Re: Parellel request from the same user in very short time

On Sun, 25 Sep 2011 02:09:46 +0200, Russell Keith-Magee
<russell@keith-magee.com> wrote:

> On Sat, Sep 24, 2011 at 10:08 PM, Martin Tiršel <django@blackpage.eu>
> wrote:
>> Hello,
>>
>> I am using Apache with mod_wsgi and following setting:
>>
>> WSGIDaemonProcess dev.xyz.com processes=1 threads=5 user=xyz group=xyz
>> display-name=dev.xyz.com
>>
>> I am observing some situations where a user sends the same request
>> multiple
>> times (double click?) where the time between these requests is very low
>> -
>> 80-100ms. Then I got these situations (bot request from the same user):
>>
>> 1. request A - SELECT ...
>> 2. request B - SELECT ...
>> 3. request A - INSERT ... (insert is based on select result from step 1)
>> 4. request B - INSERT ... (insert is based on select result from step 2)
>>
>> Because 1. and 2. gets the same data, I end with doubled (or sometimes
>> trippled) inserts what breaks my application. How do you prevent such
>> behaviour? I am using Django 1.2.x
>
> I might be missing something obvious here, but isn't this *exactly*
> what database transactions are designed to fix? Admittedly, you've got
> an unusual presentation born out of weird browser behavior, but making
> sure that two overlapping operations don't violate data integrity
> sounds like the textbook case for using transactions.
>
> Yours,
> Russ Magee %-)
>

In my case, I have a construction_queue which has an order,
building_level, game_base_id and some other fields. Order field is for
sorting the queue. When I am inserting new item, i need to determine
values for order and building_level. Order field is from SELECT COUNT...
and building_level from built_buildings plus number of buildings of the
same name in construction_queue. So when double request load the same
data, the same inserts are created. Transaction would be usable when I add
unique index on game_base_id with order. In that case when double request
comes, I got an exception and I can rollback the second request. You mean
it this way?

Martin

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