating out-of-tree volume plugins. The Container Storage Interface is a newer interface which addresses several problems with FlexVolumes.[+]
FlexVolumes enable users to write their own drivers and add support for their volumes in Kubernetes. FlexVolume driver binaries and dependencies must be installed on host machines. This requires root access. The Storage SIG suggests implementing a CSI driver if possible since it addresses the limitations with FlexVolumes.
- Helm ChartLINK
A package of pre-configured Kubernetes resources that can be managed with the Helm tool.[+]
Charts provide a reproducible way of creating and sharing Kubernetes applications. A single chart can be used to deploy something simple, like a memcached Pod, or something complex, like a full web app stack with HTTP servers, databases, caches, and so on.
- Horizontal Pod AutoscalerLINKAlso known as:HPA
An API resource that automatically scales the number of Pod replicas based on targeted CPU utilization or custom metric targets.[+]
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.
- Init ContainerLINK
One or more initialization containers that must run to completion before any app containers run.[+]
Initialization (init) containers are like regular app containers, with one difference: init containers must run to completion before any app containers can start. Init containers run in series: each init container must run to completion before the next init container begins.
An open platform (not Kubernetes-specific) that provides a uniform way to integrate microservices, manage traffic flow, enforce policies, and aggregate telemetry data.[+]
Adding Istio does not require changing application code. It is a layer of infrastructure between a service and the network, which when combined with service deployments, is commonly referred to as a service mesh. Istio's control plane abstracts away the underlying cluster management platform, which may be Kubernetes, Mesosphere, etc.
A CLI tool that helps you create, destroy, upgrade and maintain production-grade, highly available, Kubernetes clusters.[+]Note: kops has general availability support only for AWS. Support for using kops with GCE and VMware vSphere are in alpha.
kopsprovisions your cluster with:
- Fully automated installation
- DNS-based cluster identification
- Self-healing: everything runs in Auto-Scaling Groups
- Limited OS support (Debian preferred, Ubuntu 16.04 supported, early support for CentOS & RHEL)
- High availability (HA) support
- The ability to directly provision, or to generate Terraform manifests
You can also build your own cluster using Kubeadm as a building block.
kopsbuilds on the kubeadm work.
kube-proxy maintains network rules on nodes. These network rules allow network communication to your Pods from network sessions inside or outside of your cluster.
kube-proxy uses the operating system packet filtering layer if there is one and it's available. Otherwise, kube-proxy forwards the traffic itself.
Factors taken into account for scheduling decisions include: individual and collective resource requirements, hardware/software/policy constraints, affinity and anti-affinity specifications, data locality, inter-workload interference, and deadlines.
The kubelet takes a set of PodSpecs that are provided through various mechanisms and ensures that the containers described in those PodSpecs are running and healthy. The kubelet doesn't manage containers which were not created by Kubernetes.
- Kubernetes APILINK
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. The API allows configuration to be managed in a declarative way. Users can interact with the Kubernetes API directly, or via tools like
kubectl. The core Kubernetes API is flexible and can also be extended to support custom resources.
- Managed ServiceLINK
A software offering maintained by a third-party provider.[+]
A continuously active contributor in the K8s community.[+]
Members can have issues and PRs assigned to them and participate in special interest groups (SIGs) through GitHub teams. Pre-submit tests are automatically run for members' PRs. A member is expected to remain an active contributor to the community.
- Mirror PodLINK[+]
When the kubelet finds a static pod in its configuration, it automatically tries to create a Pod object on the Kubernetes API server for it. This means that the pod will be visible on the API server, but cannot be controlled from there.
(For example, removing a mirror pod will not stop the kubelet daemon from running it).
- Network PolicyLINK
A specification of how groups of Pods are allowed to communicate with each other and with other network endpoints.[+]
Network Policies help you declaratively configure which Pods are allowed to connect to each other, which namespaces are allowed to communicate, and more specifically which port numbers to enforce each policy on.
NetworkPolicyresources use labels to select Pods and define rules which specify what traffic is allowed to the selected Pods. Network Policies are implemented by a supported network plugin provided by a network provider. Be aware that creating a network resource without a controller to implement it will have no effect.
A node is a worker machine in Kubernetes.[+]
A worker node may be a VM or physical machine, depending on the cluster. It has local daemons or services necessary to run Pods and is managed by the control plane. The daemons on a node include kubelet, kube-proxy, and a container runtime implementing the CRI such as Docker.
In early Kubernetes versions, Nodes were called "Minions".
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.
- Operator patternLINK[+]
You can extend Kubernetes by adding controllers to your cluster, beyond the built-in controllers that come as part of Kubernetes itself.
If a running application acts as a controller and has API access to carry out tasks against a custom resource that's defined in the control plane, that's an example of the Operator pattern.
- Persistent VolumeLINK
An API object that represents a piece of storage in the cluster. Available as a general, pluggable resource that persists beyond the lifecycle of any individual Pod.[+]
PersistentVolumes (PVs) provide an API that abstracts details of how storage is provided from how it is consumed. PVs are used directly in scenarios where storage can be created ahead of time (static provisioning). For scenarios that require on-demand storage (dynamic provisioning), PersistentVolumeClaims (PVCs) are used instead.
- Persistent Volume ClaimLINK[+]
Specifies the amount of storage, how the storage will be accessed (read-only, read-write and/or exclusive) and how it is reclaimed (retained, recycled or deleted). Details of the storage itself are described in the PersistentVolume object.
- Platform DeveloperLINK
A person who customizes the Kubernetes platform to fit the needs of their project.[+]
A platform developer may, for example, use Custom Resources or Extend the Kubernetes API with the aggregation layer to add functionality to their instance of Kubernetes, specifically for their application. Some Platform Developers are also contributors and develop extensions which are contributed to the Kubernetes community. Others develop closed-source commercial or site-specific extensions.
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 container. It can also run optional sidecar containers that add supplementary features like logging. Pods are commonly managed by a Deployment.
- Pod Disruption BudgetLINKAlso known as:PDB
A Pod Disruption Budget allows an application owner to create an object for a replicated application, that ensures a certain number or percentage of Pods with an assigned label will not be voluntarily evicted at any point in time. PDBs cannot prevent an involuntary disruption, but will count against the budget. [+]
A Pod Disruption Budget allows an application owner to create an object for a replicated application, that ensures a certain number or percentage of Pods with an assigned label will not be voluntarily evicted at any point in time. PDBs cannot prevent an involuntary disruption, but will count against the budget.
- Pod LifecycleLINK
The sequence of states through which a Pod passes during its lifetime.[+]
- Pod Security PolicyLINK
Enables fine-grained authorization of Pod creation and updates.[+]
A cluster-level resource that controls security sensitive aspects of the Pod specification. The
PodSecurityPolicyobjects define a set of conditions that a Pod must run with in order to be accepted into the system, as well as defaults for the related fields. Pod Security Policy control is implemented as an optional admission controller.
An API object that injects information such as secrets, volume mounts, and environment variables into Pods at creation time.[+]
This object chooses the Pods to inject information into using standard selectors. This allows the podspec definitions to be nonspecific, decoupling the podspec from environment specific configuration.
In computing, a proxy is a server that acts as an intermediary for a remote service.[+]
A client interacts with the proxy; the proxy copies the client's data to the actual server; the actual server replies to the proxy; the proxy sends the actual server's reply to the client.
You can run kube-proxy as a plain userland proxy service. If your operating system supports it, you can instead run kube-proxy in a hybrid mode that achieves the same overall effect using less system resources.
- QoS ClassLINK
QoS Class (Quality of Service Class) provides a way for Kubernetes to classify Pods within the cluster into several classes and make decisions about scheduling and eviction.[+]
QoS Class of a Pod is set at creation time based on its compute resources requests and limits settings. QoS classes are used to make decisions about Pods scheduling and eviction. Kubernetes can assign one of the following QoS classes to a Pod:
A whole-number representation of small or large numbers using SI suffixes.[+]
Quantities are representations of small or large numbers using a compact, whole-number notation with SI suffixes. Fractional numbers are represented using milli units, while large numbers can be represented using kilo, mega, or giga units.
For instance, the number
1.5is represented as
1500m, while the number
1000can be represented as
1M. You can also specify binary-notation suffixes; the number 2048 can be written as
The accepted decimal (power-of-10) units are
k(kilo, intentionally lowercase),
The accepted binary (power-of-2) units are
A workload resource that manages a replicated application, ensuring that a specific number of instances of a Pod are running.[+]
The control plane ensures that the defined number of Pods are running, even if some Pods fail, if you delete Pods manually, or if too many are started by mistake.Note: ReplicationController is deprecated. See Deployment, which is similar.
Stores sensitive information, such as passwords, OAuth tokens, and ssh keys.[+]
Allows for more control over how sensitive information is used and reduces the risk of accidental exposure, including encryption at rest. A Pod references the secret as a file in a volume mount or by the kubelet pulling images for a pod. Secrets are great for confidential data and ConfigMaps for non-confidential data.
- Security ContextLINK[+]
securityContext, you can define: the user that processes run as, the group that processes run as, and privilege settings. You can also configure security policies (for example: SELinux, AppArmor or seccomp).
PodSpec.securityContextsetting applies to all containers in a Pod.
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.
- Service BrokerLINK
An endpoint for a set of Managed Services offered and maintained by a third-party.[+]
- Service CatalogLINK
An extension API that enables applications running in Kubernetes clusters to easily use external managed software offerings, such as a datastore service offered by a cloud provider.[+]
Provides an identity for processes that run in a Pod.[+]
When processes inside Pods access the cluster, they are authenticated by the API server as a particular service account, for example,
default. When you create a Pod, if you do not specify a service account, it is automatically assigned the default service account in the same Namespace.
- shuffle shardingLINK
A technique for assigning requests to queues that provides better isolation than hashing modulo the number of queues.[+]
We are often concerned with insulating different flows of requests from each other, so that a high-intensity flow does not crowd out low-intensity flows. A simple way to put requests into queues is to hash some characteristics of the request, modulo the number of queues, to get the index of the queue to use. The hash function uses as input characteristics of the request that align with flows. For example, in the Internet this is often the 5-tuple of source and destination address, protocol, and source and destination port.
That simple hash-based scheme has the property that any high-intensity flow will crowd out all the low-intensity flows that hash to the same queue. Providing good insulation for a large number of flows requires a large number of queues, which is problematic. Shuffle sharding is a more nimble technique that can do a better job of insulating the low-intensity flows from the high-intensity flows. The terminology of shuffle sharding uses the metaphor of dealing a hand from a deck of cards; each queue is a metaphorical card. The shuffle sharding technique starts with hashing the flow-identifying characteristics of the request, to produce a hash value with dozens or more of bits. Then the hash value is used as a source of entropy to shuffle the deck and deal a hand of cards (queues). All the dealt queues are examined, and the request is put into one of the examined queues with the shortest length. With a modest hand size, it does not cost much to examine all the dealt cards and a given low-intensity flow has a good chance to dodge the effects of a given high-intensity flow. With a large hand size it is expensive to examine the dealt queues and more difficult for the low-intensity flows to dodge the collective effects of a set of high-intensity flows. Thus, the hand size should be chosen judiciously.
- SIG (special interest group)LINK
Community members who collectively manage an ongoing piece or aspect of the larger Kubernetes open source project.[+]
Members within a SIG have a shared interest in advancing a specific area, such as architecture, API machinery, or documentation. SIGs must follow the SIG governance guidelines, but can have their own contribution policy and channels of communication.
Manages the deployment and scaling of a set of Pods, and provides guarantees about the ordering and uniqueness of these Pods.[+]
Like a Deployment, a StatefulSet manages Pods that are based on an identical container spec. Unlike a Deployment, a StatefulSet maintains a sticky identity for each of their Pods. These pods are created from the same spec, but are not interchangeable: each has a persistent identifier that it maintains across any rescheduling.
If you want to use storage volumes to provide persistence for your workload, you can use a StatefulSet as part of the solution. Although individual Pods in a StatefulSet are susceptible to failure, the persistent Pod identifiers make it easier to match existing volumes to the new Pods that replace any that have failed.
- Storage ClassLINK
A StorageClass provides a way for administrators to describe different available storage types.[+]
StorageClasses can map to quality-of-service levels, backup policies, or to arbitrary policies determined by cluster administrators. Each StorageClass contains the fields
reclaimPolicy, which are used when a Persistent Volume belonging to the class needs to be dynamically provisioned. Users can request a particular class using the name of a StorageClass object.
sysctlis a semi-standardized interface for reading or changing the attributes of the running Unix kernel.
On Unix-like systems,
sysctlis both the name of the tool that administrators use to view and modify these settings, and also the system call that the tool uses.
Container runtimes and network plugins may rely on
sysctlvalues being set a certain way.
Taints and tolerations work together to ensure that pods are not scheduled onto inappropriate nodes. One or more taints are applied to a node. A node should only schedule a Pod with the matching tolerations for the configured taints.
A core object consisting of three required properties: key, value, and effect. Tolerations enable the scheduling of pods on nodes or node groups that have matching taints.[+]
- Upstream (disambiguation)LINK
May refer to: core Kubernetes or the source repo from which a repo was forked.[+]
- In the Kubernetes Community: Conversations often use upstream to mean the core Kubernetes codebase, which the general ecosystem, other code, or third-party tools rely upon. For example, community members may suggest that a feature is moved upstream so that it is in the core codebase instead of in a plugin or third-party tool.
- In GitHub or git: The convention is to refer to a source repo as upstream, whereas the forked repo is considered downstream.
A Kubernetes volume lives as long as the Pod that encloses it. Consequently, a volume outlives any containers that run within the Pod, and data in the volume is preserved across container restarts.
See storage for more information.
- Volume PluginLINK
A Volume Plugin enables integration of storage within a Pod.[+]
A Volume Plugin lets you attach and mount storage volumes for use by a Pod. Volume plugins can be in tree or out of tree. In tree plugins are part of the Kubernetes code repository and follow its release cycle. Out of tree plugins are developed independently.
- WG (working group)LINK
Facilitates the discussion and/or implementation of a short-lived, narrow, or decoupled project for a committee, SIG, or cross-SIG effort.[+]
A workload is an application running on Kubernetes.[+]