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 we

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.