I work at a gaming cybercafe, and we've got a system here (smartlaunch) which keeps track of game licenses. I've written a program which interfaces with this system (actually, with it's backend MySQL database). The program is meant to be run on a client PC and (1) query the database to select an unused license from the pool available, then (2) mark this license as in use by the client PC.

The problem is, I've got a concurrency bug. The program is meant to be launched simultaneously on multiple machines, and when this happens, some machines often try and acquire the same license. I think that this is because steps (1) and (2) are not synchronised, i.e. one program determines that license #5 is available and selects it, but before it can mark #5 as in use another copy of the program on another PC tries to grab that same license.

I've tried to solve this problem by using transactions and table locking, but it doesn't seem to make any difference - Am I doing this right? Here follows the code in question:

    public LicenseKey Acquire() throws SmartLaunchException, SQLException {
    Connection conn = SmartLaunchDB.getConnection();
    int PCID = SmartLaunchDB.getCurrentPCID();

    conn.createStatement().execute("LOCK TABLE `licensekeys` WRITE");

    String sql = "SELECT * FROM `licensekeys` WHERE `InUseByPC` = 0 AND LicenseSetupID = ? ORDER BY `ID` DESC LIMIT 1";
    PreparedStatement statement = conn.prepareStatement(sql);
    statement.setInt(1, this.id);
    ResultSet results = statement.executeQuery();

    if (results.next()) {
        int licenseID = results.getInt("ID");
        sql = "UPDATE `licensekeys` SET `InUseByPC` = ? WHERE `ID` = ?";
        statement = conn.prepareStatement(sql);
        statement.setInt(1, PCID);
        statement.setInt(2, licenseID);
        statement.executeUpdate();
        statement.close();
        conn.commit();
        conn.createStatement().execute("UNLOCK TABLES");
        return new LicenseKey(results.getInt("ID"), this, results.getString("LicenseKey"), results.getInt("LicenseKeyType"));
    } else {
        throw new SmartLaunchException("All licenses of type " + this.name + "are in use");
    }
}