Kubernetes StatefulSet - obtain spec.replicas metadata and reference elsewhere in configuration - kubernetes

Question

I am configuring a StatefulSet where I want the number of replicas (spec.replicas as shown below) available to somehow pass as a parameter into the application instance. My application needs spec.replicas to determine the numer of replicas so it knows what rows to load from a MySQL table. I don't want to hard-code the number of replicas in both spec.replicas and the application parameter as that will not work when scaling the number of replicas up or down, since the application parameter needs to adjust when scaling.
Here is my StatefulSet config:
apiVersion: apps/v1beta1
kind: StatefulSet
metadata:
labels:
run: my-app
name: my-app
namespace: my-ns
spec:
replicas: 3
selector:
matchLabels:
run: my-app
serviceName: my-app
podManagementPolicy: Parallel
template:
metadata:
labels:
run: my-app
spec:
containers:
- name: my-app
image: my-app:latest
command:
- /bin/sh
- /bin/start.sh
- dev
- 2000m
- "0"
- "3" **Needs to be replaced with # replicas**
- 127.0.0.1
- "32990"
imagePullPolicy: Always
livenessProbe:
httpGet:
path: /health
port: 8081
initialDelaySeconds: 180
periodSeconds: 10
timeoutSeconds: 3
readinessProbe:
failureThreshold: 10
httpGet:
path: /ready
port: 8081
scheme: HTTP
initialDelaySeconds: 30
periodSeconds: 15
successThreshold: 1
timeoutSeconds: 3
ports:
- containerPort: 8080
protocol: TCP
resources:
limits:
memory: 2500Mi
imagePullSecrets:
- name: snapshot-pull
restartPolicy: Always
I have read the Kubernetes docs and the spec.replicas field is scoped at the pod or container level, never the StatefulSet, at least as far as I have seen.
Thanks in advance.

Answer 1

You could use a yaml anchor to do this:
Check out:
https://helm.sh/docs/chart_template_guide/yaml_techniques/#yaml-anchors
apiVersion: apps/v1beta1
kind: StatefulSet
metadata:
labels:
run: my-app
name: my-app
namespace: my-ns
spec:
replicas: &numReplicas 3
selector:
matchLabels:
run: my-app
serviceName: my-app
podManagementPolicy: Parallel
template:
metadata:
labels:
run: my-app
spec:
containers:
- name: my-app
image: my-app:latest
command:
- /bin/sh
- /bin/start.sh
- dev
- 2000m
- "0"
- *numReplicas
- 127.0.0.1
- "32990"
imagePullPolicy: Always
livenessProbe:
httpGet:
path: /health
port: 8081
initialDelaySeconds: 180
periodSeconds: 10
timeoutSeconds: 3
readinessProbe:
failureThreshold: 10
httpGet:
path: /ready
port: 8081
scheme: HTTP
initialDelaySeconds: 30
periodSeconds: 15
successThreshold: 1
timeoutSeconds: 3
ports:
- containerPort: 8080
protocol: TCP
resources:
limits:
memory: 2500Mi
imagePullSecrets:
- name: snapshot-pull
restartPolicy: Always

Answer 2

Normally you would use the downward api for this kind of thing. https://kubernetes.io/docs/tasks/inject-data-application/downward-api-volume-expose-pod-information/
However it is currently not possible for kubernetes to propagate deployment/statefulset spec data into the pod spec with the downward api, nor should it be. If you are responsible for this software I'd set up some internal functionality so that it can find it's peers and determine their count periodically.