[Rails] Concurrecy question with ActiveRecord

[Rick Bradley]

* ChongQing Xiao (CqXiao at epicsystems.com) [041209 16:33]:
> For web application, I think there are two ways to address the problem
> 
> 1. Implement some application level lock so once a patient is marked as
> locked, another user can only read the data from the web server so no
> conflict will happen. 
> 
> 2. Allow two users to change the data at the same time but roll back the
> last change if the change conflicts according the time stamp as I have
> mentioned in my previous email

This topic is well-studied and often comes down to a choice between
optimistic and pessimistic locking, but there are a lot of complicating
factors (request length, ability to resolve conflicts automatically in
the problem domain, the cost of a collision, etc.).  Martin Fowler
covers this general concurrency problem as it relates to enterprise web
applications in his _Patterns of Enterprise Application Architecture_.  [0]  

Clearly, the more you know about your particular application, its
business logic, and the nature of your session transactions the more
readily you can answer your own question and choose a suitable
implementation.  It's very difficult for a framework to solve this type
of concurrency problem generally and flexibly.  I haven't seen anything
in the Rails technologies that gets in the way of handling long
transaction concurrency, but you're not going to find a quick solution
either.

[0] http://www.martinfowler.com/books.html

Best,
Rick
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