Pass statefulset's replica count to it's pod

Question

I have a statefulset and I need to know what is the current replica count from inside the pod. To do so, I tried:

apiVersion: apps/v1
kind: StatefulSet
metadata:
  name: sample-mariadb
  namespace: demo
spec:
  replicas: 3
  template:
    spec:
      containers:
      - env:
        - name: REPLICAS
          valueFrom:
            fieldRef:
              apiVersion: v1
              fieldPath: spec.replicas

and got this error:

Warning FailedCreate 4m15s (x17 over 9m43s) statefulset-controller create Pod sample-mariadb-0 in StatefulSet sample-mariadb failed error: Pod "sample-mariadb-0" is invalid: spec.containers[1].env[3].valueFrom.fieldRef.fieldPath: Invalid value: "spec.replicas": error converting fieldPath: field label not supported: spec.replicas

How can I get current replica count from inside the pod?

That field is not available in the downward API https://kubernetes.io/docs/tasks/inject-data-application/environment-variable-expose-pod-information/ — Jonas, Jul 13 '21 at 08:00

score 3 · Answer 1 · answered Jul 13 '21 at 08:36

3

You can only send the fields that are part of Pod specification. The spec.replicas field is part of StatefulSet specification not the underlying pod's. The template part of a StatefulSet is the pod specification. Hence, you are getting this error.

answered Jul 13 '21 at 08:36

Emruz Hossain

4,764
18
26

1

would have been nice to have a solution to the problem and not just description of why its not working – Ivan Stanislavciuc Sep 15 '22 at 09:43

score 0 · Answer 2 · edited Dec 19 '22 at 09:49

I have looked for a solution, and the alternatives I could find is:

Setting the same environment variable

- name: REPLICAS
  value: "3"

Or you can update the value of the replicas as template variable. Example updating it as template.yaml:

replicas: ${num_replicas}
spec:
 - name: REPLICAS
   value: ${num_replicas}

export num_replicas=3
cat template.yaml | envsubst > dapi-stateful.yaml

Pass statefulset's replica count to it's pod

2 Answers2