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In general, we can divide a computer application into three different levels of abstraction:

 – Presentation, also known as HMI, promotes interaction between the user and the application. This layer governs keyboard and mouse input, and presents information on the screen. It is strongly recommended that this aspect be ergonomic and pleasant.

 – Processing represents the tasks to be performed by the application. They are divided into two categories:- local processing, assembling the checks performed on the conversation with the HMI (e.g. checking and input assistance);- global processing, essentially forming the application. This is what is known as the business logic layer.

 – Data, more precisely access to data, which involves assembling the tools for managing data stored by the software.

Table 1.1. The three levels of abstraction of an application6

Presentation	Presentation management
Presentation logic	Core of the application
Processing	Local	Processing logic
Global	Processing management
Data	Data logic
Data management

NOTE.– These three levels can be nested or distributed in several possible ways between multiple computers.