Andrus, Michael,
Thanks for your answers, they were very helpful.
On Apr 21, 2009, at 2:00 PM, Michael Gentry wrote:
> On Tue, Apr 21, 2009 at 4:40 PM, Ylan Segal <ylan.sega..mail.com>
> wrote:
>> Ok. That makes sense. I can see that for a web application you
>> would want to
>> have each user manipulate data on it's own, without affecting what
>> other
>> users are doing. As far as junit test cases go, I can just create a
>> new
>> DataContext without much consequence. Right?
>
> Yes. A DataContext (or ObjectContext) is fairly cheap to create, too.
> Create them as it makes sense. Some people like to create a new DC
> for every "transaction" they wish to do. Some like to have a
> session-based DC (in a web application). Some use a mixture or add in
> nested DCs. It really depends on what you are trying to accomplish.
> It is also safe in Cayenne to reuse a DC across request/response loops
> in a web application. Or even within the same request/response cycle.
> Cayenne does not disconnect objects from their database channel after
> a commit is done (unlike Hibernate). You typically do not care about
> the database channel with Cayenne, either -- the DC manages that.
>
>> In a web app where each session has a DataContext, do I need to
>> worry about
>> data caching in different DataContexts? I don't know if that is
>> clear, so
>> let me explain. Suppose we are talking about a pet store. User A
>> and B are
>> both separately
>> in their browser shopping for a cat. There is only 1 left in
>> inventory. Both
>> user A and B see that there is one left. User A makes up his mind
>> first and
>> buys the cat. The changes are committed. In the DB the inventory
>> correctly
>> has the number of cats at 0. If User B's DataContext caches the
>> data, it
>> might think that the inventory is still 1 and also allow User B to
>> buy the
>> cat. Should I worry about this or is it all taken care of by Cayenne?
>
> This actually brings up many issues. One of the issues is data
> freshness (and you can force a refresh if if you want). However,
> there is always some latency involved between two requests, so even
> that could result in a missed read. I think what you are really
> asking about here is optimistic locking:
>
> http://cwiki.apache.org/CAY/optimistic-locking-explained.html
>
> Let's say user A and B both read from the "inventory" table a value of
> 1 for "number_left". You are really wanting Cayenne to generate, as
> part of a transaction -- dataContext.commitChanges() -- an update
> similar to this:
>
> UPDATE inventory SET number_left = 0 WHERE primary_key = 587375 and
> number_left = 1;
>
> When user A commits, all is well. When user B commits, an exception
> will be thrown by Cayenne because there WHERE clause fails (the
> "number_left" value is now at 0 due to user A's commit).
>
>> Thanks for the answers. In the meantime I have been trying out the
>> modeler
>> and it seems to do a great job of creating the db schema,
>> relationship
>> mapping and Java code generation. So far I am very impressed!
>>
>> --
>> Ylan.
-- Ylan Segal ylan.sega..mail.com
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