How to pass args to pods based on Ordinal Index in StatefulSets?

Question

Is it possible to pass different args to pods based on their ordinal index in StatefulSets? Didn't find the answer on the StatefulSets documentation. Thanks!

Answer 1

Recommended way, see https://kubernetes.io/docs/tasks/run-application/run-replicated-stateful-application/#statefulset

# Generate server-id from pod ordinal index.

[[ `hostname` =~ -([0-9]+)$ ]] || exit 1
ordinal=${BASH_REMATCH[1]}

# ${ordinal} now holds the replica number

server-id=$((100 + $ordinal))

# Copy appropriate conf.d files from config-map to emptyDir.
if [[ $ordinal -eq 0 ]]; then
  # do something
else
  # do something else
fi

Answer 2

I don't know of a non-hacky way to do it, but I know a hack that works. First, each pod in a StatefulSet gets a unique predictable name. It can discover that name via the downward API or just by calling hostname. So I have shell script as then entrypoint to my container and that script gets it's pod/hostname. From there it calls the "real" executable using command line args appropriate for the specific host.

For example, one of my scripts expects the pod name to be mapped into the environment as POD_NAME via downward api. It then does something like:

#!/bin/bash

pet_number=${POD_NAME##*-}

if [ pet_number == 0 ]
then
   # stuff here
fi

# etc.

Answer 3

I found one less hacky way to pass in ordinal index into container using the lifecycle hooks

  containers:
  - name: cp-kafka
    imagePullPolicy: Always
    image: confluentinc/cp-kafka:4.0.0
    resources:
      requests:
        memory: "2Gi"
        cpu: 0.5
    ports:
    - containerPort: 9093
      name: server
      protocol: TCP
    lifecycle:
      postStart:
        exec:
          command: ["/bin/sh", "-c", "export KAFKA_BROKER_ID=${HOSTNAME##*-}"]
    env:
      - name: KAFKA_ZOOKEEPER_CONNECT
        value: zk-cs.default.svc.cluster.local:2181
      - name: KAFKA_ADVERTISED_LISTENERS
        value: PLAINTEXT://localhost:9093