Allowing only one user to acces/submit a form

[Ted Husted <te...@husted.com>]

A Stephen wrote:
 > Greetings,
 > Is there a way to lock a form if the form is in use.
 > The intention is to avoid two users submitting the same form.

You're options here are the same as with any enterprise application.

* Pessimistic locking, and
* Optimistic locking

For a pessimistic lock, you need some flag on the record that says 
someone has checked it out, along with some record of who. When an 
administrator starts the order workflow, you update the database. If 
someone else try to start the same order, you branch to a page that 
tells them they can't do that.

This pretty much works, but you also need a good routine for unlocking 
the order, since some people will invariably fail to complete the 
workflow and leave a record locked.

For an optimistic lock, you need a version flag on the record. When 
someone retrieves the record, you include the version number. Before 
saving the record, you check the version in the database. If it doesn't 
match, then someone else has beaten your user to the punch, and you 
branch to a page explaining the problem.

Optimistic locks are fine when the chance of contention is low and the 
changes are usually minimal. Pessimistic locks are better when the 
chance of contention is high and the changes are either non-minimal or 
critical.

Scott Ambler's papers provide some good background about this

* http://www.ambysoft.com/persistenceLayer.pdf
* http://www.ambysoft.com/mappingObjects.pdf

I have a telemarketing application that uses pessimistic locks to keep 
people from trying to call the same person at once. Though, for most 
things, optimistic locks work well.

-Ted.

For keeping someone from submitting the same form twice, Struts has a 
built-in token feature, but that's a different question =:0)


-- 
Ted Husted,
Struts in Action <http://husted.com/struts/book.html>


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