Maintaining database atomicity using ActiveRecord transactions

Atomicity comes from the greek word ‘a-tomos’ which means undevidable, atomic database transactions means that we want either all the database operations to happen together, or none of them at all, which means that we want them as a single undevidable operation, we would want that because not doing any of the changes at all would be much less harmful than doing the a part of the change. One of the most common examples of this can be found in ecommerce websites, when the customer buys an item and we reserve an item for the user, we want to make sure that the whole order is done till the generation of the invoice, if for any reason the flow is interrupted in the middle we want to make sure that we release the reserved item and undo any orders that was placed.

Here’s a sample (maybe unrealistic) implementation of a ecommerce website, but let it pass for the sake of the example Suppose the item was reserved successfully, but there was an error in the order creation, or maybe the order was created but there’s an error in the invoice, we would want to undo all these changes that we done, and maybe any changes that happend from these changes, like any callbacks or logs that were inserted in the database too, if you make use of those you’ll be in a lot of trouble with this unconsistent data.

In rails ActiveRecord::Base provides us with a very useful method #transaction which starts a database transaction and doesn’t commit the changes to the database until every thing is done successfully, since all models are subclasses of ActiveRecord::Base we can call #transaction on our models too for simplicity.

To protect the place_order method we created, we could wrap the statements inside it inside a transaction This way if something went wrong ActiveRecord will rollback the changes automatically, and nothing will be persisted.

Tips:

• You could fire the rollback your self if you raise inside the block, you could raise a rollback error

• Transactions could be nested, but needs a little tweaking to make it work as expected, you can read about the details here

• Transactions are just like any other block, it would return the last executed statement, could be used to return to an outer variable This way object will end up being the last array we created