I am trying to deploy and Express api on GKE, with a Mongo StatefulSet.

googlecloud_ssd.yaml

kind: StorageClass
apiVersion: storage.k8s.io/v1beta1
metadata:
  name: fast
provisioner: kubernetes.io/gce-pd
parameters:
  type: pd-ssd

mongo-statefulset.yaml

apiVersion: v1
kind: Service
metadata:
  name: mongo
  labels:
    name: mongo
spec:
  ports:
  - port: 27017
    targetPort: 27017
  clusterIP: None
  selector:
    role: mongo
---
apiVersion: apps/v1beta1
kind: StatefulSet
metadata:
  name: mongo
spec:
  serviceName: "mongo"
  replicas: 2
  template:
    metadata:
      labels:
        role: mongo
        environment: test
    spec:
      terminationGracePeriodSeconds: 10
      containers:
        - name: mongo
          image: mongo
          command:
            - mongod
            - "--replSet"
            - rs0
            - "--smallfiles"
            - "--noprealloc"
          ports:
            - containerPort: 27017
          volumeMounts:
            - name: mongo-persistent-storage
              mountPath: /data/db
        - name: mongo-sidecar
          image: cvallance/mongo-k8s-sidecar
          env:
            - name: MONGO_SIDECAR_POD_LABELS
              value: "role=mongo,environment=test"
  volumeClaimTemplates:
  - metadata:
      name: mongo-persistent-storage
      annotations:
        volume.beta.kubernetes.io/storage-class: "fast"
    spec:
      accessModes: [ "ReadWriteOnce" ]
      resources:
        requests:
          storage: 10Gi

I deployed my Express app and it works perfect, I then deployed Mongo using the above yaml config.

Having set the connection string in express as: "mongodb://mongo-0.mongo,mongo-1.mongo:27017/"

I can see the updated pod(s) not starting.

Looking at the logs for that container I see

{
 insertId:  "a9tu83g211w2a6"  
 labels: {…}  
 logName:  "projects/<my-project-id>/logs/express"  
 receiveTimestamp:  "2019-06-03T14:19:14.142238836Z"  
 resource: {…}  
 severity:  "ERROR"  
 textPayload:  "[ ERROR ] MongoError: no mongos proxy available
"  
 timestamp:  "2019-06-03T14:18:56.132989616Z"  
}

I am unsure how to debug / fix MongoError: no mongos proxy available

Edit So I scaled down my replicas to 1 on each and it's now working.

I'm confused as to why this won't work more than 1 replica.