foreign key in magento 2 code example

Example: magento 1 setup add foreign key

->addForeignKey(
     $installer->getFkName('namespace_module/shop', 'area_id', 'namespace_module/area','area_id'),
     'area_id',
     $installer->getTable('namespace_module/area'), 
     'area_id',
     Varien_Db_Ddl_Table::ACTION_CASCADE, 
     Varien_Db_Ddl_Table::ACTION_CASCADE
)