Practical Guide to SQL Transaction Isolation
One tool to add to your toolbox as a developer is named locks. Say you have a table for orders and a table for order items and a table for payments. Collectively they represent an order, but that is a higher level concept that isn't something an RDBMS understands. When the user on your site simultaneously sends two updates, such as "remove item from the cart" and "pay and finish", you will have a slew of simultaneous updates to the rows across the three tables. Unless you use the highest isolation level, you will get inconsistencies. If you do use serializable isolation level you might get a deadlock, depending on how conscientious you are about updating everything in the same order. You could do an initial read from the orders table with FOR UPDATE which will provide you an implicit lock on that order, but you still have issues with new rows being inserted into your order items table that will not be range locked.<p>So use names/advisory locks: create a named lock called "order-1234" where 1234 is tbe ID of the order. Acquire it before doing any manipulations to the order across all its tables. Then release it. Semantics of names locks differ across RDBMS's. Unsurprisingly Postgres has saner and more flexible semantics than MySQL, but both are perfectly suitable for the purpose. This technique can save you a ton of headaches.