Simple Controller in Drupal via Symfony, returns a JsonResponse cached for 30 seconds.

OpenNowCheckController.php

<?php

declare(strict_types = 1);

namespace Drupal\harlib_open_now\Controller;

use Drupal\Core\Controller\ControllerBase;
use Symfony\Component\HttpFoundation\JsonResponse;
use Symfony\Component\DependencyInjection\ContainerInterface;
use Drupal\Component\Serialization\Json;

/**
 * Class OpenNowCheckController.
 */
class OpenNowCheckController extends ControllerBase {

  const FILE_URI = 'public://libcal/open-now.json';

  /**
   * Return status of all libraries.
   *
   * @return \Symfony\Component\HttpFoundation\JsonResponse
   *   Return the open today in a JsonResponse.
   *
   * @throws \Exception;
   */
  public function getAll() : JsonResponse {
    $file = file_get_contents(self::FILE_URI);

    if (!$file) {
      throw new \Exception('File containing LibCal data does not exist.');
    }

    $content = Json::decode($file);
    $response = new JsonResponse($content);
    $response->setMaxAge(30);
    $response->setPublic();
    return $response;
  }

}