Re: Preventing race conditions when submitting forms

Tim, Simon,

Thanks for the responses. Looks like django-concurrency is a good fit, and it works the way Tim suggested (so no need to write your own mixins!)

One follow-up question: do you have any ideas for conflict resolution? The django-concurrency resolution is pretty basic, but it would be a lot of work to do something better.

Paul

On Tuesday, November 25, 2014 6:59:15 PM UTC, Tim Chase wrote:

On 2014-11-25 07:57, Paul Johnston wrote:
> Consider an e-commerce site, where Alice and Bob are both editing
> the product listings. Alice is improving descriptions, while Bob is
> updating prices. They start editing the Acme Wonder Widget at the
> same time. Bob finishes first and saves the product with the new
> price. Alice takes a bit longer to update the description, and when
> she finishes, she saves the product with her new description.
> Unfortunately, she also overwrites the price with the old price,
> which was not intended.

The common solution in this case your model would have something like
a "last_modified_timestamp" or "last_modified_counter".  This would
be sent with the form and then resubmitted back.  If the
timestamp/counter is the same, you know it's safe to update the
timestamp or increment the counter (done within the transaction to
ensure atomicity).  If it's *not* the same timestamp/counter as it was
when the form was created, you know that it's been modified in the
interim and you can present a conflict-resolution form.

I don't think Django has anything like this out of the box, but it's
fairly straightforward to implement.  I've done it enough times that
I should just create some model/form mixin to handle it for me.

-tkc

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