Rails - Jun 2006 - Avoiding race conditions

I want to be able to display a number of messages, and guarantee that  
they''re only displayed once.  Something like this :

def get_new_messages
	@messages = Message.find(:all, :limit => 5, :conditions => ''new
= 1'')
	@messages.each do |msg|
		msg.new = 0
		msg.save
	end
end


However, there''s a race condition in the above code if two users try  
to access the get_new_messages action at the same time.  How can I  
avoid this?


Thanks,
Jon

On Jun 27, 2006, at 11:19 AM, Jonathan del Strother
wrote:
> I want to be able to display a number of messages, and guarantee  
> that they''re only displayed once.  Something like this :
>
> def get_new_messages
> 	@messages = Message.find(:all, :limit => 5, :conditions =>
''new = 1'')
> 	@messages.each do |msg|
> 		msg.new = 0
> 		msg.save
> 	end
> end
>
>
> However, there''s a race condition in the above code if two users  
> try to access the get_new_messages action at the same time.  How  
> can I avoid this?
There is no race condition, but concurrent users may get the same  
records. To ensure a user has exclusive access to the selected  
records, use row locking (recently introduced to edge rails - rake  
rails:freeze:edge).

def get_new_messages
   Message.transaction do
     # select * from messages limit 5 for update
     Message.find(:all, :limit => 5, :conditions =>
''new=1'', :lock =>
true).map do |message|
       message.new = false
       message.save!
       message
     end
   end
end

(Note that mysql locks all rows touched by the index it chooses, not  
just those returned, so you''ll suffer heavy lock contention.)

A pleasant way to solve the problem is to avoid it - is there a  
better way to track unread messages?

jeremy

Hi,

2006/6/27, Jeremy Kemper <jeremy@bitsweat.net>:
> A pleasant way to solve the problem is to avoid it - is there a
> better way to track unread messages?
>
I''d say: why not associate the last message id  viewed to the user
model and retrieve only the messages after (e.g. with id greater than)
that (you could also use a record timestamp if you don''t want to rely
on ids)...

bye
Luca