R – Do you allow the Web Tier to access the DAL directly

blldata-access-layerdesign-patterns

I'm interested in perceived "best practice", tempered with a little dose of reality here.

In a web application, do you allow your web tier to directly access the DAL, or should it go through a BLL first?

I'm talking specifically of scenarios where there's no "business logic" really involved — such as a simple query : "Fetch all customers with surname of 'Atwood'". Scenarios where there's any kind of logic absolutely are gonna go through the BLL, so lets call that moo.

While you could encapsulate this method inside a BLL object, it seems to be somewhat pointless when often the signature will be exactly the same as that of the DLL object, and the code probably as simple as a one liner delegating the query off to the DLL.

If you choose the former — employing a BLL object — what do you call these objects? (Assuming they do little more than provide a query layer into the DLL). Helpers? QueryProviders?

Thoughts please.

Regards

Marty

Best Answer

In my opinion, you should ALWAYS use a BLL (Business Logic Layer) between your web tier and your DAL (Data Access Layer).

I appreciate that for some of the more "simple" queries, the BLL will closely mimic the DAL (e.g. Fetch all countries, Fetch all Product Types etc.), but to honest, even in your example:

(Fetch all customers with surname of 'Atwood')

there is "business logic" being expressed here - A desire for the data records to be filtered by surname, if nothing else!

By implementing a BLL from the start of a project it becomes incredibly easy to insert either validation or additional "logic" as and when the need may arise (and if your project is a commercial application, that need will almost certainly arise eventually if it isn't there at the beginning of the project). Adding in additional logic such as:

Fetch all customers who have spent over $10000 this year

or

Don't allow customers with the surname of 'Atwood' to purchase items over $1000

becomes significantly easier when a true BLL is involved, rather than trying to crowbar this logic into the web tier.

Bear in mind that with the kinds of queries above, we're almost certainly talking about multiple entities and database tables that will have to join together with specifically defined relationships in order to implement this functionality. Trying to achieve this by directly manipulating the DAL becomes messy since you'll be dealing with multiple entities and classes. A BLL here would greatly simplify your web tier code, since the BLL will encapsulate those entity relationships behind a greatly simplified interface.

This "separation of concerns" becomes increasing important when and if the need to change the user interface arises.

On at least two separate occasions now, I've worked on commercial web applications with a web site user interface, and have been eventually asked (due to business need arising from clients seeking greater integration within their software products) to produce a web service interface offering the exact same functionality as the web site.

Had I embedded any business logic within my web tier, I would have had to duplicate and re-write that logic when implementing my web service. As it was, I ensured that all business logic was encapsulated within BLL classes, which meant that I simply had to design a series of web service interface method calls, and plug these up against calls to methods on the BLL classes (I actually used the Facade Design Pattern in places to simplify the web service API).

In all, I can think of no reason to NOT include a BLL layer between my DAL and my web tier.

At it's easiest, when the BLL closely "mimics" the DAL, yes, there does appear to be a duplication of code and functionality, however, whilst being a little more typing, this also makes it relatively easy to implement.

When it's more involved (such as when significant business logic exists from the very beginning), the separation of concerns helps to reduce repetition (the DRY principle) whilst at the same time significantly simplifying future and ongoing maintenance.

Of course, this assumes you're doing all this "by hand". If you so desire, you can significantly simplify the DAL/BLL/UI layers by utilizing an ORM of which there are many! (i.e. LINQ-to-SQL/Entities, SubSonic, NHibernate etc.)

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POCO’s, DTO’s, DLL’s and Anaemic Domain Models

Typically, you don't want to introduce persistence into your domain objects, since it is not part of that business model (an airplane does not construct itself, it flies passengers/cargo from one location to another). You should use the repository pattern, an ORM framework, or some other data access pattern to manage the persistent storage and retreival of an object's state.

Where the anemic domain model comes in to play is when you're doing things like this:

IAirplaneService service = ...;
Airplane plane = ...;
service.FlyAirplaneToAirport(plane, "IAD");

In this case, the management of the airplane's state (whether it's flying, where it's at, what's the departure time/airport, what's the arrival time/airport, what's the flight plan, etc) is delegated to something external to the plane... the AirplaneService instance.

A POCO way of implementing this would be to design your interface this way:

Airplane plane = ...;
plane.FlyToAirport("IAD");

This is more discoverable, since developers know where to look to make an airplane fly (just tell the airplane to do it). It also allows you to ensure that state is only managed internally. You can then make things like current location read-only, and ensure that it's only changed in one place. With an anemic domain object, since state is set externally, discovering where state is changed becomes increasingly difficult as the scale of your domain increases.

R – Data Access Layer and Business Objects

Look up a concept called 'Domain Driven Design' - the biggest thing there is using what's called a repository pattern (such as your UserDB class) as an adapter to the database, as well as a factory. Your business objects, or domain objects, then incorporate business logic into themselves and can handle interactions with other business objects.

What technology are you using? Something like ActiveRecord can probably help you a lot.

Best Answer

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