K8S Depends

Motivations

This Go program aims to give Kubernetes a similar capability that Docker Compose offers : depends_on. In Docker Compose, this keyword allows to programmatically set the order in which the containers are to start.

Kubernetes does not propose this functionality as it is out of the tool's scope. Hence, this small Go program and its Docker image offers a simple solution to make sure that containers inside a Pod will not start until the specified Kubernetes service are alive.

The program accepts the following flags:

Flag	Option	Default Value
--timeout	Set the timeout in seconds for each service after which it is considered down	60
--sleep	Set the timeout in seconds between two consecutive polls of a given service	1

The rest of the arguments passed to the program are the services in form SERVICE_NAME:SERVICE_PORT you want to wait for.

Install

This program is available as a Docker image and is hosted on Docker Hub:

$ docker pull s0ders/k8s-depends

Examples

apiVersion: v1
kind: Pod
metadata:
  name: hello_world
spec:
  # Image is used as an initContainers to prevent containers from starting
  # before the given services are up and running.
  initContainers:
  - name: depends
    image: s0ders/depends
    args: ["--timeout=120", "database-svc:5432", "cache-svc:6379"]
  containers:
  - name: web_server
    image: nginx

To check the logs of your "depends" initContainers you can use the following kubectl command:

kubectl logs <pod_name> -c <init_container_name>