Explain the concept of a global session and its main purpose.

A global session is a mechanism that allows storing and managing the state of a user or process at the level of the entire application or distributed system.

The main purpose of a global session is to ensure the preservation of data that should be accessible across different components or services, as well as between different requests of the same user. This enables implementation of authentication, authorization, storage of user settings, and other data necessary for the correct operation of the application.

For example, in a web application, a global session can store the user ID and access rights so that re-authentication is not required with each request.

In Java, this can be implemented through HttpSession, distributed caches, or specialized session management systems.