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

1) This is called locking. There are two main types of locking when referring to relational databases (like MySQL): table locking and row locking. Table locking ensures only one session at at time is making changes to a table, whereas row locking ensures only one session at a time is making changes to a particular row. You can think of row locking as a more fine-grained approach to concurrent access than table locking. Row locking is more complicated, but allows multiple concurrent sessions to write to the same table (important if your database has lots of concurrent writes--table locking should be fine for 10-15 users)

2-3) MySQL takes care of concurrent access for you! It automatically implements locking in the background. The type of locking (row or table) depends on which storage engine you use. For example, MyISAM uses table locking and InnoDB uses row locking. MySQL uses an internal table to manage this. You can query the status of this table (and all locks on your database) by checking the Table_locks_immediate and Table_locks_waited variables (it uses your option number 2).
When you issue an INSERT or UPDATE statement while another session is using the table (or row), the calling application (i.e. PHP in this case) will pause for a few milliseconds until the other session is done writing.

4) Again, MySQL will automatically take care of locking, but you can manually manage table locking with the LOCK TABLES and UNLOCK TABLES commands. If you are using row locking with InnoDB, there is a host of functions you can use to manually manage concurrent access.

See MySQL's page on Internal Locking for an overview of MySQL's locking system, and Concurrent Inserts for InnoDB's row locking features.