Issue with concurrent requests in CakePHP 2.0 - mysql

Question

Thanks in advance for attempting to asssist me with this issue.
I'm using CakePHP 2 (2.10.22).
I have a system which creates applications. Each application that gets created has a unique application number. The MySQL database column that stores this application number is set to 'Not null' and 'Unique'. I'm using CakePHP to get the last used application number from the database to then build the next application number for the new application that needs to be created. The process that I have written works without any problem when a single request is received at a given point in time. The problem arises when two requests are received to create an application at the exact same time. The behaviour that I have observed is that the the request that gets picked up first gets the last application number - e.g. ABC001233 and assigns ABC001234 as the application number for the new application it needs to create. It successfully saves this application into the database. The second request which is running concurrently also gets ABC001233 as the last application number and tries to create a new application with ABC001234 as the application number. The MySQL database returns an error saying that the application number is not unique. I then put the second request to sleep for 2 seconds by which time the first application has successfully saved to the database. I then re-attempt the application creation process which first gets the last application number which should be ABC001234 but instead each database read keeps returning ABC001233 even though the first request has long been completed. Both requests have transactions in the controller. What I have noticed is that when I remove these transactions, the process works correctly where for the second request after the first attempt fails, the second attempt works correctly as the system correctly gets ABC001234 as the last application number and assigns ABC001235 as the new application number. I want to know what I need to be doing so as to ensure the process works correctly even with the transaction directives in the controller.
Please find below some basic information on how the code is structured -
Database
The last application number is ABC001233
Controller file
function create_application(){
$db_source->begin(); //The process works correctly if I remove this line.
$result = $Application->create_new();
if($result === true){
$db_source->commit();
)else{
$db_source->rollback();
}
}
Application model file
function get_new_application_number(){
$application_record = $this->find('first',[
'order'=>[
$this->name.'.application_number DESC'
],
'fields'=>[
$this->name.'.application_number'
]
]);
$old_application_number = $application_record[$this->name]['application_number'];
$new_application_number = $old_application_number+1;
return $new_application_number;
}
The above is where I feel the problem originates. For the first request that gets picked up, this find correctly finds that ABC001233 is the last application number and this function then returns ABC001234 as the next application number. For the second request, it also picks up ABC001233 as the last application number but will fail when it tries to save ABC001234 as the application number as the first request has already saved an application with that number. As a part of the second attempt for the second request (which occurs because of the do/while loop) this find is requested again, but instead of returning ABC001234 as the last application number (per the successfuly save of the first request), it keeps returning ABC001233 resulting in a failure to correctly save. If I remove the transaction from the controller, this then works correctly where it will return ABC001234 in the second attempt. I couldn't find any documentation as to why that is and what can be done about the same and is where I need some assistance. Thank you!
function create_new(){
$new_application_number = $this->get_new_application_number();
$save_attempts = 0;
do{
$save_exception = false;
try{
$result = $this->save([$this->name=>['application_number'=>$new_application_number]], [
'atomic'=>false
]);
}catch(Exception $e){
$save_exception = true;
sleep(2);
$new_application_number = $this->get_new_application_number();
}
}while($save_exception === true && $save_attempts++<5);
return !$save_exception;
}

Answer 1

You just have to lock the row with the previous number in a transaction using SELECT ... FOR UPDATE. It's much better than the whole table lock as said in the comments.
According to documentation https://book.cakephp.org/2/en/models/retrieving-your-data.html you just have to add 'lock' => true to get_new_application_number function:
function get_new_application_number(){
$application_record = $this->find('first',[
'order'=>[
$this->name.'.application_number DESC'
],
'fields'=>[
$this->name.'.application_number'
],
'lock'=>true
]);
$old_application_number = $application_record[$this->name]['application_number'];
$new_application_number = $old_application_number+1;
return $new_application_number;
}
How does it work:
The second transaction will wait on that request while the first transaction is ended.
P.S. According to documentation lock option was added in the 2.10.0 version of CakePHP.