For various reasons, we are writing a new business objects/data storage library. One of the requirements of this layer is to separate the logic of the business rules, and the actual data storage layer.

It is possible to have multiple data storage layers that implement access to the same object - for example, a main "database" data storage source that implements most objects, and another "ldap" source that implements a User object. In this scenario, User can optionally come from an LDAP source, perhaps with slightly different functionality (eg, not possible to save/update the User object), but otherwise it is used by the application the same way. Another data storage type might be a web service, or an external database.

There are two main ways we are looking at implementing this, and me and a co-worker disagree on a fundamental level which is correct. I'd like some advice on which one is the best to use. I'll try to keep my descriptions of each as neutral as possible, as I'm looking for some objective view points here.

• Business objects are base classes, and data storage objects inherit business objects. Client code deals with data storage objects.

  In this case, common business rules are inherited by each data storage object, and it is the data storage objects that are directly used by the client code.

  This has the implication that client code determines which data storage method to use for a given object, because it has to explicitly declare an instance to that type of object. Client code needs to explicitly know connection information for each data storage type it is using.

  If a data storage layer implements different functionality for a given object, client code explicitly knows about it at compile time because the object looks different. If the data storage method is changed, client code has to be updated.

• Business objects encapsulate data storage objects.

  In this case, business objects are directly used by client application. Client application passes along base connection information to business layer. Decision about which data storage method a given object uses is made by business object code. Connection information would be a chunk of data taken from a config file (client app does not really know/care about details of it), which may be a single connection string for a database, or several pieces connection strings for various data storage types. Additional data storage connection types could also be read from another spot - eg, a configuration table in a database that specifies URLs to various web services.

  The benefit here is that if a new data storage method is added to an existing object, a configuration setting can be set at runtime to determine which method to use, and it is completely transparent to the client applications. Client apps do not need to be modified if data storage method for a given object changes.

• Business objects are base classes, data source objects inherit from business objects. Client code deals primarily with base classes.

  This is similar to the first method, but client code declares variables of the base business object types, and Load()/Create()/etc static methods on the business objects return the appropriate data source-typed objects.

  The architecture of this solution is similar to the first method, but the main difference is the decision about which data storage object to use for a given business object is made by the business layer, not the client code.

I know there are already existing ORM libraries that provide some of this functionality, but please discount those for now (there is the possibility that a data storage layer is implemented with one of these ORM libraries) - also note I'm deliberately not telling you what language is being used here, other than that it is strongly typed.

I'm looking for some general advice here on which method is better to use (or feel free to suggest something else), and why.