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From Andrus Adamchik <>
Subject Re: Best Practices Question
Date Tue, 21 Apr 2009 13:54:58 GMT
Let me also point that what I am suggesting is exactly the same as  
what Michael did. The difference lies in the presence of an IoC  
container and all the data access methods moved out of DataObjects to  
the "DAO services".

In some applications picking one or the other style makes no  
difference. Where it does make a difference is when the same set of  
DAO interfaces has to work differently depending on some environment  
parameters. So you have more than one implementation of a given DAO  
all using the same shared Cayenne mapping. Simple example - a CMS  
application running in two modes: "production" and "preview", and  
there is a PageDAO that provides access to persistent Page objects.  
Production-mode DAO may avoid returning pages whose state is "not  
published", while preview mode DAO will return them to the caller.


On Apr 21, 2009, at 4:43 PM, Andrus Adamchik wrote:

> Yeah, it depends greatly on the general app architecture and usage  
> patterns... IIRC the petstore example was taken from Spring and  
> rewritten with Cayenne, so it may have some of the Spring-induced  
> artifacts (don't remember all the details).
> The pattern I am personally following lately in the systems that  
> have an IoC container (say Spring or Tapestry), is to define one  
> generic Cayenne "service" that gives access to an ObjectContext  
> depending on your needs, and a bunch of "DAO services" that  
> encapsulate specific queries and other data operations, including  
> cache strategies. This way the frontend code doesn't have to deal  
> with cache rules and other low level details. This requires some  
> thought on the design, but at the end scales best in terms of  
> complexity.
> Here is some examples:
> interface CayenneService {
>   // depending on implementation this can return application or  
> session-level shared context.
>   // often this is a read-only context...
>   ObjectContext getSharedContext();
>   // creates a new context
>   ObjectContext createNewContext();
> }
> interface AccountDAO {
>   List<Account> getActiveAccounts();
>   List<Account> getNewAccounts();
>   List<Account> searchAccounts(Stirng searchCriteria);
> }
> class AccountDAOImpl implements AccountDAO {
>   // injected
>   CayenneService cayenneService;
>   List<Account> getActiveAccounts() {
>       SelectQuery query = new SelectQuery(Account.class);
>       query.andQualifier(...);
>       query.andQualifier(...);
>       query.setCacheStrategy(...);
>       query.setCacheGroups(...);
>       return cayenneSerice. getSharedContext().performQuery(query);
>   }
> }
> Andrus
> On Apr 21, 2009, at 4:26 PM, Michael Gentry wrote:
>> I'm not familiar with the Pet Store example, so I'm not sure what was
>> meant by it not being a best practices example.  Also, people have
>> many different ideas of what they think a best practice is, so it can
>> vary greatly.
>> My typical usage is to put my custom business logic in the
>> Cayenne-generated subclass and to also put my "fetch" type methods in
>> there, too (when not creating them in Cayenne Modeler).  The more
>> interesting questions (to me) are what are the usage patterns, etc.
>> That helps determine how to use the DataContext (or ObjectContext),
>> how to do data refreshing, etc.  One application I did had a single
>> read-mostly DC that I synchronized access to for all the users (the
>> queries took a prohibitive amount of time for interactive usage, so I
>> cached things in a single DC).  Others I used one (or more) DCs per
>> user's session.  Again, it really depends upon what your requirements
>> are for data access.
>> Feel free to ask more questions.  Hope that helped just a little.
>> mrg
>> On Mon, Apr 20, 2009 at 3:30 PM, Ylan Segal <>  
>> wrote:
>>> Hi Everyone,
>>> I am new to Cayenne. I was looking at the examples in the wiki,  
>>> especially
>>> at the pet store:
>>> So far, I like what I see. The above page does mention that the  
>>> pet store
>>> example "... definitely needs a rewrite and DB redesign before we  
>>> can call
>>> it Cayenne Best Practices Demo."
>>> Why isn't it a best practice? Because of the DB design or because  
>>> of how
>>> Cayenne classes are used? The overall app design (one domain  
>>> package, one
>>> DAO package, one presentation package) seems reasonable to me. I  
>>> would be
>>> designing a DB schema from scratch for a web application (probably  
>>> with
>>> struts), and I am thinking of following the example pattern,  
>>> assuming that
>>> pattern is not what needs rewriting! Can anyone suggest best  
>>> practices for
>>> this type of situation? I am looking for more of a "what packages  
>>> should we
>>> have" discussion rather than a "this is how you save an object to  
>>> the
>>> database" discussion.
>>> I hope this makes sense.
>>> --
>>> Ylan Segal

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