Example sandboxed post-update

my_module.post_update.php

<?php

/**
 * Example sandboxed post-update.
 *
 * This shows the basic structure you can copy for long-running updates.
 */
function mymodule_post_update_example_sandbox(&$sandbox) {

  // 1. Initialisation (first run).
  if (!isset($sandbox['initialised'])) {
    $sandbox['initialised'] = TRUE;

    // Gather all entity IDs you want to process.
    $sandbox['nids'] = \Drupal::entityQuery('node')
      ->accessCheck(FALSE)
      ->execute();

    $sandbox['total'] = count($sandbox['nids']);  // Total items.
    $sandbox['processed'] = 0;                    // Counter.
  }

  // 2. Exit early if no work to do.
  if (empty($sandbox['nids'])) {
    $sandbox['#finished'] = 1;
    return t('No nodes to process.');
  }

  // 3. Work in chunks (avoid timeouts).
  $limit = 50;
  $nids_chunk = array_splice($sandbox['nids'], 0, $limit);

  // Load and process entities.
  $nodes = \Drupal::entityTypeManager()
    ->getStorage('node')
    ->loadMultiple($nids_chunk);

  foreach ($nodes as $node) {
    // Do your real work here — e.g. update fields, save, etc.
    // For demo purposes we’ll just mark as processed.
    // $node->set('field_example', 'some value');
    // $node->save();

    $sandbox['processed']++;
  }

  // 4. Tell Drupal how far we are.
  $sandbox['#finished'] = $sandbox['processed'] / max(1, $sandbox['total']);

  // 5. Return a status message that appears in Drush output.
  return t('Processed @done of @total nodes.', [
    '@done' => $sandbox['processed'],
    '@total' => $sandbox['total'],
  ]);
}