Is it possible to pass the actual number of replicas to a pod in a StatefulSet?

General Discussions
1

I created a StatefulSet and I want to pass to each of its pods the number of pods in this StatefulSet through an environment variable named REPLICASIZE. Is this possible? My yaml looks as:

---
    apiVersion: apps/v1
    kind: StatefulSet
    metadata:
      name: php-app-4-rcweb
      namespace: kube-system
      labels:
        app: mypod
    spec:
      replicas: 3
      selector:
        matchLabels:
          app: mypod
      template:
        metadata:
          labels:
            app: mypod
        spec:
          containers:
          - name: php-apache
            image: my-registry.local:5000/my-image:0.0.1  # Or your custom image

            env:
             - name: TZ
               ...
             - name: REPLICASIZE
               value: ................

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Cluster information:

Kubernetes version:

$ k3s -version
k3s version v1.33.3+k3s1 (236cbf25)
go version go1.24.4

Cloud being used: bare metal

Installation method:

curl -sfL https://get.k3s.io | sh -s -- --cluster-cidr=142.200.0.0/16 --service-cidr=142.201.0.0/16

Host OS: Ubuntu Linux 24
CNI and version: default
CRI and version: default

2

Natively, it’s not supported.

But you can create a headless svc that will return ips of all replicas, which you can use to get the count of replicas

something like this

amazinrahul@archlinux /tmp> k get svc,ep -n kube-system 
Warning: v1 Endpoints is deprecated in v1.33+; use discovery.k8s.io/v1 EndpointSlice
NAME               TYPE        CLUSTER-IP    EXTERNAL-IP   PORT(S)                  AGE
service/antrea     ClusterIP   10.104.9.31   <none>        443/TCP                  3h33m
service/kube-dns   ClusterIP   10.96.0.10    <none>        53/UDP,53/TCP,9153/TCP   3h33m
service/php-app-4-rcweb-headless      ClusterIP   None          <none>        80/TCP                   8s

NAME                 ENDPOINTS                                               AGE
endpoints/antrea     192.168.1.169:10349                                     3h33m
endpoints/kube-dns   10.244.0.2:53,10.244.0.3:53,10.244.0.2:53 + 3 more...   3h33m
endpoints/php-app-4-rcweb-headless      10.244.0.21:80,10.244.0.22:80,10.244.0.23:80            8s
amazinrahul@archlinux /tmp> k exec -it -n kube-system php-app-4-rcweb-0 -- bash
root@php-app-4-rcweb-0:/# export REPLICASIZE=$(nslookup php-app-4-rcweb-headless.kube-system.svc.cluster.local | grep -c 'Address:' | awk '{print $1-1}')
root@php-app-4-rcweb-0:/# 
root@php-app-4-rcweb-0:/# echo $REPLICASIZE
3
root@php-app-4-rcweb-0:/# nslookup php-app-4-rcweb-headless.kube-system.svc.cluster.local
;; Got recursion not available from 10.96.0.10
;; Got recursion not available from 10.96.0.10
;; Got recursion not available from 10.96.0.10
;; Got recursion not available from 10.96.0.10
Server:		10.96.0.10
Address:	10.96.0.10#53

Name:	php-app-4-rcweb-headless.kube-system.svc.cluster.local
Address: 10.244.0.21
Name:	php-app-4-rcweb-headless.kube-system.svc.cluster.local
Address: 10.244.0.22
Name:	php-app-4-rcweb-headless.kube-system.svc.cluster.local
Address: 10.244.0.23
3

Hi @joseccz ,

What happens if you scale number of pods in statefulset? What are your expectations in this case?

4

Statefulset pods are indexed as with arrays and this could be great to create code that could use those indexes.