Kubernetes readiness watcher

Kubernetes readiness watcher is a docker container whose only job is to request the status of a service using the Kubernetes API, and succeed when the endpoint is ready. It is meant to be used as an initContainer, preventing the pod to launch further containers until the required services are ready to be consumed.

Purpose

This alpine-based Docker image fulfills only one purpose: it pauses the initialization of main containers until the required services are ready (as defined in a Pods readinessProbe property).

Alternative approaches

• Testing nslookup (see the busybox example) checks service existence, not readiness
• Using kubectl cli (as k8s-wait-for does) makes the image heavier and adds complexity
• Simply pinging a service name return false negatices (TODO: understand why)

Usage

In your deployment yaml, simply add the container as an initContainer:

apiVersion: extensions/v1beta1
kind: Deployment
spec:
  template:
    spec:
      initContainers:
      - name: waitfor-myservice # Whatever name makes sense to you
        image: ykweyer/k8s-readiness-watcher
        env:
        - name: NAMESPACE
          value: kube-public
        - name: SERVICE
          value: my-service
      # or with short syntax:
      - name: waitfor-otherservice
        image: ykweyer/k8s-readiness-watcher
        args: [ "my-service", "kube-public", "30" ]
      # you can even combine both (args has prevalence):
      - name: waitfor-lastservice
        image: ykweyer/k8s-readiness-watcher
        args: [ "my-service" ]
        envFrom:
        - configMapRef:
            name: readiness-watcher-config

You can add as many initContainers as you want, depending on how many services are required. The targeted pods readiness test should be defined.

Available parameters

Short syntax

The shell script accepts following arguments (the order matters)

$ k8s-readinessProbe service-name [namespace-name [refresh-interval]]

# test for the existence of redis in "custom" every 5 seconds:
$ k8s-readinessProbe redis custom 5

# test for the existence of a mysql service in default namespace every 15 seconds (default value):
$ k8s-readinessProbe mysql

Environment variables

Env variable are used as fallback if no args are defined. You can combine both in your yaml, which allows you to define values likely similar across several pods (like MASTER_URL, or SA_CACERT) in a shared ConfigMap

Parameter	Default value
SERVICE	none
NAMESPACE	default
REFRESH	15
MASTER_URL	https://kubernetes.default.svc
SA_CACERT	/var/run/secrets/kubernetes.io/serviceaccount/ca.crt
SA_TOKEN	/var/run/secrets/kubernetes.io/serviceaccount/token

Inspired by

• Giantswarms Blog article
• Official Init Containers documentation