Batch operations

$batch = array(
  'title' => t('Exporting'),
  'operations' => array(
    array('my_function_1', array($account->id(), 'story')),
    array('my_function_2', array()),
  ),
  'finished' => 'my_finished_callback',
  'file' => 'path_to_file_containing_myfunctions',
);
batch_set($batch);
// Only needed if not inside a form _submit handler.
// Setting redirect in batch_process.
batch_process('node/1');

// Simple and artificial: load a node of a given type for a given user
function my_function_1($uid, $type, &$context) {
  // The $context array gathers batch context information about the execution (read),
  // as well as 'return values' for the current operation (write)
  // The following keys are provided :
  // 'results' (read / write): The array of results gathered so far by
  //   the batch processing, for the current operation to append its own.
  // 'message' (write): A text message displayed in the progress page.
  // The following keys allow for multi-step operations :
  // 'sandbox' (read / write): An array that can be freely used to
  //   store persistent data between iterations. It is recommended to
  //   use this instead of $_SESSION, which is unsafe if the user
  //   continues browsing in a separate window while the batch is processing.
  // 'finished' (write): A float number between 0 and 1 informing
  //   the processing engine of the completion level for the operation.
  //   1 (or no value explicitly set) means the operation is finished
  //   and the batch processing can continue to the next operation.

  $nodes = \Drupal::entityTypeManager()->getStorage('node')
    ->loadByProperties(['uid' => $uid, 'type' => $type]);
  $node = reset($nodes);
  $context['results'][] = $node->id() . ' : ' . SafeMarkup::checkPlain($node->label());
  $context['message'] = SafeMarkup::checkPlain($node->label());
}

// A more advanced example is a multi-step operation that loads all rows,
// five by five.
function my_function_2(&$context) {
  if (empty($context['sandbox'])) {
    $context['sandbox']['progress'] = 0;
    $context['sandbox']['current_id'] = 0;
    $context['sandbox']['max'] = db_query('SELECT COUNT(DISTINCT id) FROM {example}')->fetchField();
  }
  $limit = 5;
  $result = db_select('example')
    ->fields('example', array('id'))
    ->condition('id', $context['sandbox']['current_id'], '>')
    ->orderBy('id')
    ->range(0, $limit)
    ->execute();
  foreach ($result as $row) {
    $context['results'][] = $row->id . ' : ' . SafeMarkup::checkPlain($row->title);
    $context['sandbox']['progress']++;
    $context['sandbox']['current_id'] = $row->id;
    $context['message'] = SafeMarkup::checkPlain($row->title);
  }
  if ($context['sandbox']['progress'] != $context['sandbox']['max']) {
    $context['finished'] = $context['sandbox']['progress'] / $context['sandbox']['max'];
  }
}

function my_finished_callback($success, $results, $operations) {
  // The 'success' parameter means no fatal PHP errors were detected. All
  // other error management should be handled using 'results'.
  if ($success) {
    $message = \Drupal::translation()->formatPlural(count($results), 'One post processed.', '@count posts processed.');
  }
  else {
    $message = t('Finished with an error.');
  }
  drupal_set_message($message);
  // Providing data for the redirected page is done through $_SESSION.
  foreach ($results as $result) {
    $items[] = t('Loaded node %title.', array('%title' => $result));
  }
  $_SESSION['my_batch_results'] = $items;
}