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I've been having a debate within my team on what constitutes a Data Access Layer vs Data Functions vs Business Layers.

My thoughts is all database access is done in a data access layer with Repository classes. The DAL contains utility classes & methods that populate a DataSet, List<>, POCO, Execute SQL, but as internal classes using your choice of Access method: ADO, EntityFramework, nHibernate, etc. The Business layer then interacts with the DAL without knowing any of SQL or data access methodology.

A teammate has this approach. The stored procedure names and SQL are in the business classes. Then they are passed to the DAL which then populates and executes the SQL.

Which is the best practice?

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2 Answers

up vote 2 down vote accepted

There's no use for a BAL if all it is going to do is feed database access strings. You may also find that there is too much business logic contained in the stored procedures.

If you want data access in you business logic, call it something else.

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When you're dealing with an architecture that has a separate DAL and business logic layer, stored procedures and SQL do not belong in the business logic layer. Strictly speaking, that layer should be agnostic to how the data is stored and retrieved, and should only be responsible for manipulating the instances returned from the DAL, enforcing business rules, and performing validation.

As far as what's practical, though, the lines often blur, as it's often difficult to balance efficiency (how fast the code runs) and expediency (how fast you can produce working code) against ideal design. If you have complex filters as the result of rules in your business layer, it would be extremely inefficient to return everything from your DAL and filter in the business layer, although that would follow the strict separation between layers. And it would take too long and be a maintenance nightmare to write a separate function for every possible way to filter your data. For a large project, the solution is often to introduce a way to build a custom "request" (or "query") class that gets passed to the DAL, which returns results based on the custom object, but this is overkill for a small project (unless you use an ORM such as Entity Framework, which exposes your repository as a collection of IQueryable<T>, which lets you build your query without concern for how it's actually going to be executed against your data store).

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