Zero-downtime Node deploys using parent process proxy, early exploration for https://kofi.sexy/blog/zero-downtime-render-disk

proxy.js

import { spawn } from 'child_process'
import { createServer, request } from 'http'
import pidtree from 'pidtree'

const deployCommand = ['npm', ['start']]
const deployWait = 30 * 1000
const deploySecretPath = '/~deploy-01HPZ2ACGJSJJ0NGX9MT1RYRDB'

let server = startServer(8001)

const proxy = createServer((clientRequest, clientResponse) => {
  if (clientRequest.url === deploySecretPath) {
    restartServer()
    clientResponse.writeHead(202).end()
    return
  }
  const forward = {
    method: clientRequest.method,
    headers: clientRequest.headers,
    path: clientRequest.url,
    port: server.PORT,
  }
  const serverRequest = request(forward, (serverResponse) => {
    clientResponse.writeHead(serverResponse.statusCode, serverResponse.headers)
    serverResponse.pipe(clientResponse, { end: true })
  })
  serverRequest.on('error', (error) => {
    handleError(error)
    clientResponse.writeHead(502).end()
  })
  clientRequest.pipe(serverRequest, { end: true })
})

function startServer(PORT) {
  return {
    process: spawn(...deployCommand, { stdio: 'inherit', env: { ...process.env, PORT } }).on('error', console.error),
    PORT,
  }
}

function restartServer() {
  if (server.stopping)
    return
  server.stopping = true
  const oldServer = server
  const newServer = startServer(server.PORT + 1)
  setTimeout(
    () => {
      server = newServer
      pidtree(oldServer.process.pid, { root: true }).then((pids) => pids.map((pid) => process.kill(pid)), console.error)
    },
    deployWait,
  )
}

proxy.listen(10000)