Question

I have a php script that will be requested several times “at the same time” I also have a field in a table let’s call it persons as a flag for active/inactive. I want when the first instance of the script runs to set that field to inactive so that the rest instances will die when they check that field. Can someone provide a solution for that? How can I ensure that this script will run only once?

PHP, PDO, MySQL

Thank you very much in advance.

Answer 1

Your script should fetch the current flag within a transaction using a locking read, such as SELECT ... FOR UPDATE:

$dbh = new PDO("mysql:dbname=$dbname", $username, $password);
$dbh->setAttribute(PDO::ATTR_EMULATE_PREPARES, FALSE);

$dbh->beginTransaction();

// using SELECT ... FOR UPDATE, MySQL will hold all other connections
// at this point until the lock is released
$qry = $dbh->query('SELECT persons FROM my_table WHERE ... FOR UPDATE');

if ($qry->fetchColumn() == 'active') {
  $dbh->query('UPDATE my_table SET persons = "inactive" WHERE ...');
  $dbh->commit();  // releases lock so others can see they are inactive

  // we are the only active connection
} else {
  $dbh->rollBack();
  // we are inactive
}