Labels and Selectors (2024)

Labels are key/value pairs that are attached toobjects such as Pods.Labels are intended to be used to specify identifying attributes of objectsthat are meaningful and relevant to users, but do not directly imply semanticsto the core system. Labels can be used to organize and to select subsets ofobjects. Labels can be attached to objects at creation time and subsequentlyadded and modified at any time. Each object can have a set of key/value labelsdefined. Each Key must be unique for a given object.

"metadata": { "labels": { "key1" : "value1", "key2" : "value2" }}

Labels allow for efficient queries and watches and are ideal for use in UIsand CLIs. Non-identifying information should be recorded usingannotations.

Motivation

Labels enable users to map their own organizational structures onto system objectsin a loosely coupled fashion, without requiring clients to store these mappings.

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 rigidhierarchies determined by the infrastructure rather than by users.

Example labels:

• "release" : "stable", "release" : "canary"
• "environment" : "dev", "environment" : "qa", "environment" : "production"
• "tier" : "frontend", "tier" : "backend", "tier" : "cache"
• "partition" : "customerA", "partition" : "customerB"
• "track" : "daily", "track" : "weekly"

These are examples ofcommonly used labels;you are free to develop your own conventions.Keep in mind that label Key must be unique for a given object.

Syntax and character set

Labels are key/value pairs. Valid label keys have two segments: an optionalprefix and name, separated by a slash (/). The name segment is required andmust be 63 characters or less, beginning and ending with an alphanumericcharacter ([a-z0-9A-Z]) with dashes (-), underscores (_), dots (.),and alphanumerics between. The prefix is optional. If specified, the prefixmust be a DNS subdomain: a series of DNS labels separated by dots (.),not longer than 253 characters in total, followed by a slash (/).

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 labelsto end-user objects must specify a prefix.

The kubernetes.io/ and k8s.io/ prefixes arereserved for Kubernetes core components.

Valid label value:

• must be 63 characters or less (can be empty),
• unless empty, must begin and end with an alphanumeric character ([a-z0-9A-Z]),
• could contain dashes (-), underscores (_), dots (.), and alphanumerics between.

For example, here's a manifest for a Pod that has two labelsenvironment: production and app: nginx:

apiVersion: v1kind: Podmetadata: name: label-demo labels: environment: production app: nginxspec: containers: - name: nginx image: nginx:1.14.2 ports: - containerPort: 80

Label selectors

Unlike names and UIDs, labelsdo not provide uniqueness. In general, we expect many objects to carry the same label(s).

Via a label selector, the client/user can identify a set of objects.The label selector is the core grouping primitive in Kubernetes.

The API currently supports two types of selectors: equality-based and set-based.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 separatoracts as a logical AND (&&) operator.

The semantics of empty or non-specified selectors are dependent on the context,and API types that use selectors should document the validity and meaning ofthem.

Note: For some API types, such as ReplicaSets, the label selectors of two instances mustnot overlap within a namespace, or the controller can see that as conflictinginstructions and fail to determine how many replicas should be present.

Caution: For both equality-based and set-based conditions there is no logical OR (||) operator.Ensure your filter statements are structured accordingly.

Equality-based requirement

Equality- or inequality-based requirements allow filtering by label keys and values.Matching objects must satisfy all of the specified label constraints, though they mayhave additional labels as well. Three kinds of operators are admitted =,==,!=.The first two represent equality (and are synonyms), while the latter represents inequality.For example:

environment = productiontier != frontend

The former selects all resources with key equal to environment and value equal to production.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. One could filter for resources in productionexcluding frontend using the comma operator: environment=production,tier!=frontend

One usage scenario for equality-based label requirement is for Pods to specifynode selection criteria. For example, the sample Pod below selects nodes withthe label "accelerator=nvidia-tesla-p100".

apiVersion: v1kind: Podmetadata: name: cuda-testspec: containers: - name: cuda-test image: "registry.k8s.io/cuda-vector-add:v0.1" resources: limits: nvidia.com/gpu: 1 nodeSelector: accelerator: nvidia-tesla-p100

Set-based requirement

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:

environment in (production, qa)tier notin (frontend, backend)partition!partition

• The first example selects all resources with key equal to environment and valueequal to production or qa.
• The second example selects all resources with key equal to tier and values otherthan frontend and backend, and all resources with no labels with the tier key.
• The third example selects all resources including a label with key partition;no values are checked.
• The fourth example selects all resources without a label with key partition;no values are checked.

Similarly the comma separator acts as an AND operator. So filtering resourceswith a partition key (no matter the value) and with environment differentthan qa can be achieved using partition,environment notin (qa).The set-based label selector is a general form of equality sinceenvironment=production is equivalent to environment in (production);similarly for != and notin.

Set-based requirements can be mixed with equality-based requirements.For example: partition in (customerA, customerB),environment!=qa.

API

LIST and WATCH filtering

LIST and WATCH operations may specify label selectors to filter the sets of objectsreturned using a query parameter. Both requirements are permitted(presented here as they would appear in a URL query string):

• equality-based requirements: ?labelSelector=environment%3Dproduction,tier%3Dfrontend
• set-based requirements: ?labelSelector=environment+in+%28production%2Cqa%29%2Ctier+in+%28frontend%29

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:

kubectl get pods -l environment=production,tier=frontend

or using set-based requirements:

kubectl get pods -l 'environment in (production),tier in (frontend)'

As already mentioned set-based requirements are more expressive.For instance, they can implement the OR operator on values:

kubectl get pods -l 'environment in (production, qa)'

or restricting negative matching via notin operator:

kubectl get pods -l 'environment,environment notin (frontend)'

Set references in API objects

Some Kubernetes objects, such as servicesand replicationcontrollers,also use label selectors to specify sets of other resources, such aspods.

Service and ReplicationController

The set of pods that a service targets is defined with a label selector.Similarly, the population of pods that a replicationcontroller shouldmanage is also defined with a label selector.

Label selectors for both objects are defined in json or yaml files using maps,and only equality-based requirement selectors are supported:

"selector": { "component" : "redis",}

or

selector: component: redis

This selector (respectively in json or yaml format) is equivalent tocomponent=redis or component in (redis).

Resources that support set-based requirements

Newer resources, such as Job,Deployment,ReplicaSet, andDaemonSet,support set-based requirements as well.

selector: matchLabels: component: redis matchExpressions: - { key: tier, operator: In, values: [cache] } - { key: environment, operator: NotIn, values: [dev] }

matchLabels is a map of {key,value} pairs. A single {key,value} in thematchLabels map is equivalent to an element of matchExpressions, whose keyfield is "key", the operator is "In", and the values array contains only "value".matchExpressions is a list of pod selector requirements. Valid operators includeIn, NotIn, Exists, and DoesNotExist. The values set must be non-empty in the case ofIn and NotIn. All of the requirements, from both matchLabels and matchExpressionsare ANDed together -- they must all be satisfied in order to match.

Selecting sets of nodes

One use case for selecting over labels is to constrain the set of nodes onto whicha pod can schedule. See the documentation onnode selection for more information.

Using labels effectively

You can apply a single label to any resources, but this is not always thebest practice. There are many scenarios where multiple labels should be used todistinguish resource sets from one another.

For instance, different applications would use different values for the app label, but amulti-tier application, such as the guestbook example,would additionally need to distinguish each tier. The frontend could carry the following labels:

labels: app: guestbook tier: frontend

while the Redis master and replica would have different tier labels, and perhaps even anadditional role label:

labels: app: guestbook tier: backend role: master

and

labels: app: guestbook tier: backend role: replica

The labels allow for slicing and dicing the resources along any dimension specified by a label:

kubectl apply -f examples/guestbook/all-in-one/guestbook-all-in-one.yamlkubectl get pods -Lapp -Ltier -Lrole

NAME READY STATUS RESTARTS AGE APP TIER ROLEguestbook-fe-4nlpb 1/1 Running 0 1m guestbook frontend <none>guestbook-fe-ght6d 1/1 Running 0 1m guestbook frontend <none>guestbook-fe-jpy62 1/1 Running 0 1m guestbook frontend <none>guestbook-redis-master-5pg3b 1/1 Running 0 1m guestbook backend masterguestbook-redis-replica-2q2yf 1/1 Running 0 1m guestbook backend replicaguestbook-redis-replica-qgazl 1/1 Running 0 1m guestbook backend replicamy-nginx-divi2 1/1 Running 0 29m nginx <none> <none>my-nginx-o0ef1 1/1 Running 0 29m nginx <none> <none>

kubectl get pods -lapp=guestbook,role=replica

NAME READY STATUS RESTARTS AGEguestbook-redis-replica-2q2yf 1/1 Running 0 3mguestbook-redis-replica-qgazl 1/1 Running 0 3m

Updating labels

Sometimes you may want to relabel existing pods and other resources before creatingnew resources. This can be done with kubectl label.For example, if you want to label all your NGINX Pods as frontend tier, run:

kubectl label pods -l app=nginx tier=fe

pod/my-nginx-2035384211-j5fhi labeledpod/my-nginx-2035384211-u2c7e labeledpod/my-nginx-2035384211-u3t6x labeled

This first filters all pods with the label "app=nginx", and then labels them with the "tier=fe".To see the pods you labeled, run:

kubectl get pods -l app=nginx -L tier

NAME READY STATUS RESTARTS AGE TIERmy-nginx-2035384211-j5fhi 1/1 Running 0 23m femy-nginx-2035384211-u2c7e 1/1 Running 0 23m femy-nginx-2035384211-u3t6x 1/1 Running 0 23m fe

This outputs all "app=nginx" pods, with an additional label column of pods' tier(specified with -L or --label-columns).

For more information, please see kubectl label.

What's next

• Learn how to add a label to a node
• Find Well-known labels, Annotations and Taints
• See Recommended labels
• Enforce Pod Security Standards with Namespace Labels
• Read a blog on Writing a Controller for Pod Labels