Design Patterns discussion

[Andrew Chelyabin <chelyabin AT sctest.vrn.ru>]

Hi,

Zhai, is it required to mix various startegies of access?
For example. We have default parameter value stored in remote database.
And user can override that value with new one described in local XML
configuration file (or even set this parameter from comman line :).

If it is so then you may try to use "Chain of Responsibility" instead of
"Strategy" pattern. So we have chain of access strategies configured
from the driver, the sequence in the chain determines priotity of values
lookup (first in database then in local XML and so on).

On Thu, 4 Mar 2004 10:54:37 +0800 (CST)
Zhai 
<myzhai AT yahoo.com.cn>
 wrote:

> Hi,
>   Thank all of you.
>   Now it seems that the context is important. And I
> did not mention it at beginning.
>   In fact, we want to provide an API class for
> parameter management in our system, like MICROSOFT's
> Configuration Management Application Block for .NET(
>   I do not know it before). The abstract class
> "CParaAccess" is 
>   the interface. Clients can use the interface to
> access their parameters form a file or a databse, even
> from a registry.
>   In order to do so, they would instantiate a
> CFileParaAccess or a CDatabaseParaAccess, decided by a
> configuration file. Moreover, a client can access some
> parameters form a file and  the other parameters from
> a database. So it is the client that determines which
> subclass to instantiate. And in run time, the client
> can define a variable "paraPtr" to point to the
> instance and change its content. that is, paraPtr can
> be changed from "new CFileParaAccess" to "new
> CDatabaseParaAccess", or inversely. we only provide a
> library with the client to invoke. According to Chris
> Finlayson<cfinlayson AT vls-inc.com>,
>  it's the Strategy.
>   Additionally, can we use the Factory Method design
> pattern to help clients to create the instances? we
> can design an abstract class CParaAccessFactory and
> two subclasses "CFileParaAccessFactory" 
>   and "CDatabaseParaAccessFactory", then clients can
> use these factories to create their needed class. That
> is, we use Factory Method 
>   and Strategy design patterns together to design our
> library.
> thx.
> 
> 
> 
> 
>  --- Chris Finlayson 
> <cfinlayson AT vls-inc.com>
> ____________> Hi:
> > 
> > Microsoft has an example of just such a thing in
> > their "Microsoft
> > Application Blocks", which is basically examples of
> > applying design patterns
> > to specific problem domains in .NET languages. 
> > Regarldess of whether you're
> > using .NET, it's a good example that deals with just
> > what you're looking
> > for.
> > 
> > In particular, get the "Configuration Management"
> > application block.  This
> > is an example of one having to get or set parameters
> > where the storage
> > provider could be XML, a relational DB, or the
> > registry.  It's unclear
> > exactly how the storage provider is determined in
> > Zhai's context.  In the
> > Configuration Management application block, it is
> > determined by a
> > user-configurable application settings file, but it
> > need not be; it can be
> > anything you want.
> > 
> > As for as what design pattern is used, it's the
> > Strategy.
> > 
> > They have interfaces all storage providers use for
> > reading/writing data (one
> > for just reading/one for writing&reading)
> > The interface expects the parameter to come in as an
> > XML Node, regardless of
> > the underlying storage provider
> > Specific storage providers (Registry, XML, DB) are
> > The concrete storage provider is set at runtime
> > based on the configuration
> > file.
> > The storage provider takes the XML node, and
> > transforms it however it needs
> > to be transformed to the underlying storage
> > mechanism.
> > 
> > --Chris.
> > 
> > 
> > 
> > > -----Original Message-----
> > > From: 
> > > gang-of-4-patterns-admin AT cs.uiuc.edu
> > > [mailto:gang-of-4-patterns-admin AT cs.uiuc.edu]On
> > Behalf Of
> > > Santana, Luis
> > > Sent: Wednesday, March 03, 2004 12:02 PM
> > > To: 
> > > gang-of-4-patterns AT cs.uiuc.edu;
> > >  Pablo Schor
> > > Cc: Zhai
> > > Subject: RE: [gang-of-4-patterns] discuss the
> > usage of Strategy DP
> > >
> > >
> > > Hi Pablo,
> > > I see your point and I think it all comes down to
> > the context and the
> > > requirements for Zhai's actual implementation...
> > Have you noticed that
> > > originally Zahi mentioned the Strategy Pattern and
> > then
> > > Abstract Factory and
> > > the Bridge patterns appeared in the picture? ..
> > Strategy is a
> > > behavioral,
> > > Abstract Factory is a creational and Bridge is an
> > structural
> > > pattern which
> > > means that all of them have different
> > intentions...
> > >
> > > Zhai mentions two different implementations
> > (CFileParaAccess and
> > > CDatabaseParaAccess) of the same abstract class
> > > (CParaAccess)... How will
> > > the application decide when to instantiate a
> > CFileParaAccess
> > > and when to use
> > > a CDatabaseParaAccess? The suggestion for the
> > Abstract
> > > Factory comes from
> > > this perspective... But you guys are right, if it
> > is not
> > > necessary to create
> > > anything then there is no need for an Abstract
> > Factory...
> > >
> > > Cheers,
> > >
> > > Luis Santana.
> > >
> > > -----Original Message-----
> > > From: Pablo Schor
> > [mailto:pablo.schor AT lobruno.com.ar]
> > > Sent: Wednesday, March 03, 2004 12:38 PM
> > > To: Santana, Luis; 
> > > gang-of-4-patterns AT cs.uiuc.edu
> > > Cc: Zhai
> > > Subject: Re: [gang-of-4-patterns] discuss the
> > usage of Strategy DP
> > >
> > >
> > > Luis, I would go with Zhai.
> > >
> > > I would use the Strategy design pattern, since the
> > interface
> > > is the same,
> > > and easier to develop and implement. Abstract
> > Factory, as its
> > > name states,
> > > is a factory that is abstract, means that you use
> > it to
> > > create something
> > > that you're going to use.  In this scenario, as
> > far as I
> > > understand, you
> > > don't need to create anything, and in addition
> > Abstract
> > > Factory is more
> > > complex and has more overhead.
> > >
> > > I'd analyze the Bridge pattern, for me was always
> > the best
> > > way to separate
> > > interface from implementation.
> > >
> > >
> > > Pablo
> > >
> > >
> > >
> > > ----- Original Message -----
> > > From: "Santana, Luis" 
> > > <luis.santana AT eds.com>
> > > To: 
> > > <gang-of-4-patterns AT cs.uiuc.edu>
> > > Cc: "Zhai" 
> > > <myzhai AT yahoo.com.cn>
> > > Sent: Wednesday, March 03, 2004 2:31 PM
> > > Subject: RE: [gang-of-4-patterns] discuss the
> > usage of Strategy DP
> > >
> > >
> > > > I agree with Eduardo, without knowing much about
> > the
> > > context, the best
> > > > suitable pattern would be "Abstract Factory"...
> > The 'family' of your
> > > product
> > > > is "CParaAccess" and your "concrete" products
> > would be
> > > > "CFileParaAccess"
> > > and
> > > > "CDatabaseParaAccess".
> > > >
> > > > Regards,
> > > >
> > > > Luis Santana
> > > >
> > > > -----Original Message-----
> > > > From: Zhai 
> > > > [mailto:myzhai AT yahoo.com.cn]
> > > > Sent: Wednesday, March 03, 2004 8:24 AM
> > > > To: Eduardo Franco;
> > gang-of-4-patterns AT cs.uiuc.edu
> > > > Subject: Re: [gang-of-4-patterns] discuss the
> > usage of Strategy DP
> > > >
> > > >
> > > > At first, I try to use Abstract Factory to
> > design the
> > > > system. According to the intend of Abstract
> > Factory,
> > > > it can produce a family of products. But in our
> > > > system, we do not know what the family of
> > products is.
> > > > So we next consider to use Strategy design
> > pattern.
> > > > we need more advice.
> > > >
> > > > --- Eduardo Franco 
> > > > <eduardo.franco AT pulso.com.br>
> > > > ____________> I don't think the Strategy is the best
> > > > pattern for
> > > > > your problem. I don't
> > > > > even know much about the context your are
> > developing
> > > > > the system, but
> > > > > AFAICS the best suitable pattern is the
> > "Abstract
> > > > > Factory".
> > > > >
> > > > > [..]
> > > > >
> > > > > "Provide an interface for creating families of
> > > > > related or dependent
> > > > > objects without specifying their concrete
> > classes."
> > > > >
> > > > > [..]
> > > > >
> > 
> === message truncated === 
> 
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