master-discovery.yaml

---
# regular service
apiVersion: v1
kind: Service
metadata:
  name: echo-master
spec:
  selector:
    app: echo-master
  ports:
  - port: 8080
    targetPort: 8080
---
# master discovery service
apiVersion: v1
kind: Service
metadata:
  name: echo-master-discovery
  annotations:
    qoqo.dev/service-mode: active-standby
spec:
  clusterIP: None
  selector:
    app: echo-master
---
# http servers that echo the elected master's IP in a response header
apiVersion: apps/v1
kind: Deployment
metadata:
  name: echo-master
spec:
  replicas: 3
  selector:
    matchLabels:
      app: echo-master
  template:
    metadata:
      labels:
        app: echo-master
    spec:
      volumes:
      - name: master
        emptyDir: {}
      containers:
      - image: alpine
        name: echo-master
        command:
        - sh
        - -c
        - |
          trap 'jobs -p | xargs kill -TERM' TERM
          apk add -U socat
          socat -s tcp-listen:8080,fork system:"echo 'HTTP/1.1 200 OK'; printf 'Master: '; cat /tmp/master; echo" &
          wait
        volumeMounts:
        - name: master
          mountPath: /tmp
        readinessProbe:
          tcpSocket:
            port: 8080
      - image: alpine
        name: master-checker
        command:
        - sh
        - -c
        - |
          trap 'jobs -p | xargs kill -TERM' TERM
          self="$(hostname -i)"
          discover_master() { getent hosts echo-master-discovery | cut -d' ' -f1; }

          # try to resolve the master discovery service for a new master
          # update the /tmp/master file for the app container
          # if we're the master, just chill
          # otherwise, ping the master to make sure it's up
          # if the ping fails, repeat
          while :; do
              master="$(discover_master)"
              echo "$master" > /tmp/master
              if [ "$self" = "$master" ]; then
                  tail -f /dev/null
              else
                  while ping -c1 -W1 "$master" >/dev/null; do sleep 1; done
              fi
          done &

          # in addition, periodically discover the master regardless of pings
          while :; do discover_master > /tmp/master; sleep 5; done &

          wait
        volumeMounts:
        - name: master
          mountPath: /tmp