Rails core - Dec 2012 - Isolation level for transactions initiated by ActiveRecord::Base#save

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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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 <jonasba@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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> You received this message because you are subscribed to the Google Groups
> "Ruby on Rails: Core" group.
> To view this discussion on the web visit
> https://groups.google.com/d/msg/rubyonrails-core/-/5WfQkkKQWvsJ.
> To post to this group, send email to rubyonrails-core@googlegroups.com.
> To unsubscribe from this group, send email to
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> For more options, visit this group at
> http://groups.google.com/group/rubyonrails-core?hl=en.
>
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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. :)

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