index.js

// @ts-check

const express = require("express");
const app = express();
const fs = require("fs");
const bodyParser = require('body-parser');

// volume mount path
const mountPath = "/etc/config";

// ConfigMap filename
const configMapFile = "..data";

// watching file
fs.watch(`${mountPath}`, (event, filename) => {
  // only listen to event "rename".
  // kubelet will rename the reference folder on ConfigMap update.
  if (event === "rename" && filename) {
    if (filename === configMapFile) {
      console.log(`process.env BEFORE: ${JSON.stringify(process.env)}`);
      
      // get all files from `..data` directory
      const dir = fs.readdirSync(`${mountPath}/${configMapFile}`);

      console.log(`Env list: ${dir}`);

      // read all files inside `..data` directory
      dir.forEach(env => {
        // inject the new envar value to `process.env` object (not recommended)
        process.env[env] = fs.readFileSync(
          `${mountPath}/${configMapFile}/${env}`
        );
      });

      console.log(`process.env AFTER: ${JSON.stringify(process.env)}`);
    }
  }
});

// for readinessProbe and livelinessProbe Kubernetes
app.get("/info", (req, res) => {
  res.sendStatus(200);
});

app.get("/isMaintenance", (req, res) => {
  const isMaintenace = process.env.ENV_IS_MAINTENANCE;

  res.status(200).send(`Is it maintenance? ${isMaintenace}`);
})

app.listen(3000, err => {
  if (err) {
    console.error(err);
    process.exit(1);
  }

  console.log(`Server is up on port 3000`);
});