cayenne-user mailing list archives

Site index · List index
Message view « Date » · « Thread »
Top « Date » · « Thread »
From Ylan Segal <ylan.se...@gmail.com>
Subject Re: Best Practices Question
Date Tue, 21 Apr 2009 22:56:34 GMT
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.segal@gmail.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.segal@gmail.com


Mime
View raw message