[Chicago] appengine session locking

[Cosmin Stejerean]

On Mon, May 12, 2008 at 1:50 PM, Massimo Di Pierro
<mdipierro at cs.depaul.edu> wrote:
> This is the problem.
>  In order to lock a session I need first to check that it has not been
> locked already.
>  I can only mark it as locked after I have performed the check.
>  If if perform the check and then the update in separate statements I enter
> in a race condition.
>  As far as I know there is no API to perform a conditional update.
>
>  Am I wrong?
>

Are you familiar with datastore transactions?
http://code.google.com/appengine/docs/datastore/transactions.html

here's an example that should work

def lock_session(key):
  session = db.get(key)
  if session.locked:
    raise ValueError("Session with key %s is already locked" % key)
  else:
    session.locked = True
    session.put()

db.run_in_transaction(lock_session, session_key_name)

If there are concurrent updates to the same object while your
transaction is running Google will restart your transaction a fixed
number of times. If the function raises an exception Google will abort
the transaction.


-- 
Cosmin Stejerean
http://blog.offbytwo.com