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13

3

From Kubernetes documentation

The selector field defines how the Deployment finds which Pods to
manage.

But, when creating deployment, I already specify the pod template as part of the deployment. So, why will I need the selectors as well?

Is it supposed to be used like services, where pods are already being started separately, but later brought under the umbrella of Deployment to be managed together?

kubernetes

share|improve this question

asked Jun 19 '18 at 18:45

PaddyPaddy

18511 silver badge88 bronze badges

add a comment | 

13

3

From Kubernetes documentation

The selector field defines how the Deployment finds which Pods to
manage.

But, when creating deployment, I already specify the pod template as part of the deployment. So, why will I need the selectors as well?

Is it supposed to be used like services, where pods are already being started separately, but later brought under the umbrella of Deployment to be managed together?

kubernetes

share|improve this question

asked Jun 19 '18 at 18:45

PaddyPaddy

18511 silver badge88 bronze badges

add a comment | 

From Kubernetes documentation

The selector field defines how the Deployment finds which Pods to
manage.

But, when creating deployment, I already specify the pod template as part of the deployment. So, why will I need the selectors as well?

Is it supposed to be used like services, where pods are already being started separately, but later brought under the umbrella of Deployment to be managed together?

kubernetes

share|improve this question

asked Jun 19 '18 at 18:45

PaddyPaddy

18511 silver badge88 bronze badges

share|improve this question

asked Jun 19 '18 at 18:45

PaddyPaddy

18511 silver badge88 bronze badges

share|improve this question

asked Jun 19 '18 at 18:45

PaddyPaddy

18511 silver badge88 bronze badges

asked Jun 19 '18 at 18:45

PaddyPaddy

18511 silver badge88 bronze badges

asked Jun 19 '18 at 18:45

PaddyPaddy

18511 silver badge88 bronze badges

1 Answer
1

active

oldest

votes

5

Answer for this question we can find in section Deployments from kubernetes.io

So, why will I need the selectors as well?

Quotes below from documentation for k8s v 1.14

.spec.selector is an required field that specifies a label selector
for the Pods targeted by this deployment.

.spec.selector must match .spec.template.metadata.labels, or it
will be rejected by the API.

In API version apps/v1, .spec.selector and .metadata.labels do not
default to .spec.template.metadata.labels if not set. So they must be
set explicitly. Also note that .spec.selector is immutable after
creation of the Deployment in apps/v1.

A Deployment may terminate Pods whose labels match the selector if
their template is different from .spec.template or if the total number
of such Pods exceeds .spec.replicas. It brings up new Pods with
.spec.template if the number of Pods is less than the desired number.

Pods are already being started separately, but later brought under the umbrella of Deployment to be managed together?

Simply speaking, No

Note: You should not create other pods whose labels match this
selector, either directly, by creating another Deployment, or by
creating another controller such as a ReplicaSet or a
ReplicationController. If you do so, the first Deployment thinks
that it created these other pods. Kubernetes does not stop you from
doing this. If you have multiple controllers that have overlapping
selectors, the controllers will fight with each other and won’t behave
correctly.

share|improve this answer

edited Jun 10 at 11:59

answered Jun 19 '18 at 19:17

alexander.polomodovalexander.polomodov

1,02633 gold badges77 silver badges1212 bronze badges

• 
  
  
  

  
  4
  

  
  
  
  
  
  

  
  
  So what is it used for? It seems like you should always make the selector match the labels of the spec… Is there some case where it is useful not to do so?
  
  – Victor Noël
  Aug 28 '18 at 10:11
  

  
  
  
  


• 
  
  
  

  
  3
  

  
  
  
  
  
  

  
  
  Doesn't really answer the question Why? - Why matchLabels exist and must match .spec.template.metadata.labels? What's the point of it, as spec is anyway defined below Deployment so it's clear what Pods are started for the Deployment.
  
  – Ivan
  Jan 15 at 20:07
  
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  Anyone who bumps into this discussion should check the latest documentation. For example, currently, .spec.selector is not an optional field. It is a required field. kubernetes.io/docs/concepts/workloads/controllers/deployment
  
  – SoftwareTheory
  Jun 10 at 11:34
  
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Thanks a lot for your comment. I think that in new documentation authors have corrected optional to required, because in old docs this field was told as optional but on next line authors added that this field must be set explicitly:) So effectively this field also was required
  
  – alexander.polomodov
  Jun 10 at 12:06
  

  
  
  
  

add a comment | 

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5

Answer for this question we can find in section Deployments from kubernetes.io

So, why will I need the selectors as well?

Quotes below from documentation for k8s v 1.14

.spec.selector is an required field that specifies a label selector
for the Pods targeted by this deployment.

.spec.selector must match .spec.template.metadata.labels, or it
will be rejected by the API.

In API version apps/v1, .spec.selector and .metadata.labels do not
default to .spec.template.metadata.labels if not set. So they must be
set explicitly. Also note that .spec.selector is immutable after
creation of the Deployment in apps/v1.

A Deployment may terminate Pods whose labels match the selector if
their template is different from .spec.template or if the total number
of such Pods exceeds .spec.replicas. It brings up new Pods with
.spec.template if the number of Pods is less than the desired number.

Pods are already being started separately, but later brought under the umbrella of Deployment to be managed together?

Simply speaking, No

Note: You should not create other pods whose labels match this
selector, either directly, by creating another Deployment, or by
creating another controller such as a ReplicaSet or a
ReplicationController. If you do so, the first Deployment thinks
that it created these other pods. Kubernetes does not stop you from
doing this. If you have multiple controllers that have overlapping
selectors, the controllers will fight with each other and won’t behave
correctly.

share|improve this answer

edited Jun 10 at 11:59

answered Jun 19 '18 at 19:17

alexander.polomodovalexander.polomodov

1,02633 gold badges77 silver badges1212 bronze badges

• 
  
  
  

  
  4
  

  
  
  
  
  
  

  
  
  So what is it used for? It seems like you should always make the selector match the labels of the spec… Is there some case where it is useful not to do so?
  
  – Victor Noël
  Aug 28 '18 at 10:11
  

  
  
  
  


• 
  
  
  

  
  3
  

  
  
  
  
  
  

  
  
  Doesn't really answer the question Why? - Why matchLabels exist and must match .spec.template.metadata.labels? What's the point of it, as spec is anyway defined below Deployment so it's clear what Pods are started for the Deployment.
  
  – Ivan
  Jan 15 at 20:07
  
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  Anyone who bumps into this discussion should check the latest documentation. For example, currently, .spec.selector is not an optional field. It is a required field. kubernetes.io/docs/concepts/workloads/controllers/deployment
  
  – SoftwareTheory
  Jun 10 at 11:34
  
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Thanks a lot for your comment. I think that in new documentation authors have corrected optional to required, because in old docs this field was told as optional but on next line authors added that this field must be set explicitly:) So effectively this field also was required
  
  – alexander.polomodov
  Jun 10 at 12:06
  

  
  
  
  

add a comment | 

5

Answer for this question we can find in section Deployments from kubernetes.io

So, why will I need the selectors as well?

Quotes below from documentation for k8s v 1.14

.spec.selector is an required field that specifies a label selector
for the Pods targeted by this deployment.

.spec.selector must match .spec.template.metadata.labels, or it
will be rejected by the API.

In API version apps/v1, .spec.selector and .metadata.labels do not
default to .spec.template.metadata.labels if not set. So they must be
set explicitly. Also note that .spec.selector is immutable after
creation of the Deployment in apps/v1.

A Deployment may terminate Pods whose labels match the selector if
their template is different from .spec.template or if the total number
of such Pods exceeds .spec.replicas. It brings up new Pods with
.spec.template if the number of Pods is less than the desired number.

Pods are already being started separately, but later brought under the umbrella of Deployment to be managed together?

Simply speaking, No

Note: You should not create other pods whose labels match this
selector, either directly, by creating another Deployment, or by
creating another controller such as a ReplicaSet or a
ReplicationController. If you do so, the first Deployment thinks
that it created these other pods. Kubernetes does not stop you from
doing this. If you have multiple controllers that have overlapping
selectors, the controllers will fight with each other and won’t behave
correctly.

share|improve this answer

edited Jun 10 at 11:59

answered Jun 19 '18 at 19:17

alexander.polomodovalexander.polomodov

1,02633 gold badges77 silver badges1212 bronze badges

• 
  
  
  

  
  4
  

  
  
  
  
  
  

  
  
  So what is it used for? It seems like you should always make the selector match the labels of the spec… Is there some case where it is useful not to do so?
  
  – Victor Noël
  Aug 28 '18 at 10:11
  

  
  
  
  


• 
  
  
  

  
  3
  

  
  
  
  
  
  

  
  
  Doesn't really answer the question Why? - Why matchLabels exist and must match .spec.template.metadata.labels? What's the point of it, as spec is anyway defined below Deployment so it's clear what Pods are started for the Deployment.
  
  – Ivan
  Jan 15 at 20:07
  
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  Anyone who bumps into this discussion should check the latest documentation. For example, currently, .spec.selector is not an optional field. It is a required field. kubernetes.io/docs/concepts/workloads/controllers/deployment
  
  – SoftwareTheory
  Jun 10 at 11:34
  
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Thanks a lot for your comment. I think that in new documentation authors have corrected optional to required, because in old docs this field was told as optional but on next line authors added that this field must be set explicitly:) So effectively this field also was required
  
  – alexander.polomodov
  Jun 10 at 12:06
  

  
  
  
  

add a comment | 

Answer for this question we can find in section Deployments from kubernetes.io

So, why will I need the selectors as well?

Quotes below from documentation for k8s v 1.14

.spec.selector is an required field that specifies a label selector
for the Pods targeted by this deployment.

.spec.selector must match .spec.template.metadata.labels, or it
will be rejected by the API.

In API version apps/v1, .spec.selector and .metadata.labels do not
default to .spec.template.metadata.labels if not set. So they must be
set explicitly. Also note that .spec.selector is immutable after
creation of the Deployment in apps/v1.

A Deployment may terminate Pods whose labels match the selector if
their template is different from .spec.template or if the total number
of such Pods exceeds .spec.replicas. It brings up new Pods with
.spec.template if the number of Pods is less than the desired number.

Pods are already being started separately, but later brought under the umbrella of Deployment to be managed together?

Simply speaking, No

Note: You should not create other pods whose labels match this
selector, either directly, by creating another Deployment, or by
creating another controller such as a ReplicaSet or a
ReplicationController. If you do so, the first Deployment thinks
that it created these other pods. Kubernetes does not stop you from
doing this. If you have multiple controllers that have overlapping
selectors, the controllers will fight with each other and won’t behave
correctly.

share|improve this answer

edited Jun 10 at 11:59

answered Jun 19 '18 at 19:17

alexander.polomodovalexander.polomodov

1,02633 gold badges77 silver badges1212 bronze badges

share|improve this answer

edited Jun 10 at 11:59

answered Jun 19 '18 at 19:17

alexander.polomodovalexander.polomodov

1,02633 gold badges77 silver badges1212 bronze badges

answered Jun 19 '18 at 19:17

alexander.polomodovalexander.polomodov

1,02633 gold badges77 silver badges1212 bronze badges

answered Jun 19 '18 at 19:17

alexander.polomodovalexander.polomodov

1,02633 gold badges77 silver badges1212 bronze badges