A Drupal 8 batch job to load large amounts of data into a Pantheon web site. Relies on Batch Services sandbox module: https://www.drupal.org/sandbox/acrosman/3025562

DataLoaderBatchService.php

<?php

namespace Drupal\example_pantheon_loader;

use Drupal\Core\Database\Connection;
use Drupal\batch_service_interface\AbstractBatchService;
use Drupal\Core\Logger\LoggerChannelFactoryInterface;
use Drupal\Core\StringTranslation\TranslationInterface;
use Drupal\file\Entity\File;
use Drupal\Core\File\FileSystem;
use Drupal\Core\Queue\QueueFactory;

/**
 * Class DataLoaderBatchService.
 */
class DataLoaderBatchService extends AbstractBatchService {

  /**
   * Drupal\Core\Database\Connection definition.
   *
   * @var \Drupal\Core\Database\Connection
   */
  protected $database;
  /**
   * Drupal\user\UserDataInterface definition.
   *
   * @var \Drupal\user\UserDataInterface
   */
  protected $userData;
  /**
   * Drupal\Core\File\FileSystem definition.
   *
   * @var \Drupal\Core\File\FileSystem
   */
  protected $fileSystem;

  /**
   * Drupal\Core\Queue\QueueFactory Defition.
   *
   * @var \Drupal\Core\Queue\QueueFactory
   */
  protected $queueFactory;

  /**
   * Constructs a new UserUpdateBatchService object.
   */
  public function __construct(
    LoggerChannelFactoryInterface $logger_factory,
    TranslationInterface $stringTranslation,
    Connection $database_service,
    FileSystem $file_system,
    QueueFactory $queue_factory
  ) {
    parent::__construct($logger_factory, $stringTranslation);
    $this->database = $database_service;
    $this->fileSystem = $file_system;
    $this->queueFactory = $queue_factory;
  }

  /**
   * {@inheritdoc}
   */
  public static function getServiceId() {
    return 'example_pantheon_loader.data';
  }

  /**
   * Data is the values from the form, which should include the file reference.
   */
  public function generateBatchJob($data = NULL) {

    // Clear any tracking data.
    $this->database->truncate('example_pantheon_loader_tracker')->execute();

    // If you are processing a queue make sure there is a valid queue prepped.
    // $queue = $this->queueFactory->get('example_pantheon_loader_remap');
    // $queue->createQueue();

    $fid = array_pop($data['file']);
    $fileEntity = File::load($fid);
    $ops = [];

    if (empty($fileEntity)) {
      $this->logger->error('Unable to load file data for processing.');
      return [];
    }
    $filePath = $this->fileSystem->realpath($fileEntity->getFileUri());

    $ops = ['processData' => [$filePath]];

    return $this->prepBatchArray($this->t('Loading Data for Processing'), $this->t('Starting file processing...'), $ops);
  }

  /**
   * Batch processing function to process large files.
   *
   * @param array $data
   *   Path to file.
   * @param array $context
   *   Drupal batch context array.
   */
  public function processData(array $data, array &$context) {

    $filePos = 0;
    $header = [];
    if (isset($context['sandbox']['file_position'])) {
      $filePos = $context['sandbox']['file_position'];
      $header = $context['sandbox']['file_header'];
    }

    // Old-school file handling.
    $path = array_pop($data);
    $file = fopen($path, "r");

    if (empty($file)) {
      $this->logger->error('File lost during processing.');
      $context['finished'] = 1;
      $context['results']['success'] = FALSE;
      return;
    }

    if (!feof($file)) {
      if (empty($header)) {
        $header = fgetcsv($file);
        $context['finished'] = 0.00001;
      }
      else {
        fseek($file, $filePos);

        // Each pass we process 100 lines, if you have to do something complex
        // you might want to reduce the run.
        for ($i = 0; $i < 100; $i++) {
          $row = fgetcsv($file);
          if (!empty($row)) {
            $data = array_combine($header, $row);
            $member['timestamp'] = time();
            $rowData = [
              'col_one' => $data['field_name'],
              'data' => serialize($data),
              'timestamp' => time(),
            ];
            $row_id = $this->database->insert('example_pantheon_loader_tracker')
              ->fields($rowData)
              ->execute();

            // If you're setting up for a queue you include something like this.
            // $queue = $this->queueFactory->get('example_pantheon_loader_remap');
            // $queue->createItem($row_id);
          }
          else {
            break;
          }
        }
        $filePos = (float) ftell($file);
        $context['finished'] = $filePos / filesize($path);
      }
    }
    else {
      $context['finished'] = 1;
    }

    $context['sandbox']['file_position'] = ftell($file);
    $context['sandbox']['file_header'] = $header;

    fclose($file);
  }

  /**
   * {@inheritdoc}
   */
  public function doFinishBatch($success, $results, $operations) {

    if ($success) {
      $message = $this->t('All provided data prepped.');
    }
    else {
      $message = $this->t('An error occurred during processing.');
    }

    drupal_set_message($message);
    $this->logger->info($message);
  }

}

PrepForm.php

<?php

namespace Drupal\example_pantheon_loader\Form;

use Drupal\Core\Form\FormBase;
use Drupal\Core\Form\FormStateInterface;
use Drupal\Core\Database\Connection;
use Symfony\Component\DependencyInjection\ContainerInterface;
use Drupal\example_pantheon_loader\DataLoaderBatchService;

/**
 * Class PrepForm.
 */
class PrepForm extends FormBase {

  /**
   * Drupal\example_pantheon_loader\UserUpdateBatchService definition.
   *
   * @var \Drupal\example_pantheon_loader\DataLoaderBatchService
   */
  protected $dataLoaderBatchService;

  /**
   * Drupal\Core\Database\Connection definition.
   *
   * @var \Drupal\Core\Database\Connection
   */
  protected $database;

  /**
   * Constructs a new PrepForm object.
   */
  public function __construct(
    DataLoaderBatchService $data_loader_service,
    Connection $database_service
  ) {
    $this->dataLoaderBatchService = $data_loader_service;
    $this->database = $database_service;
  }

  /**
   * {@inheritdoc}
   */
  public static function create(ContainerInterface $container) {
    return new static(
      $container->get('example_pantheon_loader.users'),
      $container->get('database')
    );
  }

  /**
   * {@inheritdoc}
   */
  public function getFormId() {
    return 'example_pantheon_loader_form';
  }

  /**
   * {@inheritdoc}
   */
  public function buildForm(array $form, FormStateInterface $form_state) {

    $form['intro'] = [
      '#type' => 'html_tag',
      '#tag' => 'div',
      '#value' => $this->t('This is example code requires additional support.'),
    ];

    $batchRunners = $this->getBatchOptions();
    $options = [];
    foreach ($batchRunners as $runner => $details) {
      $options[$runner] = $details['description'];
    }

    $form['file'] = [
      '#type' => 'managed_file',
      '#name' => 'data_file',
      '#title' => $this->t('Data file'),
      '#description' => $this->t('CSV format for this example.'),
      '#upload_location' => 'private://example_pantheon_loader_data/',
      '#upload_validators' => [
        'file_validate_extensions' => ['csv'],
      ],
    ];
    $form['submit'] = [
      '#type' => 'submit',
      '#value' => $this->t('Load Data'),
    ];
    $form['truncate'] = [
      '#type' => 'submit',
      '#value' => $this->t('Truncate Table'),
    ];

    return $form;
  }

  /**
   * {@inheritdoc}
   */
  public function validateForm(array &$form, FormStateInterface $form_state) {
    parent::validateForm($form, $form_state);
  }

  /**
   * {@inheritdoc}
   */
  public function submitForm(array &$form, FormStateInterface $form_state) {
    $input = $form_state->getUserInput();
    if ($input['op'] == 'Truncate Tables') {
      $this->truncateTables();
      return;
    }

    $batch = $this->dataLoaderBatchService->generateBatchJob($form_state->getValues());
    batch_set($batch);
  }

  /**
   * Truncate the tracker table.
   */
  private function truncateTables() {
    $this->database->truncate('example_pantheon_loader_tracker')->execute();
    drupal_set_message('Truncated Pantheon Load Tracker');
  }

}