UI, Business Logic Layer, Data Layer and where to put web services

Question

We are developing a web application. We want to possibly reuse the work we do here for a different application that will use the same database, and use the same business rules for reading and writing to said database.

Which design would be more correct

1. Having the UI call web services, which would use business objects containing the business logic, which would talk to the data access layer.

2. Have the UI use business objects containing the business logic, which would call web services, which would then talk to the data access layer.

3. Have the UI user business objects containing the business logic, which would talk to the data access layer.

Answer 1

From your description, you haven't provided a reason why you would need the use of a web service layer. Assuming your database is reachable by your UI system, i.e. within the same network behind your firewall, a basic business-object layer that your website UI code (server-side, I'm assuming) will employ meets your requirements.

Bring in a web service tier when the distance between your UI system and your data layer starts to cross boundaries that a Data access layer or Business logic layer would begin to encounter difficulties.

Answer 2

In terms of the design being "correct" or not, it's not really possible to give a 100% answer to the correctness of a design without the full context. What are the requirements (functional and non-functional)? What design goals do you want to fulfill? How important is each goal?

The only goal your question mentions is that you want to reuse the business logic with another application. When I want to reuse the business logic of an application in a standard way I choose web services. So based solely on your one requirement I would say that option 1 ( UI->Web Service->Business Layer->Data Layer ) is a good choice.

Answer 3

Check Out: http://www.icemanind.com/layergen.aspx

The way it should go is, you have your UI layer on top, your data layer on the bottom and your business layer in between the two. Each layer can only communicate with the layer below it. So the UI talks to the business layer only...the business layer talks to the data layer only. Your UI should never talk with the data layer and your data layer should never interact with your UI.

Unless you have a reason to use a web service, then I wouldn't.

Answer 4

I would say the 3rd one. I tend to think of web services as another presentation layer.

Think of it this way: you have a web UI, which calls your business layer code to do things like create a new user (User.Add), find all products that match a given description (Products.FindByDescription), etc.

You can now re-use that same business layer code to build a set of public-facing web services for 3rd parties to make use of. There can be a method which adds a user - that calls your internal User.Add() method, another one to find products, etc..

What you get is a parallel set of presentations/interfaces to the same underlying data and business logic.

Behind the scenes (totally out of the scope of web services or UI layers), the business layer calls a data access layer that takes care of physically querying the database. If you were to change to a different DBMS, you should ideally (and in theory) be able to rebuild the data layer for the new database and have everything simply work.

Your business layer contains the rules like a username has to be 4 to 15 characters long; users are only allowed to search for and load products that are at a store they have access to; etc.

If you decide to change a business rule - like a user is allowed to search for products in any store in their state - then you change it in once place, and don't have to touch the web service or UI to make it work.

Answer 5

Hello,

Don't mix logical design with physical design. Logical design operates over layers and physical design - tiers. Web Service is not a layer. It is simply a tier. In logical design there is standard approach: UI layer-> BL layer -> DAL In physical design all layers can reside within one client-side application connecting local database, or can be distributed over the remote tiers. But for distributed applications usually is added one more layer: Application layer, which hides from BL layer communication over the wire.