detecting stale session data

Hi,

I'm pretty new to Django and am having a problem with a race conditions while modifying my session data. I'm using the standard session backend in Django 1.4.1,, backed by Mysql.

I have view A, which can take a long time to process, and view B, which is usually faster. I store multiple data fields, fieldA and fieldB, in the session dictionary. View A modifies fieldA, and view B modifies fieldB.

Sometimes, view A starts up with a copy of the session data as it exists at t1. Then, while it's processing, view B starts up. It completes quickly, modifying fieldB in the session data at time t2. Then view A finally completes, modifying its field in the session data at time t3. But what I wind up with is fieldA at time t3 and fieldB at time t1. In other words, when view A completes its write, it's storing the stale version of fieldB that it read when it started. What I want in that situation is t3's version of fieldA and t2's version of fieldB.

My questions: (1) is there a way to check if your copy of the session dictionary is stale? And if so, is there a way to do it atomically? (2) Is there a way to selectively write just part of the session data, instead of the whole thing? (3)  Or, is there a better solution besides either of these ideas?

The only thing I can think of using RAM memory to keep track of a last_write timestamp for each session -- but that's not a scalable solution. Is there a better way?

Thanks in advance,

Spork

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