What is the best method to make sure two people don't edit the same row on my web app?

Question

I have a PHP/jQuery/AJAX/MySQL app built for managing databases. I want to implement the ability to prevent multiple users from editing the same database row at the same ti

Answer 1

There are two general approaches-- optimistic and pessimistic locking.

Optimistic locking is generally much easier to implement in a web-based environment because it is fundamentally stateless. It scales much better as well. The downside is that it assumes that your users generally won't be trying to edit the same set of rows at the same time. For most applications, that's a very reasonable assumption but you'd have to verify that your application isn't one of the outliers where users would regularly be stepping on each other's toes. In optimistic locking, you would have some sort of last_modified_timestamp column that you would SELECT when a user fetched the data and then use in the WHERE clause when you go to update the date, i.e.

UPDATE table_name
   SET col1 = <>,
       col2 = <>,
       last_modified_timestamp = <>
 WHERE primary_key = <>
   AND last_modified_timestamp = <>

If that updates 1 row, you know you were successful. Otherwise, if it updates 0 rows, you know that someone else has modified the data in the interim and you can take some action (generally showing the user the new data and asking them if they want to overwrite but you can adopt other conflict resolution approaches).

Pessimistic locking is more challenging to implement particularly in a web-based application particularly when users can close their browser without logging out or where users may start editing some data and go to lunch before hitting Submit. It makes it harder to scale and generally makes the application more difficult to administer. It's really only worth considering if users will regularly try to update the same rows or if updating a row takes a large amount of time for a user so it's worth letting them know up front that someone else has locked the row.