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From "Carlos Peix" <peix-lis...@praxia.com.ar>
Subject RE: Kind of architecture question
Date Tue, 08 May 2007 15:37:32 GMT
Hi Tom,
 
Yes, I know that it's not IBatis concern, I'm not asking Gilles to add support
for this.
 
But this operation mode (to stop calling this an issue :-) not only gives me
different instances of the same object (I can fall in the trap modifying one
object and reading another unmodified instance in may code) but it have
performance implications.
 
Think for a minute in the example I posted (it's below you answer in this post).
Imagine that the customer holds a reference to the collection of orders,
something like:
 
Customer {
    // Mapped collection, with getter and setter, of course.
    private IList orders;
}
 
and the order has a reference to the Customer, just like the example below.
 
if I code this way:
 
Customer customer = CustomerRepository.GetById( 1 );
 
and then I do (in another method, down the code path):
 
foreach ( Order order in customer.Orders ) {
    Console.WriteLine( "Order #{0} from {1}", order.Number, order.Customer.Name
);
}
 
I'm loading the customer again with all the orders (I know I can use lazy load
for this or that I can send the original customer instance).
 
I just want to know how other people is handling this.
 
Thanks
 
Carlos


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From: Nguyen, Tom [mailto:Tom.Nguyen@rels.info] 
Sent: Martes, 08 de Mayo de 2007 11:28 a.m.
To: user-cs@ibatis.apache.org
Subject: RE: Kind of architecture question



I agreed with Sam that this should not be a concern for iBatis.  I've also
battled with the same issues in the pass, but I've found that it's much easier
to use SQL for controlling of identity and concurrency (through various methods
such as comparison of OriginalValue or LastModifiedDate).

 

Repository implementation is a can of worm in itself.  I prefer to use light
weight object and don't have to worry about issues such as multi-threading,
caching and releasing references for Garbage Collector when it's not being
reference.  Also, you can't really ensure identical object references in a
webfarm or multiple application server SOA implementation.

 

On the other hand, I had a very specific need of repository base on lookup
business objects such as OrderType, BusinessType, ContactType, etc.  In this
case, I implement custom in-memory caching (repository).  It's a very specific
implementation which allow me to lazy load objects by identity (key) and type,
cache, timely clearing of cache for GC, etc..

 

Regards,


Tom Nguyen 
Sr. Developer



 

On 5/7/07, Carlos Peix <peix-listas@praxia.com.ar> wrote: 

Hi Shane, thanks for taking the time to answer.

 

Yes, you are talking about an Identity Map I guess, but this solution is harder
to implement and tune. It's my last resort though.

 

Of course, definitely the select="CustomerById" needs to be removed. In fact,
the command CustomerById uses a different cache that the command CustomerAll so,
if we use cache for both commands we can stop IBatis building different
instances.

 

One approach I used is for resolving this is implementing the
CustomerRepository.GetAll() with an IBatis query (cached) and the
CustomerRepository.GetById() resolved in memory. But I'm not very happy with
this solution either

 

Of course, the Order ResultMap just get the Customer Id and I resolve the
property getter or setter that way.

 

class Order {

 

    // This field is mapped in IBatis

    object customerId;

 

    // this is a transient field

    private Customer customer;

 

    public Customer Customer

    {

        get {

            if ( customer == null )

                customer = CustomerRepository.GetById( customerId );

            return customer;

        }

        set { customerId = value.Id; }

    }

 

}

 

Carlos

 


  _____  


From: Shane Courtrille [mailto:shanecourtrille@gmail.com] 
Sent: Lunes, 07 de Mayo de 2007 07:03 p.m.
To: user-cs@ibatis.apache.org; peix-listas@praxia.com.ar
Subject: Re: Kind of architecture question

I'm just learning iBatis.Net now but I have used the repository <-> mapper
pattern in the past.  Usually what I have done is have the repository contain a
reference to a cache.  It checks the cache before using the mapping layer to
retrieve the item.  The problem I see is your Order select="CustomerById".
Without knowing more about iBatis I would suggest you may need to remove that
and instead have your repository fill in the reference after it gets the Order.
Definitely not a solution I'm in love with though. 

Shane

On 5/7/07, Carlos Peix <peix-listas@praxia.com.ar> wrote: 

Hi all,

 

I used IBatis.NET with success in various projects now but I am still not very
happy with my implementations. I'll try to explain my concerns.

 

My environment is .NET 1.1 but I think .NET 2.0 is the same thing. I should say
that I try to work guided by the DDD principles, so I access the final store
through repositories.

 

I have, for example, an entity Customer (aggregate root or simply a persistent
identifiable object) and a CustomerRepository with the following interface:

 

Customer CustomerRepository.GetById( object id );

IList CustomerRepository.GetAll(); 

...

 

and I also have a Order and OrderRepository with the following interface

 

OrderRepository.GetByNumber( int number );

...

 

The problem I face all the time with IBatis is the I get different instances if
I do:

 

// implemented with a Mapper.QueryForObject( "CustomerById", id );

Customer customer1 = CustomerRepository.GetById( object id );

 

// implemented with a Mapper.QueryForList( "CustomerAll" );

Customer customer2 = CustomerRepository.GetAll()[0]; 

 

// implemented with a Mapper.QueryForObject( "OrderByNumber", number );

// and Customer mapped this way in the OrderResultMap:

//  <result property="customer" column="CustomerId" select="CustomerById" />
Customer customer3 = OrderRepository.GetByNumber( 100 ).Customer;

 

But customer1.Id, customer2.Id and customer3.Id are the same.

 

Ok, this is a situation that need to be controlled, otherwise I could modify or
check different instances of the same object. I was told previously that this is
not an IBatis problem and I see why (in fact IBatis doesn't know anything about
object identity, so it can't control this).

 

The question is: how are you structuring your code to control that situation?
there are some "recommended practices"? I started with some ideas (one of them
include IBatis cache configuration) but I'm not happy with any of them.

 

Sorry about the long post and thanks in advance.

 

Carlos Peix

 

 





 


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