monkjunior / pvc-watch

My first dummy K8S cli tool

A simple k8s cli tool for watching PVC

Teaching myself how to write a simple K8S tool using Golang.

References:

• Building stuff with the K8S APIs

Exploring API objects

I spent almost a year to learn K8S and working with K8S ecosystem. Soon, I realized that there are tons of tools in this ecosystem. It's pretty overwhelming to decide which tool will be good in which position for beginner like me.

Because I have learned the fundamentals of K8S for a year, now I decided to gain my strength of knowledge about it by building simple tools for my own purposes.

K8S is an API-first architecture

Get list of all API's path

$ kubectl get --raw /
{
  "paths": [
    "/api",
    "/api/v1",
    "/apis",
    ...
    "/ui",
    "/ui/",
    "/version"
  ]
}

Example: Get list of Nodes's info

kubectl get --raw /api/v1/nodes | jq
{
  // A tons of infomations
}

The most common tool to working with K8S's APIs is kubectl. We can use it to manipulate many resources, it is a good example for my first tools.

Note: Most of the time, I use yaml files to define K8S object, but we can also use json files.

Access to the API server directly

Using commend kubectl proxy

$ kubectl proxy --port=8080
Starting to serve on 127.0.0.1:8080

Then you can try to use cURL, a browser or your favourite REST tool

$ curl -XGET http://127.0.0.1/8080/
{
  "paths": [
    "/api",
    "/api/v1",
    "/apis",
...
    "/ui",
    "/ui/",
    "/version"
  ]
}

By accessing the K8S's APIs in this way, you don't need to provide authorization credential.

Direct server access

If kubectl proxy is not your option, you can try access to the API server directly with the proper authorization credentials.

API_SERVER=$(kubectl config view -o jsonpath='{.clusters[*].cluster.server}')
TOKEN=$(kubectl get secrets -o jsonpath='{.items[?(@.type=="kubernetes.io/service-account-token")].data.token}' \
    | base64 --decode)
curl $API_SERVER/version --header "Authorization: Bearer $TOKEN" --insecure

Building my first cli tool using Go

We will use client-go project. It is the oldest, of the K8S client frameworks, and therefore come with more knobs and features.

The project is a collection of packages that can accommodate different programming needs from REST-style primitives to more sophisticated clients.

It's being maintained by sig-api-machinery.

pvcwatch, my first dummy K8S cli tool

The example will highlight several aspects of the K8S Go client including:

• Connectivity

• Resource list retrieval and walk through

• Object watch

The K8S Go client framework supports the ability to watch the cluster for specified API object lifecycle events including ADDED, MODIFIED, DELETED.

For our simple cli tool, we will use this watch ability to monitor and trigger some automated actions (current version will just log notification to the standard output).