Foeign Key Locking Transaction timeout when a second database connection is opened - mysql

Question

I'm having a problem involving a database transaction in one class that is timing out due to a secondary database connection being opened within the transaction; the problem started occurring when I added a foreign key constraint. And, testing using:
SET foreign_key_checks = 0;
I've been able to confirm this.
My database class looks like this (I've left off all of the methods):
class Db {
function __construct($config) {
$this->config = $config;
}
private function connect($config) {$dsn = 'mysql:host=' . $config['host'] . ';dbname=' . $config['dbname'] . ';charset=utf8';
$options = array(
// PDO::ATTR_PERSISTENT => true,
PDO::ATTR_EMULATE_PREPARES => false,
PDO::ATTR_ERRMODE => PDO::ERRMODE_EXCEPTION,
PDO::ATTR_DEFAULT_FETCH_MODE => PDO::FETCH_ASSOC
);
$dbh = new PDO($dsn, $config['username'], $config['password'], $options);
$dbh->exec("SET NAMES utf8;");
return $dbh;
}
}
My model looks like this:
class Model {
function __construct() {
$this->db = new Db(array('host'=>DB_HOST,'dbname'=>DB_NAME,'username'=>DB_USERNAME,'password'=>DB_PASSWORD));
}
}
The code below then performs a little bit of logic, then an insert into the question_orders table: question_orders has a column question_id, with a foreign key index, which references the parent table questions; I think that the problem is that Assessment_Question_Orders extends the Model and creates a new database connection? Any thoughts on how to maintain both the transaction and foreign key aspects would be appreciated.
class This_Is_A_Problem extends Model() {
public function __construct() {
parent::construct();
}
public function problemFunction() {
/*variable init code left out*/
$this->db->beginTransaction();
$db_result = false;
try {
$db_result = $this->db->insert('questions', $questions_data);
$new_insert_id = $this->db->lastInsertId();
$assessment_question_orders = new Assessment_Question_Orders();
$question_number = $assessment_question_orders->insertSingleQuestionOrder($module_id, $new_insert_id);
$db_result = $this->db->commit();
}
} catch (PDOException $e) {
$this->db->rollBack();
}}}

Answer 1

One thread should (usually) have only one connection to the database. So I recommend one of these patterns:
Plan A: a single $db passed into all classes:
$db = new PDO(...);
$my_obj = new My_Class($db); -- $db is saved in $this->db for use within the methods of My_Class.
Plan B: a singleton Db class with a getter method:
// Singleton (of sorts)
class Db
{
private static $db;
function __construct()
{
self::$db = new PDO(...);
// A variant would include "lazy" instantiation of self::$Db.
}
function Get_Db() { return self::$db; } // All calls get the same `db`
}
class My_class
{
function My_Method()
{
$db = Db::Get_Db();
$db->...
}
}
new Db(); // one time call at start of program
There is only rarely a need to have two db connections in a single program. Plan A easily allows for such. (But see if you can avoid it -- you are in trouble now because of it.)