Session Management

Definitions

A session is a group of applications running, each of which has a particular state. The session is controlled by a service called the session manager. The applications participating in the session are called session clients.

The session manager issues commands to its clients on behalf of the user. These commands may cause clients to commit unsaved changes (for example by saving open files), to preserve their state for future sessions or to terminate gracefully. The set of these operations is called session management.

In the common case, a session consists of all applications that a user runs on his desktop at a time. Under Unix/X11, however, a session may as well run on different computers, span multiple displays or just consists of a few applications.

Shutting a session down

A session is shut down by the session manager, usually on behalf of the user when she wants to log out. A system might, however, also perform an automatic shutdown in an emergency case, when for example the power is about to be lost. Clearly there is one big difference between both shutdowns. During the first, the user may want to interact with the application, telling exactly which files should be saved and which should be discarded. In the latter case, there's no time for interaction. There may not even be a user sitting in front of the machine!

Protocols and support on different platforms

On MS-Windows, there is nothing like complete session management for applications yet, i.e. no restoring of previous sessions. Windows does, however, support graceful logouts where applications have the chance to cancel the process after getting confirmation from the user. This is the functionality that corresponds to the QApplication::commitData() method.

X11 on the other hand supports complete session management since X11R6. Within the life-time of Qt-2.0, virtually all popular Unix/X11 desktops will very likely support the XConsortium standard.

Getting session management to work with Qt

In any case you should reimplement QApplication::commitData() to enable your application to take part in the graceful logout process. If you target the MS-Windows platform only, this is all you can and have to provide. Ideally, your application should provide a shutdown dialog similar to the following one:

Example code to this dialog can be found in the documentation of QSessionManager::allowsInteraction().

For complete session management yet only supported on X11R6, you also have to take care of saving the state of the application and potentially restore the state in the next life cycle of the session. This saving is done by reimplementing QApplication::saveState(). All state data you are saving in this function, shall be marked with the session identifier QApplication::sessionId(). This application specific identifier is globally unique, so no clashes will happen.

Restauration is usually done in the application's main() function. Check if QApplication::isSessionRestored() is TRUE. If that's the case, use the session identifier QApplication::sessionId() again to access your state date and restore the state of the application.

Important: In order to allow the window manager to restore window attributes such as stacking order or geometry information, you have to identify your toplevel widgets with an application-wide unique object name (see QObject::setName() ). When restoring the application, you'll have to ensure that all restored toplevel widgets end up with their prior object name again.

Testing and debugging session management

As mentioned earlier, session management support on Windows is fairly limited due to the lack of this functionality in the Windows operating system itself. Simply shut the session down and verify that your application behaves as wanted. It may be a good idea to launch another application, usually the integrated development environment, before starting your application. This other application will get the shutdown message afterwards, thus permitting you to cancel the shutdown. Otherwise you would have to log in again after each test run, which is not a problem per se but time consuming.

On Unix you can either use a desktop environment that supports standard X11R6 session management or, the recommended method, use the session manager reference implementation provided by the X Consortium. This sample manager is dubbed xsm and is part of a standard X11R6 installation. As always with X11, a useful and informative manual page is provided. Using xsm is straightforward (apart from the clumsy Athena-based user interface). Here's a simple approach:

• Run X11R6.
• Create a dot file .xsmstartup in your home directory which contains the single line

  xterm
  

  This tells xsm that the default/failsafe session is just an xterm and nothing else. Otherwise xsm would try to invoke lots of clients including the windowmanager twm, which isn't very helpful.

• Now launch xsm from another terminal window. Both a session manager window and the xterm will appear. The xterm has a nice property that sets it apart from all the other shells you are currently running: Within its shell, the SESSION_MANAGER environment variable points to the session manager you just started.
• Launch your application from the new xterm window. It will connect itself automatically to the session manager. You can check with the ClientList push button whether the connect was successful.
  Note: Never keep the ClientList open when you start or end session managed clients! Otherwise xsm is likely to crash.
• Use the session manager's Checkpoint and Shutdown buttons with different settings and see how your application behaves. The save type local means that the clients should save their state. It corresponds to the QApplication::saveState() function. The global save type asks application to save their unsaved changes in the permanent, globally accessible storage. It invokes QApplication::commitData(), respectively.
• Whenever something crashes, blame xsm and not Qt. xsm is far from being a usable session manager on a user's desktop. It is, however, stable and useful enough to serve as testing environment.