Approach extending ContentEntityBase and using BaseFieldDefinition::create

baseDefinitionApproach.php

public static function baseFieldDefinitions(EntityTypeInterface $entity_type) {

    $fields['id'] = BaseFieldDefinition::create('integer')
      ->setLabel(t('ID'))
      ->setDescription(t('The ID of the Voting API Vote entity.'))
      ->setReadOnly(TRUE);

    $fields['uuid'] = BaseFieldDefinition::create('uuid')
      ->setLabel(t('UUID'))
      ->setDescription(t('The UUID of the Voting API Vote entity.'))
      ->setReadOnly(TRUE);

    $fields['target_entity_type'] = BaseFieldDefinition::create('string')
      ->setLabel(t('Entity Type'))
      ->setSettings(array(
        'max_length' => 64
      ))
      ->setDefaultValue('node');

    $fields['target_entity_id'] = BaseFieldDefinition::create('entity_reference')
      ->setLabel(t('Voted entity'))
      ->setRevisionable(FALSE)
      ->setSetting('handler', 'default')
      ->setDefaultValue(0);

    $fields['value'] = BaseFieldDefinition::create('float')
      ->setLabel(t('Value'))
      ->setDefaultValue(0);

    $fields['value_type'] = BaseFieldDefinition::create('string')
      ->setLabel(t('Value Type'))
      ->setSettings(array(
        'max_length' => 64
      ))
      ->setDefaultValue('percent');

    $fields['tag'] = BaseFieldDefinition::create('string')
      ->setLabel(t('Tag'))
      ->setSettings(array(
        'max_length' => 64,
      ))
      ->setDefaultValue('vote');

    $fields['user_id'] = BaseFieldDefinition::create('entity_reference')
      ->setLabel(t('Authored by'))
      ->setDescription(t('The user ID of author of the Voting API Vote entity.'))
      ->setRevisionable(FALSE)
      ->setSetting('target_type', 'user')
      ->setSetting('handler', 'default')
      ->setTranslatable(FALSE)
      ->setDefaultValue(0);

    $fields['timestamp'] = BaseFieldDefinition::create('created')
      ->setLabel(t('Created'))
      ->setDescription(t('The time that the entity was created.'))
      ->setDefaultValue(0);

    $fields['vote_source'] = BaseFieldDefinition::create('string')
      ->setLabel(t('Value Type'))
      ->setSettings(array(
        'max_length' => 255
      ));

    return $fields;
  }