Question

I’m new to database applications and I’m trying to use Datamapper to make a ruby web application.

I stumbled across this piece of code which I don’t understand:

transaction do |txn|
    link = Link.new(:identifier => custom)
    link.url = Url.create(:original => original)
    link.save   
end

I have a few questions: What exactly are transactions? And why was this preferred instead of just doing:

link = Link.new(:identifier => custom)
link.url = Url.create(:original => original)
link.save   

When should I consider using transactions? What are the best use-cases? Is there any resource available online where I can read more about such concepts.

Thanks!

Answer 1

Transaction is an indivisible unit of work. The idea comes from the database world and is connected with the problems of data selection/update. Consider the following situation:

1. user A asks for object O in order to change it.
2. While A was doing his/her stuff, user B asked for the same object. Object O currently is equal for both users.
3. Then A puts the update to the database, with changed property O1 to the object O. User B hasn’t got this change – his object O is still the same as it was before.
4. B puts the update to the database with changed property O2 to the object O. The change to O1 is effectively lost.

Basically, it has to do with multi-user access and changes – there are several kinds of problems that arise.

Transaction are also used to couple different operations together into one logical processing statement. For example, you need to delete User with all his/her associated Photos.

The topic is really vast to cover in one post, so I’d recommend reading following articles: wiki#1 and wiki#2.