GDM3 (Display Manager의 한 종류)

00 개요

• NICE DCV를 설치할 때 GDM3 desktop manager 사용할 경우 Wayland protocol을 비활성화해야 한다고 함 (NICE DCV는 Wayland protocol을 지원하지 않기 때문)  (출처: https://docs.aws.amazon.com/dcv/latest/adminguide/setting-up-installing-linux-prereq.html)
•  GDM3와 Wayland가 무엇인지 알아보고자 함
• 간단히 설명하자면 GDM3 는 Display Manager 종류 중 하나이며 Wayland는 Display Server와 Client가 통신할 수 있게 만든 프로토콜, 즉 Desktop Environment (데스크탑 환경)을 구현하기 위한 프로토콜 (C 사용)
• 여기서는 GDM3만 다룸

01 Display Manager (DM)란

1. 정의

• a graphical user interface (GUI) front-end for login information entry, which then often directs to an actual full-fledged GUI → manages the graphical user interface (GUI) of operating systems (OS)
• a sofrware components that aid the management of user authentication (log-ins), graphical sessions, etc.
• act as a link between users and the display server in question, managing both the login process and the graphical session
• play the role of creating both visual appeal and a user-friendly interface
• Various types of display managers serve different Desktop Environments

2. 작동 원리

• Initialization and startup is the first function where display managers launch during the boot process to start the GUI environment.
• They also start together with the operating system and other necessary components.
• Display Managers work to initiate the X Window System or the Wayland compositor, based on the display server that’s used.
• As soon as the display server starts running, display managers offer the user the login screen.
• The login screen lets the user access the computer by entering their credentials, which are usually the username and password to access the system.
• This display manager authenticates the credentials put in against the database of the system user, hence upping the security level.
• On successful authentication, the display managers load the desktop environment or a window manager. 
• The manager sets up the best variables in the environment, starts the graphical session, and ensures that the needed resources for the session are allocated. 
• In the display customization and configuration role, display managers offer the option to tailor the different options in the GUI. 
• We can also customize their graphical environment based on our preferences. 
• The hardware abilities can also be customized through the configuration interface. 
• Display Managers also offer login screen customization options where we can pick the background, themes, and wallpapers for personal aesthetics.
• To better the usability and inclusivity, display managers include more accessibility features.
• The features help the users, especially those with visual and other types of disabilities.
• Some of the most common accessibility features include screen readers, magnifiers, high-contrast themes, and keyboard navigation.

3. Display 와 GUI Hierarchy

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• Display Manager (DM) is a utility that starts during system booting. It presents the user with a login screen and potentially a selector on what to display after a successful login. We usually employ a DM to automate the access to our system and to get a graphical user interface (GUI): either with a windows manager or a desktop environment
  • In a way, Display Managers are login managers, as they are daemons that often comprise several components: 
    • login screen (i.e., greeter) 
    • optional WM or DE selector 
    • optional session type selector 
    • remote options 
    • etc.
• Windows Manager (WM) is a piece of software only in charge of placing the windows of each program on the screen. It may also control what appearance these windows have. As they mostly work in conjunction with a desktop environment, we will focus on the setup of desktop environments themselves. However, the procedure for WM is similar, as a WM shares many features with desktop environments.
• Desktop Environment (DE) is a bunch of programs running in the same system to control its GUI. Some elements of a DE are windows (controlled with WM), toolbars, folders, widgets, etc. They provide an alternative interaction mechanism to the command-line interface.

4. Display Manager 종류

1) GDM (GNOME Display Manager)

• 'GNOME Display Manager'의 약자
• GNOME: 'GNU Network Object Model Environment'의 약자로, a free and open-source Desktop Environment for Unix-like operating systems
• GDM3는 GDM의 가장 최신 버전
• Specifically made for the GNOME Desktop Environment
• Visually appealing and offers a log-in experience filled with features.
• Some of the main features of GDM include:
  • graphical login screen: GDM has an appealing login screen for the user to key in their username and password to gain access to the Desktop Environment
  • remote logins
  • multiple user support: GDM is able to handle various user accounts, hence different users can access their accounts simultaneously, carrying out different sessions
  • session management: GDM allows users to have individual and secure environments for their activities
  • automatic session fallbacks
  • customization: with themes, background images, and log-in screen images, GDM can tailor the user’s desktop based on their preferences, as well as accessibility features like screen readers and high-contrast themes
• 예) Ubuntu에서 GDM
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2) LightDM

• a cross-desktop Display Manager, which aims to be both flexible and lightweight
• works well with Desktop Environments such as LXDE, Xfce, and Unity
• Especially works as the default manager for most Linux distros
• LightDM’s configuration files are mainly located in the /etc/lightdm directory
  • The files let the administrator change different aspects of the Display Manager’s behavior.
  • These include the default session settings or timeout value.
• Significant advantage: provides a modular architecture that allows for easy customization and theming
• allows for various multiple graphical toolkits such as Qt and web-based interfaces
• has additional features such as remote login and multi-seater support
• It’s necessary to note that individual distributions can tailor LightDM’s appearance to meet their specific needs
• 예) Ubuntu에서 LightDM
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참조

1. Display Manager란 https://www.baeldung.com/linux/display-managers-explained
2. Display Manager란 https://www.baeldung.com/linux/display-managers-install-uninstall
3. Display Manager란 https://en.wikipedia.org/wiki/Windowing_system
4. Display와 GUI Hierarchy https://www.baeldung.com/linux/default-desktop-environment-start-up#display-manager-windows-manager-and-desktop-environment
5.