n-tier design with website and backend transaction processor.

Question

We have a website, where transactions are entered in and put through a workflow. We are going to follow the standard BLL(Business Logic Layer), DTO(Data Transfer Object), DAL(Data Access Layer) etc. for a tiered application. We have the need to separate everything out because some transactions will cross multiple applications with different business logic.

We also have a backend processor. It handles our transactions once the workflow has been completed. It works with various third party systems, some of which are unstable, or the interface to them is unstable, and then reports the status of the transaction. Each website will have its own version of the backend processor.

Now the question, with N-Tier, they suggest a new BLL for each application. With the layout of the application above, it can be argued that the backend processor and website is one application acting in unison, or two applications with different business logic. What would be the ideal way to handle this? Have it act like one system, or two?

Answer 1

The "Ideal" way to do this depends on the project at hand and the various requirements of the system.

My default design is to have it act as one app. But if there are more heavyweight processes taking place, I like to create a batching process where the parameters of the requested job are stored and acted upon by a seperate process.

Answer 2

If you separate your concerns well then I think that you will be able to view them as the same application with a single business logic layer, there is no point writing the same code twice. The trick will be forcing the separation of concerns between the user interface portions of the website and the business logic in your BLL library.

Performance is going to be an issue as well, you have to ensure that your batch processing doesn't block your website from performing tasks that it needs to perform due to your resources. This may be an argument to keep them more separate, however as they're likely sharing a database anyway (or some other file based resource) then that may be an issue regardless.

I would keep a common business logic library programmed to interfaces and fully separated from your other concerns.

Answer 3

You might consider partitioning the functionality to reflect the organization of the stakeholders. Usually if you have two distinct organizational groups, then development and administration requirements are easier to manage if the functionality is similarly partioned. And vise versa.

Most of us don't spend that much time writing applications that explore the outer boundaries of hardware and software capabilities.

Answer 4

One thing that I picked up on while learning MVC over the last couple years is the difference between what I call application logic and domain logic. I don't like the term business logic anymore, because it has too much baggage from all the conflicting theories and practices that have used that term too loosely.

Domain logic is the "traditional" business logic, how things are supposed to act, what they require (validation), etc. Application logic is anything that is specific to a given presentation of your domain, IE when the user clicks this submit button in your web app then they are directed to this web page over here (note that this has nothing to do with how a WinForms app or a background processor would work). Application logic should live in your application. Domain logic should live in your BLL and lower, and be reusable across the different applications that may use your common "business logic".

Kind of a general answer, but I hope that helps.