Hi,
I think that I do require "instance locking" for the DataStore to solve the
integrity problem in my scenario.
It's a long story but I will try to put it as brief as possible:
1) Suppose there is a FIFO queue, Q.
2) To consume the queue, there are many clients ( say, the flash movie in
the browser) connect to the GAE for consuming the queue.
3) For every consumption from the client side, the dispatching CGI has to
3.1) query the DataStore for all entries,
3.2) sorting the entries to get the 1st item in the queue.
3.3) mark the item been consumed. then, the item moves to its next
phase.
3.4) update the queue.(remove the item from queue)
The problem will happen if :
a) when client-A requests for consumption, the dispatching CGI handles the
request. but the DataStore got stuck in doing 3.2 on occasion.
b) During that stuck period, client-B requests for consumption, this time,
the DataStore runs very well, without any delay.
c) The client-A and client-B reaches the 3.3 at the same time, they both got
the same item in the queue. That is the problem.
Either the client-A or the client-B overrides the other's effort to the
same item in queue.
I am thinking about to utilize the memcache to do the locking, but I was
told that the memcache is not guaranteed.
I also assume that the "locking" should not depends on the DataStore which
is the origin of uncertainty.
(aka to put a BooleanProperty "LOCK" to the instance)
The other suggestion is the "transaction". I encountered many exceptions
while putting the step 3.2, 3.3 to a transaction.
Such as "nested transaction is not allowed","Cannot operate on different
entity groups","must be an ancestor...".
Because the transaction has so much limitation, it seems not realistic to
use transaction.
Whereas to lock/release the entry (object) is simpler and more concise.
It would be nice if the item is lock-able, or the queue is lock-able.
Iap
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