Container execution checks utility for use with InitContainers/readiness/liveness probes (proposal)

[namliz]

Handling initialization is typically shown with simple commands such as 'wget' or 'cat' and is rather straightforward.

However, for non trivial conditionals this can get hairy.

A contrived example

Consider an InitContainer that succeeds when a service responds with 3 DNS endpoints.
At first glance it is a simple nslookup servicename -ge 3 one liner. That is until you happen to use an image that doesn't bundle nslookup so you'd getent hosts servicename -ge 3 instead.

Writing bash one liners is suboptimal

• What utilities can one safely rely on for the one-liner munging?
• No sane style guide
• Maintainability

In reality past the simple one liner people should (and do) reach for the scripting language of their choice. However, now you went from a tiny busybox InitContainer to a 300MB container that bundles python to avoid writing a little bash.

The executing checks do belong in the project yaml/json file instead of being baked into some one-off image on the side. Most of these checks for most projects probably fall into two dozen or some common patterns.

So I purpose a utility in the spirit of bc.

apiVersion: v1
kind: Pod
metadata:
  name: nginx
  annotations:
    pod.alpha.kubernetes.io/init-containers: '[
        {
            "name": "install",
            "image": "busybox",
            "command": ["k", "service", "name", "at least", "3"]
        }
]'

To be written in Go, with a small core, and extensible (so users can add custom checks via a volume).