I recently build a web site with a small-medium profile of user access (around 150.000 pageviews/month) and sometimes, in the middle of the day I get the following error:

PDOException: SQLSTATE[08004] [1040] Too many connections in lock_may_be_available()

The Drupal application is hosted with the MySQL on the same server, with an 2 CPUs Intel Xeon Quad Core and 8 GB of RAM.

I'm new on the Drupal world, and someone recomended me to use db_query() to throw my queries to the database. I don't know if this is the best pratice or even if this is the root of the problem. Can anyone help me?


You need to make sure the number of MySQL connections (max_connections in my.cnf) is greater than the number of PHP processes (pm.max_children if you use php-fpm, or MaxClients if you're using Apache mod_php).

The default for Apache is often 300, and the MySQL default is only 100, so you can run out of connections pretty quickly if these values aren't tuned.


Actually, lock_may_be_available() is using db_query(); if that would not cause the error you are seeing, then you should not see it when lock_may_be_available() is called.

It is suggested to use db_query() when the query that is being executed doesn't need to be altered from other modules; in that case, it is preferable to use db_select().

If you look at the code of db_query(), and db_select(), you will notice that both call Database::getConnection to obtain the database connection. The cause of that error is surely not the use of a function instead of the other.

function db_query($query, array $args = array(), array $options = array()) {
  if (empty($options['target'])) {
    $options['target'] = 'default';

  return Database::getConnection($options['target'])->query($query, $args, $options);
function db_select($table, $alias = NULL, array $options = array()) {
  if (empty($options['target'])) {
    $options['target'] = 'default';
  return Database::getConnection($options['target'])->select($table, $alias, $options);
  • Yes, we can see that it opens a database connection. But does it actually close it when finished? If so, presumably it happens behind the scenes in PDO?
    – sam452
    Apr 20 '16 at 14:21
  • I take the connection gets closed, considering the number of calls to sb_query() done from the modules. I think there are some settings to keep the connection open, but they are set in the settings.php file, together the data for the connection. And yes, those settings are the ones PDO allows you to set.
    – apaderno
    Apr 20 '16 at 14:31

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