Xfconf -- A New Configuration Storage System

October 6, 2008 at 11:25

I suppose others have written in broad terms about Xfconf, Xfce 4.6’s new configuration storage/retrieval system, but I guess an in-depth explanation is overdue and is probably up to me.

First, some background.

Xfce has used a configuration system called MCS (Multi-Channel Settings) for some time now, since 4.0 was released near the end of 2003. It’s inflexible and very monolithic. Only a single application (xfce-mcs-manager) is allowed to actually modify settings; all other applications can only query values. That means that the GUI settings dialogs have to run in the xfce-mcs-manager process. The GUI dialogs are implemented in shared libraries that are linked into the xfce-mcs-manager process at runtime. To add more confusion as to its purpose, xfce-mcs-manager also acts as an XSETTINGS manager, automatically proxying the settings stored in a “special” channel.

I had a few loosely-defined goals when designing Xfconf.

  • Keep the familiar “channels” concept from MCS, but make the namespaces for settings storage inside channels hierarchical.

  • The configuration storage daemon should be able to run without a GUI environment, and the client library shouldn’t have GUI dependencies.

  • Get the XSETTINGS manager out of the configuration daemon.

  • Integrate “kiosk mode” functionality directly into the configuration storage mechanism.

  • Native support for arrays/structs in the configuration store.

  • Provide a client library that’s easy to use and exposes commonly-used functionality.

  • No client application is special: any app can read or write any setting.

There were probably others, but those seem to be the ones that stick out in my mind after the fact.

MCS used a protocol involving a special X window, X selections, and X window properties to pass settings around at runtime. Obviously this kind of thing isn’t suitable for a configuration system that can run without a GUI, or even on a non-X11 system. So, that was right out. Given momentum over the past couple years, D-Bus seemed to make the most sense as an IPC method. So, the D-Bus API for Xfconf was born. (Please note that I don’t consider this frozen until we release 4.6.0 final. I’ll do my best not to remove or change existing interfaces, though.)

The rest of the story might sound rather gconf-like, and I guess it probably is. There’s a settings daemon (xfconfd) and a client library (libxfconf). Apps use the client library to fetch settings from and store settings to xfconfd. The daemon stores the settings on disk in a format of its choosing and manages the D-Bus interface. D-Bus gives us a few nice things for free; for example, xfconfd starts via D-Bus activation, so there’s no need to include it in any startup scripts or have client applications specifically start it. D-Bus also ensures that two instances of xfconfd aren’t running, fighting over the config store (well, unless you start a separate session bus daemon).

As for settings themselves, you have a channel that holds an arbitrary number of hierarchical properties. Channel names are simple ASCII text strings, like “xfwm4” or “xfce4-desktop”. Property names look a bit like path names and are, as I said, hierarchical. You can use this to logically organise properties inside your application. Xfdesktop is a good example of this, prefixing menu-related preferences with “/desktop-menu/” and backdrop-related prefs with “/backdrop/”. Xfwm4 is a terrible example of this, with all preferences prefixed with “/general/” for no apparent reason.

Anyhow, we support a variety of different data types: basically all of those supported and marshalable by dbus-glib, plus special 16-bit signed and unsigned types (though those two are pretty hacky at the moment and I’d like to be able to do those in a better way). The client library, libxfconf, lets you set commonly-used data types directly, and also has a generic interface for setting the others.

One thing I really like about libxfconf is its incredibly flexible support for array properties. Any property can be set to an array of values rather than a single value. The values in the array need not even be of the same type. Libxfconf has native support for setting array types, and also can directly map C structs to and from array types, automatically.

A final bit of usefulness is in libxfconf’s bindings mechanism. Libxfconf can “bind” an Xfconf property to a GObject property in your application. If the GObject property changes, then libxfconf will automatically update the value in the Xfconf store. If the value in the Xfconf store changes, libxfconf will automatically update the GObject property. This can greatly simplify the Xfconf code in your application. In your settings dialog, you simply have a single line of code to – for example – connect a GtkCheckButton’s “toggled” property to a boolean Xfconf property. If the user toggles the checkbox, Xfconf gets updated automatically. If the setting gets changed outside the application somehow while the settings dialog is open, the dialog gets updated automatically too. You can also use this functionality on the other “end” of the equation, too: if you use GObjects in your application, and your settings map to GObject properties on those objects, you can bind the properties there, too, and not have to manually take action when the user sets a setting in the settings dialog. Unfortunately, libxfconf only supports setting scalar properties now (not arrays), except for the special case of the GdkColor struct. Hopefully this will change in a future release.

We also include an “xfconf-query” application in the xfconf package, written mostly by Stephan Arts, which is a simple command-line Xfconf client. As you might guess, you can use it to query the values of Xfconf properties from the command-line, or from scripts. However, despite the “query” part of its name, it can modify Xfconf properties as well.

In Xfce 4.4 and below, we had the “XfceKiosk” system, which would allow you to lock down a desktop install so certain settings can and can’t be changed by particular users. It worked decently well, but was sometimes confusing to configure, and the application had to do special things to support it. Xfconf integrates a “locking” system whereby the system administrator can install a normally-formatted Xfconf configuration file with directives that instruct xfconfd to consider some properties unmodifiable by the user (and allows the sysadmin to set defaults too). This takes the burden away from applications to support a particular locking framework, as it’s natively built into the configuration system. (Unfortunately, as of this writing, the locking system isn’t working properly. It’ll definitely be finished by 4.6.0 final.)

So, I guess that’s it. If you’d like to get started with Xfconf, a good place to look would be the API documentation (please note that the API will not be frozen until 4.6.0 final). Otherwise, feel free to ask questions on the xfce4-dev mailing list.