Health check script for Drupal 8

health.php

<?php

/**
 * @file
 * Health check controller for the API Indexer.
 */

use Drupal\Core\Database\Database;
use Drupal\Core\DrupalKernel;
use Drupal\Core\Installer\InstallerKernel;
use Symfony\Component\HttpFoundation\JsonResponse;
use Symfony\Component\HttpFoundation\Request;

$autoloader = require 'autoload.php';
$request = Request::createFromGlobals();
$kernel = DrupalKernel::createFromRequest($request, $autoloader, 'prod');
$result = ['status' => TRUE, 'message' => 'APIs are up and ready !'];

try {
  // Initialize the DI container.
  $kernel->boot();

  // Check if database is ready.
  if (!Database::getConnectionInfo() && !InstallerKernel::installationAttempted() && PHP_SAPI !== 'cli') {
    throw new \Exception('Deployment in progress... Thank you for your patience.');
  }

  // Check if site is installed like the core do.
  // This means checking for the session table.
  // @see Drupal\Core\Installer\InstallerRedirectTrait::shouldRedirectToInstaller().
  $connection = $kernel->getContainer()->get('database');
  if (!$connection->schema()->tableExists('sessions')) {
    throw new \Exception('Deployment in progress... Thank you for your patience.');
  }

  // Get maintenance mode.
  $maintenance = $kernel->getContainer()->get('state')->get('system.maintenance_mode');
  if ($maintenance) {
    throw new \Exception('Website is currently under maintenance. We should be back shortly. Thank you for your patience.');
  }
} catch (\Exception $e) {
  $result = ['status' => FALSE, 'message' => $e->getMessage()];
}

// Return the JSON data.
$response = new JsonResponse($result);
$response->send();

$kernel->terminate($request, $response);