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From Michael Gentry <mgen...@masslight.net>
Subject Re: Best Practices Question
Date Tue, 21 Apr 2009 21:00:42 GMT
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.

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