/**
 * Implements hook_cron().
 */
function {{ machine_name }}_cron() {
  // Short-running operation example, not using a queue:
  // Delete all expired records since the last cron run.
  $expires = \Drupal::state()->get('mymodule.last_check', 0);
  \Drupal::database()->delete('mymodule_table')
    ->condition('expires', $expires, '>=')
    ->execute();
  \Drupal::state()->set('mymodule.last_check', REQUEST_TIME);

  // Long-running operation example, leveraging a queue:
  // Queue news feeds for updates once their refresh interval has elapsed.
  $queue = \Drupal::queue('aggregator_feeds');
  $ids = \Drupal::entityTypeManager()->getStorage('aggregator_feed')->getFeedIdsToRefresh();
  foreach (Feed::loadMultiple($ids) as $feed) {
    if ($queue->createItem($feed)) {
      // Add timestamp to avoid queueing item more than once.
      $feed->setQueuedTime(REQUEST_TIME);
      $feed->save();
    }
  }
  $ids = \Drupal::entityQuery('aggregator_feed')
    ->condition('queued', REQUEST_TIME - (3600 * 6), '<')
    ->execute();
  if ($ids) {
    $feeds = Feed::loadMultiple($ids);
    foreach ($feeds as $feed) {
      $feed->setQueuedTime(0);
      $feed->save();
    }
  }
}