How to guarantee data integrity for concurrent Rails/Active Record operations

I need to implement a feature for a rails site that will involve
reading and exporting most of my database.

I know this operation is going to take a while. That's fine-- I've got
delayed job for that.

What I'm worried about is the data changing during the running of the
job, and the resulting export being corrupted because of that.

My initial thought was to do all of the reads within a transaction.
However, I would also like to be running the reads concurrently.
ActiveRecord docs say that Transactions cannot be shared between
Connections, and Connections cannot be shared between Threads. So it
looks as though I am restricted to a single thread with this approach.

Any suggestions for a workaround? Is there another way to give the job
a consistent view of the data that doesn't involve transactions? Or is
there some alternative to ActiveRecord/Mysql out there that can
distribute transactions across threads?

One possibility would be to have a slave replicating from the master.
You can halt replication, do all your data dumping from the slave and
then resume replicating. This might also be a good idea because it
would mean that your export stuff wouldn't be impacting the main stuff
your site does.

Fred

If this is a MySQL database just how long does it take to dump it from
the command line? I would guess that the system supplied tools would
be faster than anything you could possibly write in Ruby.

Thanks for the suggestion Fred-- that sounds like a good solution.

I've never setup slave replication before, but I'm hoping
http://dev.mysql.com/doc/refman/5.1/en/replication.html will have the
information I need.

~Rafe