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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?

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

So, why will I need the selectors as well?

.spec.selector is an optional 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.

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    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
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    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

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