Two Kubernetes Deployments with exactly the same pod labels - kubernetes

Question

Let's say I have two deployments which are exactly the same apart from deployment name:
apiVersion: apps/v1
kind: Deployment
metadata:
name: nginx-d
spec:
replicas: 3
selector:
matchLabels:
app: mynginx
template:
metadata:
labels:
app: mynginx
spec:
containers:
- name: nginx
image: nginx
---
apiVersion: apps/v1
kind: Deployment
metadata:
name: nginx-d2
spec:
replicas: 3
selector:
matchLabels:
app: mynginx
template:
metadata:
labels:
app: mynginx
spec:
containers:
- name: nginx
image: nginx
Since these two deployments have the same selectors and the same pod template, I would expect to see three pods. However, six pods are created:
# kubectl get pods --show-labels
NAME READY STATUS RESTARTS AGE LABELS
nginx-d-5b686ccd46-dkpk7 1/1 Running 0 4m16s app=mynginx,pod-template-hash=5b686ccd46
nginx-d-5b686ccd46-nz7wf 1/1 Running 0 4m16s app=mynginx,pod-template-hash=5b686ccd46
nginx-d-5b686ccd46-vdtfr 1/1 Running 0 4m16s app=mynginx,pod-template-hash=5b686ccd46
nginx-d2-5b686ccd46-nqmq7 1/1 Running 0 4m16s app=mynginx,pod-template-hash=5b686ccd46
nginx-d2-5b686ccd46-nzrlc 1/1 Running 0 4m16s app=mynginx,pod-template-hash=5b686ccd46
nginx-d2-5b686ccd46-qgjkn 1/1 Running 0 4m16s app=mynginx,pod-template-hash=5b686ccd46
Why is that?

Answer 1

Consider this: The pods are not directly managed by a deployment, but a deployment manages a ReplicaSet.
This can be validated using
kubectl get rs
NAME DESIRED CURRENT READY AGE
nginx-d-5b686ccd46 3 3 3 74s
nginx-d2-7c76fbbbcb 3 3 0 74s
You choose which pods to consider for a replicaset or deployment by specifying the selector. In addition to that each deployment adds its own label to be able to discriminate which pods are managed by its own replicaset and which are managed by other replicasets.
You can inspect this as well:
kubectl get pods --show-labels
NAME READY STATUS RESTARTS AGE LABELS
nginx-d-5b686ccd46-7j4md 1/1 Running 0 4m app=mynginx,pod-template-hash=5b686ccd46
nginx-d-5b686ccd46-9j7tx 1/1 Running 0 4m app=mynginx,pod-template-hash=5b686ccd46
nginx-d-5b686ccd46-zt4ls 1/1 Running 0 4m app=mynginx,pod-template-hash=5b686ccd46
nginx-d2-5b686ccd46-ddcr2 1/1 Running 0 75s app=mynginx,pod-template-hash=5b686ccd46
nginx-d2-5b686ccd46-fhvm7 1/1 Running 0 79s app=mynginx,pod-template-hash=5b686ccd46
nginx-d2-5b686ccd46-q99ww 1/1 Running 0 83s app=mynginx,pod-template-hash=5b686ccd46
These are added to the replicaset as match labels:
spec:
replicas: 3
selector:
matchLabels:
app: mynginx
pod-template-hash: 5b686ccd46
Since even these are identical you can inspect the pods and see that there is an owner reference as well:
kubectl get pod nginx-d-5b686ccd46-7j4md -o yaml
apiVersion: v1
kind: Pod
metadata:
creationTimestamp: "2021-10-28T14:53:17Z"
generateName: nginx-d-5b686ccd46-
labels:
app: mynginx
pod-template-hash: 5b686ccd46
name: nginx-d-5b686ccd46-7j4md
namespace: default
ownerReferences:
- apiVersion: apps/v1
blockOwnerDeletion: true
controller: true
kind: ReplicaSet
name: nginx-d-5b686ccd46
uid: 7eb8fdaf-bfe7-4647-9180-43148a036184
resourceVersion: "556"
More information on this can be found here: https://kubernetes.io/docs/concepts/overview/working-with-objects/owners-dependents/
So a deployment (and replicaset) can disambiguate which pods are managed by which and each ensure the desired number of replicas.