Which part of mvc design pattern represents the business logic?

Question

I am a little confused about the definition of the business logic in programming because I used to develop without paying an attention to any of these terminologies but now I want to be a good developer.

While I was reading in Wiki about the definition of the business logic I have read the following definition:

In computer software, business logic or domain logic is the part of the program that encodes the real-world business rules that determine how data can be created, displayed, stored, and changed.

and in another website I have read the following definition with example:

Business logic is that portion of an enterprise system which determines how data is: Transformed and/or calculated. For example, business logic determines how a tax total is calculated from invoice line items. Routed to people or software systems, aka workflow.

So I am wondering about which part of the MVC represents the business logic, is it the controller or the model or another part could be in the MVC?

I know that the controller is responsible for sending commands to the model but is it responsible for applying the rules of the business?

For example let's take the above example of the tax:

Suppose that we got the invoice data from a form in a view, the data will be directed to the controller but where the tax will be calculated, will we calculate it in the controller or will we ask for the help of an external class to compute it or will we compute it in the model before updating the database?

Could Example would be appreciated.

Answer 1

In Enterprise software development, must of time we use N-tier applications. They are 3 or more tiers.
1- Data tier:
2- Application tier (business logic, logic tier, or middle tier)
3- Presentation tier: it is a layer which users can access directly such as a web page, including simple control and user input validation or an operating systems GUI. , .

MVC is one of the seminal insights in the early development of graphical user interfaces, and one of the first approaches to describe and implement software constructs in terms of their responsibilities This said that MVC is use as a presentation layer. They are others like MVP, MVVM..
Sometime time in small application, an MVC structure is use to separate layer where the model is use as data layer, the controller as Logic layer.

Answer 2

You can put the tax calculation logic in the Controller, but you're better off putting it in the Model as it is more loosely coupled. It is reasonable to want to calculate tax on lots of pages, so don't put it in lots of controllers, put it in a model that can be re-used.

If you hear someone talking about "Fat" Controllers vs "Thin" Controllers, that is what they're talking about. Most devs will advocate having very little code in their controllers (making them "thin") and really just acting as a relay/orchestrator off to the Model.

I do think the term Model is a bit confusing though because in Service Oriented Architecture (and functional languages for that matter), they stress trying to have "dumb" objects that don't have any functionality and they frequently refer those dumb objects as "Models". But when it comes to MVC, the "Model" really refers to the Business Model, which incorporates dumb objects that hold values AND the services that work on top of them.