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Stop munger produce the link to a release version doc if it doesn't exit in the release branch
217 lines
10 KiB
Markdown
217 lines
10 KiB
Markdown
<!-- BEGIN MUNGE: UNVERSIONED_WARNING -->
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<!-- BEGIN STRIP_FOR_RELEASE -->
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<img src="http://kubernetes.io/img/warning.png" alt="WARNING"
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width="25" height="25">
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<img src="http://kubernetes.io/img/warning.png" alt="WARNING"
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width="25" height="25">
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<img src="http://kubernetes.io/img/warning.png" alt="WARNING"
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width="25" height="25">
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<img src="http://kubernetes.io/img/warning.png" alt="WARNING"
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width="25" height="25">
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<img src="http://kubernetes.io/img/warning.png" alt="WARNING"
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width="25" height="25">
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<h2>PLEASE NOTE: This document applies to the HEAD of the source tree</h2>
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If you are using a released version of Kubernetes, you should
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refer to the docs that go with that version.
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<!-- TAG RELEASE_LINK, added by the munger automatically -->
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<strong>
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The latest release of this document can be found
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[here](http://releases.k8s.io/release-1.1/docs/user-guide/labels.md).
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Documentation for other releases can be found at
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[releases.k8s.io](http://releases.k8s.io).
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</strong>
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--
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<!-- END STRIP_FOR_RELEASE -->
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<!-- END MUNGE: UNVERSIONED_WARNING -->
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# Labels
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**Table of Contents**
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<!-- BEGIN MUNGE: GENERATED_TOC -->
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- [Labels](#labels)
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- [Motivation](#motivation)
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- [Syntax and character set](#syntax-and-character-set)
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- [Label selectors](#label-selectors)
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- [_Equality-based_ requirement](#equality-based-requirement)
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- [_Set-based_ requirement](#set-based-requirement)
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- [API](#api)
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- [LIST and WATCH filtering](#list-and-watch-filtering)
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- [Set references in API objects](#set-references-in-api-objects)
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- [Service and ReplicationController](#service-and-replicationcontroller)
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- [Job and other new resources](#job-and-other-new-resources)
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<!-- END MUNGE: GENERATED_TOC -->
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_Labels_ are key/value pairs that are attached to objects, such as pods.
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Labels are intended to be used to specify identifying attributes of objects that are meaningful and relevant to users, but which do not directly imply semantics to the core system.
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Labels can be used to organize and to select subsets of objects. Labels can be attached to objects at creation time and subsequently added and modified at any time.
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Each object can have a set of key/value labels defined. Each Key must be unique for a given object.
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```json
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"labels": {
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"key1" : "value1",
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"key2" : "value2"
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}
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```
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We'll eventually index and reverse-index labels for efficient queries and watches, use them to sort and group in UIs and CLIs, etc. We don't want to pollute labels with non-identifying, especially large and/or structured, data. Non-identifying information should be recorded using [annotations](annotations.md).
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## Motivation
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Labels enable users to map their own organizational structures onto system objects in a loosely coupled fashion, without requiring clients to store these mappings.
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Service deployments and batch processing pipelines are often multi-dimensional entities (e.g., multiple partitions or deployments, multiple release tracks, multiple tiers, multiple micro-services per tier). Management often requires cross-cutting operations, which breaks encapsulation of strictly hierarchical representations, especially rigid hierarchies determined by the infrastructure rather than by users.
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Example labels:
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* `"release" : "stable"`, `"release" : "canary"`
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* `"environment" : "dev"`, `"environment" : "qa"`, `"environment" : "production"`
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* `"tier" : "frontend"`, `"tier" : "backend"`, `"tier" : "cache"`
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* `"partition" : "customerA"`, `"partition" : "customerB"`
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* `"track" : "daily"`, `"track" : "weekly"`
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These are just examples; you are free to develop your own conventions.
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## Syntax and character set
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_Labels_ are key value pairs. Valid label keys have two segments: an optional prefix and name, separated by a slash (`/`). The name segment is required and must be 63 characters or less, beginning and ending with an alphanumeric character (`[a-z0-9A-Z]`) with dashes (`-`), underscores (`_`), dots (`.`), and alphanumerics between. The prefix is optional. If specified, the prefix must be a DNS subdomain: a series of DNS labels separated by dots (`.`), not longer than 253 characters in total, followed by a slash (`/`).
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If the prefix is omitted, the label key is presumed to be private to the user. Automated system components (e.g. `kube-scheduler`, `kube-controller-manager`, `kube-apiserver`, `kubectl`, or other third-party automation) which add labels to end-user objects must specify a prefix. The `kubernetes.io/` prefix is reserved for Kubernetes core components.
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Valid label values must be 63 characters or less and must be empty or begin and end with an alphanumeric character (`[a-z0-9A-Z]`) with dashes (`-`), underscores (`_`), dots (`.`), and alphanumerics between.
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## Label selectors
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Unlike [names and UIDs](identifiers.md), labels do not provide uniqueness. In general, we expect many objects to carry the same label(s).
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Via a _label selector_, the client/user can identify a set of objects. The label selector is the core grouping primitive in Kubernetes.
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The API currently supports two types of selectors: _equality-based_ and _set-based_.
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A label selector can be made of multiple _requirements_ which are comma-separated. In the case of multiple requirements, all must be satisfied so the comma separator acts as an _AND_ logical operator.
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An empty label selector (that is, one with zero requirements) selects every object in the collection.
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A null label selector (which is only possible for optional selector fields) selects no objects.
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### _Equality-based_ requirement
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_Equality-_ or _inequality-based_ requirements allow filtering by label keys and values. Matching objects must satisfy all of the specified label constraints, though they may have additional labels as well.
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Three kinds of operators are admitted `=`,`==`,`!=`. The first two represent _equality_ (and are simply synonyms), while the latter represents _inequality_. For example:
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```
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environment = production
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tier != frontend
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```
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The former selects all resources with key equal to `environment` and value equal to `production`.
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The latter selects all resources with key equal to `tier` and value distinct from `frontend`, and all resources with no labels with the `tier` key.
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One could filter for resources in `production` excluding `frontend` using the comma operator: `environment=production,tier!=frontend`
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### _Set-based_ requirement
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_Set-based_ label requirements allow filtering keys according to a set of values. Three kinds of operators are supported: `in`,`notin` and exists (only the key identifier). For example:
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```
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environment in (production, qa)
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tier notin (frontend, backend)
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partition
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!partition
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```
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The first example selects all resources with key equal to `environment` and value equal to `production` or `qa`.
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The second example selects all resources with key equal to `tier` and values other than `frontend` and `backend`, and all resources with no labels with the `tier` key.
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The third example selects all resources including a label with key `partition`; no values are checked.
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The fourth example selects all resources without a label with key `partition`; no values are checked.
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Similarly the comma separator acts as an _AND_ operator. So filtering resources with a `partition` key (no matter the value) and with `environment` different than `qa` can be achieved using `partition,environment notin (qa)`.
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The _set-based_ label selector is a general form of equality since `environment=production` is equivalent to `environment in (production)`; similarly for `!=` and `notin`.
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_Set-based_ requirements can be mixed with _equality-based_ requirements. For example: `partition in (customerA, customerB),environment!=qa`.
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## API
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### LIST and WATCH filtering
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LIST and WATCH operations may specify label selectors to filter the sets of objects returned using a query parameter. Both requirements are permitted:
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* _equality-based_ requirements: `?labelSelector=environment%3Dproduction,tier%3Dfrontend`
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* _set-based_ requirements: `?labelSelector=environment+in+%28production%2Cqa%29%2Ctier+in+%28frontend%29`
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Both label selector styles can be used to list or watch resources via a REST client. For example targeting `apiserver` with `kubectl` and using _equality-based_ one may write:
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```console
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$ kubectl get pods -l environment=production,tier=frontend
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```
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or using _set-based_ requirements:
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```console
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$ kubectl get pods -l 'environment in (production),tier in (frontend)'
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```
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As already mentioned _set-based_ requirements are more expressive. For instance, they can implement the _OR_ operator on values:
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```console
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$ kubectl get pods -l 'environment in (production, qa)'
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```
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or restricting negative matching via _exists_ operator:
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```console
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$ kubectl get pods -l 'environment,environment notin (frontend)'
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```
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### Set references in API objects
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Some Kubernetes objects, such as [`service`s](services.md) and [`replicationcontroller`s](replication-controller.md), also use label selectors to specify sets of other resources, such as [pods](pods.md).
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#### Service and ReplicationController
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The set of pods that a `service` targets is defined with a label selector. Similarly, the population of pods that a `replicationcontroller` should manage is also defined with a label selector.
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Labels selectors for both objects are defined in `json` or `yaml` files using maps, and only _equality-based_ requirement selectors are supported:
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```json
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"selector": {
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"component" : "redis",
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}
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```
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or
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```yaml
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selector:
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component: redis
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```
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this selector (respectively in `json` or `yaml` format) is equivalent to `component=redis` or `component in (redis)`.
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#### Job and other new resources
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Newer resources, such as [job](jobs.md), support _set-based_ requirements as well.
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```yaml
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selector:
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matchLabels:
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component: redis
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matchExpressions:
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- {key: tier, operator: In, values: [cache]}
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- {key: environment, operator: NotIn, values: [dev]}
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```
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`matchLabels` is a map of `{key,value}` pairs. A single `{key,value}` in the `matchLabels` map is equivalent to an element of `matchExpressions`, whose `key` field is "key", the `operator` is "In", and the `values` array contains only "value". `matchExpressions` is a list of pod selector requirements. Valid operators include In, NotIn, Exists, and DoesNotExist. The values set must be non-empty in the case of In and NotIn. All of the requirements, from both `matchLabels` and `matchExpressions` are ANDed together -- they must all be satisfied in order to match.
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<!-- BEGIN MUNGE: GENERATED_ANALYTICS -->
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[]()
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<!-- END MUNGE: GENERATED_ANALYTICS -->
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