Application Deployment

A Deployment provides declarative updates for a ReplicaSet, which in turn manages Pods. The ownership chain is Deployment -> ReplicaSet -> Pods. You describe the desired state in the Deployment spec and the controller reconciles actual state toward it.

A Deployment uses apiVersion apps/v1 and kind Deployment. The apps/v1 group is stable and is also used by ReplicaSet, StatefulSet, and DaemonSet.

spec.replicas (desired Pod count), spec.selector.matchLabels (how the Deployment finds the Pods it owns), and spec.template (the Pod template used to create Pods). The selector must match the labels in the template.

The selector tells the ReplicaSet which Pods it owns. If the template's metadata.labels do not match the selector, the apiserver rejects the Deployment because the controller could not adopt the Pods it creates.

No. spec.selector is immutable for apps/v1 Deployments. You cannot change matchLabels after creation; you must delete and recreate the Deployment to change the selector.

kubectl create deployment web --image=nginx --replicas=3. The --replicas flag sets the initial desired count; omit it and the default is 1.