Re: [Rails-core] Isolation level for transactions initiated by ActiveRecord::Base#save

[Rafael Mendonça França]

It is already implemented and will be shipped with Rails 4.

See https://github.com/rails/rails/commit/392eeecc11a291e406db927a18b75f41b2658253

Rafael Mendonça França
http://twitter.com/rafaelfranca
https://github.com/rafaelfranca

On Sun, Dec 23, 2012 at 6:33 PM, Jonas Amundsen <jon...@gmail.com> wrote:

Hi,

I am currently developing a website for selling tickets for events. A big issue here is concurrency. Multiple user should not be able to buy tickets at the exact same time and create a inconsistent database where more tickets have been sold than what was actually available.

The act of purchasing is a two step process:

 1. Check that there are enough available tickets left.

 2. Create ticket records representing the users tickets.

This is a critical part of the system and mutual exclusion must occur. At the moment, I have created a file based semaphore. Here is an isolated example of how I do this. The example shows mutual exclusion on a per-event basis, enabling multiple purchases to occur at the same time, as long as they are for seperate events.

https://gist.github.com/4366276

This works swimmingly. (And it's so cool!) But I would much rather like to utilize database transactions with an isolation level of serializable. I understand that ActiveRecord::Base#save already [creates a transaction](https://github.com/rails/rails/blob/master/activerecord/lib/active_record/transactions.rb#L257).

To my knowledge, there's nothing in Rails resembling the possibility to set the isolation level of an transaction. I think that is an issue. Isolation level is, in my opinion, a huge part of what transactions are. At least I could use a feature like that, in my case.

It would be so awesome if one for instance could do

transaction.save :isolation => :serializable

or

transaction :isolation => :serializable do
  # …
end

What do you think?

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[Jonas Amundsen]

On Monday, December 24, 2012 12:08:48 AM UTC+1, Rafael Mendonça França wrote:

It is already implemented and will be shipped with Rails 4.

See https://github.com/rails/rails/commit/392eeecc11a291e406db927a18b75f41b2658253

Oh, that's great! It didn't occur to me to look in edge. I just messed around with 3.2.9.

I'll be looking forward to Rails 4. :)