How can we create service dependencies using kubernetes

Question

I have 2 services. One containing 2 pod replicas for a web application which is dependent on another backend service having pod (2 replicas) for MySQL containers.

The web application uses environment variables set by backend DB service. I've all the json inside the same directory.

Is there any way to express the dependencies so that kubectl always create (and run) backend pods and services before it starts the web application service? I have used kubedeam to create the cluster.

Answer 1

I can suggest two solutions:

First, to attach an init container to the web servers that waits until MySQL is up and running. The deployment would be something like this:

apiVersion: apps/v1
kind: Deployment
metadata:
  name: web
spec:
  selector:
    matchLabels:
      app: web
  replicas: 2
  template:
    metadata:
      labels:
        app: web
    spec:
      initContainers:
      - name: init-wait
        image: alpine
        command: ["sh", "-c", "for i in $(seq 1 300); do nc -zvw1 mysql 3306 && exit 0 || sleep 3; done; exit 1"]
      containers:
      - name: web
        image: web-server
        ports:
        - containerPort: 80
          protocol: TCP

It uses netcat to try to start a TCP connection to the mysql service on the port 3306 every 3 seconds. Once it achieves to connect, the init-container ends and the web server starts normally.

The second option is to use Mirantis AppController. It allows you to create dependency objects as you need between server and database deployments. Check their repo for a full documentation.

Answer 2

Having the same problem and as what they recommend, using the k8s initContainers solved my problem.

Update

Added sample code.

kind: Service
apiVersion: v1
metadata:
  name:  postgres-service
spec:
  # ...
---
apiVersion: apps/v1
kind: Deployment
# ...
spec:
  # ...
  template:
    # ...
    spec:
      # wait for postgres-service to run first
      initContainers:
      - name: init-wait-for-db
        image: alpine
        command: ["/bin/sh", "-c", "for i in $(seq 1 300); do nc -zvw1 postgres-service 5432 && exit 0 || sleep 3; done; exit 1"]
      containers:
      - name: my-django-app
        image: dockerhubuser/my-django-app
        command: ["/bin/sh", "-c", "python /root/django/manage.py migrate && python /root/django/manage.py runserver 0.0.0.0:8000 --noreload"]
        ports:
        - containerPort: 8000
        env:
          # ...

Answer 3

Use readiness probe or init container, refer to here