Hi Dmitry,

>In the 3-tier application you have a bunch of BIZ objects, for different entities. That is clear. Is it correct to assume that there should be only one DA object that all BIZ objects use to update the database?

Depends on what the DA means to you. Often you have the following layers (UI aside):

Business object: Contains all business logic on an abstracted level. There's absolutely no database specific dependency in this layer including any of the SQL data types.

- Data access layer: Is specific to the BO and the backend. The DAL contains all the data related parts. It handles the translation between .NET types and SQL types, transforms object dependencies into related entities, etc. There's one DAL object for every BO and every supported back end. The DAL does not talk directly with the database

- Database access: handles the actual access to the database. This layer manages connections, permissions, transactions, sends SQL commands, etc. There's only one Database Access object for each back end that all DAL objects talk to. Well, usually this is more than a single object as transactions, connections, etc. are involved.