X Session Manager - Overview

Overview

From the point of view of an X session manager, a session is a “state of the desktop” at a given time: a set of windows with their current content. More precisely, a session is the set of clients managing these windows or related to them and the information that allows these applications to restore the condition of these windows if required.

The most recognizable effect of using a session manager is the possibility of logging out from an interactive session and then finding exactly the same windows in the same state when logging in again. For this to work, the session manager program stores the names of applications that are running at logout and starts them again at login. Moreover, for the state of the applications to be restored as well (which is needed to restore the content of windows), the applications must be able to save their state of execution upon request from the session manager and load it back when started again.

In general, a session can be saved or loaded at any time, even if the user is not logging in or out. It is also possible to save a number of different sessions and loading one of them at user's choice. Sessions can also be specified by providing the list of applications that compose the session. As a result, the user has the possibility of saving a set of different sessions, either by storing the state of execution of the currently running applications or by explicitly listing the applications that compose a session. This way, the user can later decide to load a given session.

In order for a session to include the state of an application, the application must be able to store and load its current state when appropriate. A protocol named X Session Management Protocol (XSMP) specifies how applications and session managers interact. Of particular importance is that the window manager is able to communicate with the session manager, as the window manager is responsible for the placement of windows and the existence of icons. Applications that cannot store their state can be included in a session, but they do not preserve their state across sessions.

The X Window System includes a default session manager called xsm. Other session managers have been developed for specific desktop systems: for example, ksmserver is the default session manager of KDE.