多个客户端同时执行SQL

Question

I have a sample data (Slots = 1) The system must provide 1 slot for a client. If the client A gets a slot, sample data will be Slots = 0.

What if Client A and Client B execute the SQL query at the same time? The Sample data will be Slots = -1.

I tried to prevent it using PHP,

if($Slots > 0){
execute....
}

But clients still executing the SQL query. How can I make ONLY ONE can get the slot even if they execute it at the same time?

Answer 1

To start, make sure your table engine is InnoDB , as MyISAM can only do table level locking instead of row locking.

Then you can use the SELECT .. FOR UPDATE command to get exclusive access to the row.

This answer on this thread on dba.stackexchange.com explains it perfectly: https://dba.stackexchange.com/a/15864

Once the row is locked, MySQL will throw an error to any other process that tries to lock or update that row. So to tell a user the slots are already taken, you'll need to catch any errors and alert the user accordingly.

Here's a simple example:

<?php
$db = new PDO('mysql:host=localhost;dbname=test;charset=utf8', 'root', 'password');

// Ask PDO to throw Exceptions on error
$db->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);

try
{
    $db->beginTransaction();

    // If the row is locked, mysql will wait for a time-out. By default the timeout is 50 seconds
    // so to make sure the flow of the script works, we set the time-out to 1
    $db->query('SET innodb_lock_wait_timeout = 1;');

    // Select the row we want to update and tell MySQL to lock it until we are done updating
    // by adding the "FOR UPDATE" command to the query.
    // Note that if the row is already locked by another process, this query will throw an error
    // which we'll catch below. It means that someone else already has the slot and is updating it.
    $result = $db->query('SELECT slots FROM table WHERE id = 1 FOR UPDATE');

    if ($result !== false)
    {
        $object = $result->fetchObject();

        if ($object->slots > 0) 
        {
            $db->query('UPDATE table SET slots = slots - 1 WHERE id = 1');

            echo "You got the slot!";
        }
        else echo "Sorry, no more slots available.";
    }

    // Commit the transaction and release the lock, and move on.
    $db->commit();
}
catch (Exception $e)
{
    // An error was thrown, so rollback the transaction
    $db->rollback();

    // and tell the user he couldn't get the slot
    echo "Failed to reserve a slot for you: (" . $e->getMessage() . ")";
    exit();
}

It's easy to test this locally. Just open up a second MySQL session via command-line and execute these commands to set the lock:

{10:34}[5.4.36]/tmp ➭ mysql -uroot -p 
Welcome to the MySQL monitor.  Commands end with ; or \g.

..

mysql> USE test;
Reading table information for completion of table and column names
You can turn off this feature to get a quicker startup with -A

Database changed
mysql> BEGIN;
Query OK, 0 rows affected (0.00 sec)

mysql> SELECT slots FROM table WHERE id = 1 FOR UPDATE;
+-------+
| slots |
+-------+
|    87 |
+-------+
1 row in set (0.00 sec)

..
the lock is set, if you now run the example script you should not be able to update the slots and get an error
.. 

mysql> COMMIT;

Take a look at the MySQL docs for more info: https://dev.mysql.com/doc/refman/5.0/en/innodb-locking-reads.html