Page last modified on November 15, 2018 at 2:05 PM PST by Initial commit (Page history)
This glossary is intended to be a comprehensive, standardized list of Kubernetes terminology. It includes technical terms that are specific to K8s, 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.
A key-value pair that is used to attach arbitrary non-identifying metadata to objects.
[+]The metadata in an annotation can be small or large, structured or unstructured, and can include characters not permitted by labels. Clients such as tools and libraries can retrieve this metadata.
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.
Terms under which a contributorSomeone who donates code, documentation, or their time to help the Kubernetes project or community. grants a license to an open source project for their contributions.
[+]CLAs help resolve legal disputes involving contributed material and intellectual property (IP).
Container network interface (CNI) plugins are a type of Network plugin that adheres to the appc/CNI specification.
[+]Cloud Controller Manager is an alpha feature in 1.8. In upcoming releases it will be the preferred way to integrate Kubernetes with any cloud.
[+]Kubernetes v1.6 contains a new binary called cloud-controller-manager. cloud-controller-manager is a daemon that embeds cloud-specific control loops. These cloud-specific control loops were originally in the kube-controller-manager. Since cloud providers develop and release at a different pace compared to the Kubernetes project, abstracting the provider-specific code to the cloud-controller-manager binary allows cloud vendors to evolve independently from the core Kubernetes code.
Cloud provider is a company that offers cloud computing platform that can run Kubernetes clusters.
[+]Cloud providers or sometime called Cloud Service Provider (CSPs) provides cloud computing platforms. They may offer services such as Infrastructure as a Service (IaaS) or Platform as a Service (PaaS). Cloud providers host the Kubernetes cluster and also provide services that interact with the cluster, such as Load Balancers, Storage Classes etc.
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 person who develops and contributes code to the Kubernetes open source codebase.
[+]They are also an active community memberA continuously active contributor in the K8s community. who participates in one or more Special Interest Groups (SIGs)Community members who collectively manage an ongoing piece or aspect of the larger Kubernetes open source project. .
An API object used to store non-confidential data in key-value pairs. Can be consumed as environment variables, command-line arguments, or config files in a volumeA directory containing data, accessible to the containers in a pod. .
[+]Allows you to decouple environment-specific configuration from your container imagesA lightweight and portable executable image that contains software and all of its dependencies. , so that your applications are easily portable. When storing confidential data use a Secret.
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] ContainersA lightweight and portable executable image that contains software and all of its dependencies. . For example, file system, information about the container itself and other cluster resources such as service endpoints, etc.
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 ClassA StorageClass provides a way for administrators to describe different available storage types. that uses that CSI driver.
Someone who donates code, documentation, or their time to help the Kubernetes project or community.
[+]Contributions include pull requests (PRs), issues, feedback, special interest groups (SIG)Community members who collectively manage an ongoing piece or aspect of the larger Kubernetes open source project. participation, or organizing community events.
A control loop that watches the shared state of the cluster through the apiserverComponent on the master that exposes the Kubernetes API. It is the front-end for the Kubernetes control plane. and makes changes attempting to move the current state towards the desired state.
[+]Examples of controllers that ship with Kubernetes today are the replication controller, endpoints controller, namespace controller, and serviceaccounts controller.
Ensures a copy of a PodThe smallest and simplest Kubernetes object. A Pod represents a set of running containers on your cluster. is running across a set of nodes in a clusterA set of machines, called nodes, that run containerized applications managed by Kubernetes. .
[+]Used to deploy system daemons such as log collectors and monitoring agents that typically must run on every NodeA node is a worker machine in Kubernetes. .
An API object that manages a replicated application.
[+]Each replica is represented by a PodThe smallest and simplest Kubernetes object. A Pod represents a set of running containers on your cluster. , and the Pods are distributed among the nodes of a cluster.
May refer to: Application DeveloperA person who writes an application that runs in a Kubernetes cluster. , Code ContributorA person who develops and contributes code to the Kubernetes open source codebase. , or Platform DeveloperA person who customizes the Kubernetes platform to fit the needs of their project. .
[+]This overloaded term may have different meanings depending on the context
May refer to: code in the Kubernetes ecosystem that depends upon the core Kubernetes codebase or a forked repo.
[+]Allows users to request automatic creation of storage VolumesA directory containing data, accessible to the containers in a pod. .
[+]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, StorageClassA StorageClass provides a way for administrators to describe different available storage types. , referring to a Volume PluginA Volume Plugin enables integration of storage within a Pod. that provisions a VolumeA directory containing data, accessible to the containers in a pod. and the set of parameters to pass to the Volume Plugin.
Flexvolume is an interface for creating out-of-tree volume plugins. The Container Storage InterfaceThe Container Storage Interface (CSI) defines a standard interface to expose storage systems to containers. 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 CSIThe Container Storage Interface (CSI) defines a standard interface to expose storage systems to containers. driver if possible since it addresses the limitations with Flexvolumes.
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.
An API resource that automatically scales the number of pod replicas based on targeted CPU utilization or custom metric targets.
[+]HPA is typically used with Replication ControllersKubernetes service that ensures a specific number of instances of a pod are always running. , DeploymentsAn API object that manages a replicated application. , or Replica Sets. It cannot be applied to objects that cannot be scaled, for example DaemonSetsEnsures a copy of a Pod is running across a set of nodes in a cluster. .
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.
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 finite or batch task that runs to completion.
[+]Creates one or more PodThe smallest and simplest Kubernetes object. A Pod represents a set of running containers on your cluster. objects and ensures that a specified number of them successfully terminate. As Pods successfully complete, the Job tracks the successful completions.
A CLI tool that helps you create, destroy, upgrade and maintain production-grade, highly available, Kubernetes clusters. NOTE: Officially supports AWS only, with GCE and VMware vSphere in alpha.
[+]kops provisions your cluster with:
You can also build your own cluster using KubeadmA tool for quickly installing Kubernetes and setting up a secure cluster.
as a building block. kops builds on the kubeadm work.
A command line tool for communicating with a Kubernetes APIThe application that serves Kubernetes functionality through a RESTful interface and stores the state of the cluster. 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.
[+]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.
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.
Tags objects with identifying attributes that are meaningful and relevant to users.
[+]Labels are key/value pairs that are attached to objects such as PodsThe smallest and simplest Kubernetes object. A Pod represents a set of running containers on your cluster. . They are used to organize and to select subsets of objects.
A software offering maintained by a third-party provider.
[+]Some examples of Managed Services are AWS EC2, Azure SQL Database, and GCP Pub/Sub, but they can be any software offering that can be used by an application. Service Catalog provides a way to list, provision, and bind with Managed Services offered by Service BrokersAn endpoint for a set of Managed Services offered and maintained by a third-party. .
A continuously active contributorSomeone who donates code, documentation, or their time to help the Kubernetes project or community. in the K8s community.
[+]Members can have issues and PRs assigned to them and participate in special interest groups (SIGs)Community members who collectively manage an ongoing piece or aspect of the larger Kubernetes open source project. through GitHub teams. Pre-submit tests are automatically run for members’ PRs. A member is expected to remain an active contributor to the community.
An abstraction used by Kubernetes to support multiple virtual clusters on the same physical clusterA set of machines, called nodes, that run containerized applications managed by Kubernetes. .
[+]Namespaces are used to organize objects in a cluster and provide a way to divide cluster resources. Names of resources need to be unique within a namespace, but not across namespaces.
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. NetworkPolicy resources 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 machine may be a VM or physical machine, depending on the cluster. It has the ServicesAn API object that describes how to access applications, such as a set of Pods, and can describe ports and load-balancers. necessary to run PodsThe smallest and simplest Kubernetes object. A Pod represents a set of running containers on your cluster. and is managed by the master components. The ServicesAn API object that describes how to access applications, such as a set of Pods, and can describe ports and load-balancers. on a node include Docker, kubelet and kube-proxy.
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 PodThe smallest and simplest Kubernetes object. A Pod represents a set of running containers on your cluster. .
[+]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.
Claims storage resources defined in a PersistentVolume so that it can be mounted as a volume in a container.
[+]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 in the PersistentVolume specification.
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 contributorsSomeone who donates code, documentation, or their time to help the Kubernetes project or community. 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 containersA lightweight and portable executable image that contains software and all of its dependencies. 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 DeploymentAn API object that manages a replicated application. .
Enables fine-grained authorization of PodThe smallest and simplest Kubernetes object. A Pod represents a set of running containers on your cluster. creation and updates.
[+]A cluster-level resource that controls security sensitive aspects of the Pod specification. The PodSecurityPolicy objects 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.
Manages authorization decisions, allowing admins to dynamically configure access policies through the Kubernetes APIThe application that serves Kubernetes functionality through a RESTful interface and stores the state of the cluster. .
[+]RBAC utilizes roles, which contain permission rules, and role bindings, which grant the permissions defined in a role to a set of users.
ReplicaSet is the next-generation Replication Controller.
[+]ReplicaSet, like ReplicationController, ensures that a specified number of pods replicas are running at one time. ReplicaSet supports the new set-based selector requirements as described in the labels user guide, whereas a Replication Controller only supports equality-based selector requirements.
Kubernetes service that ensures a specific number of instances of a pod are always running.
[+]Will automatically add or remove running instances of a pod, based on a set value for that pod. Allows the pod to return to the defined number of instances if pods are deleted or if too many are started by mistake.
Provides constraints that limit aggregate resource consumption per NamespaceAn abstraction used by Kubernetes to support multiple virtual clusters on the same physical cluster. .
[+]Limits the quantity of objects that can be created in a namespace by type, as well as the total amount of compute resources that may be consumed by resources in that project.
Community membersA continuously active contributor in the K8s community. 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.
For more information, see the kubernetes/community repo and the current list of SIGs and Working Groups.
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 PodThe smallest and simplest Kubernetes object. A Pod represents a set of running containers on your cluster. 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.
The securityContext field defines privilege and access control settings for a Pod or Container, including the runtime UID and GID.
[+]The securityContext field in a PodThe smallest and simplest Kubernetes object. A Pod represents a set of running containers on your cluster. (applying to all containers) or container is used to set the user (runAsUser) and group (fsGroup), capabilities, privilege settings, and security policies (SELinux/AppArmor/Seccomp) that container processes use.
Allows users to filter a list of resources based on labels.
[+]Selectors are applied when querying lists of resources to filter them by LabelsTags objects with identifying attributes that are meaningful and relevant to users. .
An API object that describes how to access applications, such as a set of PodsThe smallest and simplest Kubernetes object. A Pod represents a set of running containers on your cluster. , and can describe ports and load-balancers.
[+]The access point can be internal or external to the cluster.
Provides an identity for processes that run in a PodThe smallest and simplest Kubernetes object. A Pod represents a set of running containers on your cluster. .
[+]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 NamespaceAn abstraction used by Kubernetes to support multiple virtual clusters on the same physical cluster.
.
An endpoint for a set of Managed ServicesA software offering maintained by a third-party provider. offered and maintained by a third-party.
[+]Service BrokersAn endpoint for a set of Managed Services offered and maintained by a third-party. implement the Open Service Broker API spec and provide a standard interface for applications to use their Managed Services. Service Catalog provides a way to list, provision, and bind with Managed Services offered by Service Brokers.
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.
[+]It provides a way to list, provision, and bind with external Managed ServicesA software offering maintained by a third-party provider. from Service BrokersAn endpoint for a set of Managed Services offered and maintained by a third-party. without needing detailed knowledge about how those services are created or managed.
Manages the deployment and scaling of a set of PodsThe smallest and simplest Kubernetes object. A Pod represents a set of running containers on your cluster. , and provides guarantees about the ordering and uniqueness of these Pods.
[+]Like a DeploymentAn API object that manages a replicated application. , 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.
A StatefulSet operates under the same pattern as any other Controller. You define your desired state in a StatefulSet object, and the StatefulSet controller makes any necessary updates to get there from the current state.
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 provisioner, parameters, and reclaimPolicy, which are used when a Persistent VolumeAn 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.
belonging to the class needs to be dynamically provisioned. Users can request a particular class using the name of a StorageClass object.
May refer to: core Kubernetes or the source repo from which a repo was forked.
[+]A directory containing data, accessible to the containers in a podThe smallest and simplest Kubernetes object. A Pod represents a set of running containers on your cluster. .
[+]A Kubernetes volume lives as long as the podThe smallest and simplest Kubernetes object. A Pod represents a set of running containers on your cluster. that encloses it. Consequently, a volume outlives any containersA lightweight and portable executable image that contains software and all of its dependencies. that run within the podThe smallest and simplest Kubernetes object. A Pod represents a set of running containers on your cluster. , and data is preserved across containerA lightweight and portable executable image that contains software and all of its dependencies. restarts.
A Volume Plugin enables integration of storage within a PodThe smallest and simplest Kubernetes object. A Pod represents a set of running containers on your cluster. .
[+]A Volume Plugin lets you attach and mount storage volumes for use by a PodThe smallest and simplest Kubernetes object. A Pod represents a set of running containers on your cluster. . 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.
Facilitates the discussion and/or implementation of a short-lived, narrow, or decoupled project for a committee, SIGCommunity members who collectively manage an ongoing piece or aspect of the larger Kubernetes open source project. , or cross-SIG effort.
[+]Working groups are a way of organizing people to accomplish a discrete task, and are relatively easy to create and deprecate when inactive.
For more information, see the kubernetes/community repo and the current list of SIGs and working groups.
Docker 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).
Consistent and highly-available key value store used as Kubernetes’ backing store for all cluster data.
[+]Always have a backup plan for etcd’s data for your Kubernetes cluster. For in-depth information on etcd, see etcd documentation.
Component on the master that exposes the Kubernetes API. It is the front-end for the Kubernetes control plane.
[+]It is designed to scale horizontally – that is, it scales by deploying more instances. See Building High-Availability Clusters.
Component on the master that runs controllersA control loop that watches the shared state of the cluster through the apiserver and makes changes attempting to move the current state towards the desired state. .
[+]Logically, each controllerA control loop that watches the shared state of the cluster through the apiserver and makes changes attempting to move the current state towards the desired state. is a separate process, but to reduce complexity, they are all compiled into a single binary and run in a single process.
Component on the master that watches newly created pods that have no node assigned, and selects a node for them to run on.
[+]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.