gistfile1.php

<?php
// DatabaseStorateController::save()
  public function save(EntityInterface $entity) {
    $transaction = $this->database->startTransaction();
    try {
      // Load the stored entity, if any.
      if (!$entity->isNew() && !isset($entity->original)) {
        $entity->original = entity_load_unchanged($this->entityType, $entity->id());
      }

      $entity->preSave($this);
      $this->invokeHook('presave', $entity);

      if (!$entity->isNew()) {
        // It's about his part. The entity is send directly to the drupal_write_record function
        $return = drupal_write_record($this->entityInfo->getBaseTable(), $entity, $this->idKey);
        $this->resetCache(array($entity->id()));
        $entity->postSave($this, TRUE);
        $this->invokeHook('update', $entity);
      }
      else {
        // .. doing stuff
      }

      // .. doing more stuff
    }
    catch (\Exception $e) {
      // .. more
    }
  }

// FieldableDatabaseStorageController::save()
  public function save(EntityInterface $entity) {
    // .. doing stuff

      // Create the storage record to be saved.
      $record = $this->mapToStorageRecord($entity);

      if (!$entity->isNew()) {
        if ($entity->isDefaultRevision()) {
          // Here there is a record inserted instead of the entity.
          $return = drupal_write_record($this->entityInfo->getBaseTable(), $record, $this->idKey);
        }
        else {
          // .. doing stuff
        }
     // .. doing a lot!
  }