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Elektra 0.11.0
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This plugin is a notification plugin, which sends a signal to D-Bus when the key database (KDB) has been modified.
See installation. The package is called libelektra5-dbus
.
libdbus-1-dev
A preferred way to interconnect desktop applications and even embedded system applications on mobile devices running Linux is D-Bus. The idea of D-Bus accords to that of Elektra: to provide standards to let software work together more tightly. D-Bus provides a simple and lightweight IPC (Inter-Process Communication) system to be used within desktop systems. Next to RPC (Remote Procedure Call), which is not used in this plugin, it supports signals which can notify an arbitrary number of other applications about changes. Given software like a D-Bus library, notification itself is a rather easy task, but it involves additional library dependences. So it is the perfect task to be implemented as a plugin. The information about the channels to be used can be stored in the global key database.
D-Bus supports a system-wide bus and a session bus. The system configuration can be accessed by each user and the user configuration is limited to a single user. Both buses can immediately be used for the system and user configuration notification updates to get pleasing results. But, there is a problem with the session bus: It is possible within D-Bus that a user starts several sessions. The user configuration should be global to the user and is not aware of these sessions. So if several sessions are started, some of the user's processes will miss notification updates.
The namespaces are mapped to the buses the following way:
Following signal names are used to notify about changes in the Elektra’s KeySet:
Alternatively, (with the option announce=once) only a single message is send:
The recommended way is to globally mount the plugin:
Alternatively one can mount the plugin additionally to a storage plugin, e.g.:
For openicc one would use (mounts with announce=once):
Then we can receive the notification events using:
Or via the supplied test program:
We can trigger a message with:
Note that changes in user
fire on the dbus session
, and changes in namespace system
in the dbus system
bus. To receive system
changes we will use:
And then fire it with:
See the full example here.
Here a small example for QDBusConnection:
Place this in your Qt class header:
Put this in your Qt class, e.g. the constructor:
Here comes the org.libelektra signals:
In Python the DBus notifications can be used as follows
Today, programs are often interconnected in a dense way. Such applications should always be informed when something in their environment changes. For user interactive software, notification about configuration changes is expected. The only alternative is polling, which wastes resources. It additionally is no option for interactive software, where the latency needs to be low. Instead, the software which changes the configuration has to notify all other interested applications that can reread their configuration without significant delay. In Elektra, a notification plugin ensures that a notification is actually sent on each change.
Applications can wait for such a notification with hand-written code. Bindings, however, allow for better integration. It is a common approach for toolkits to provide a main loop. Applications using such toolkits can integrate notification services into this main loop.
The actions that occur in such events are application or toolkit specific because of the non-invasive nature of Elektra. Software reacts in many different ways to update events. Hence, the frequency of update events should be kept at a minimum. Changes are kept atomic with a single attempt to write out configuration. Notification callbacks shall not change configuration because this can lead to a longer chain of unwanted modifications. That might not be true, however, if a programmer of the whole system knows that a chain of reactions will terminate. When doing such event-driven programming, care is needed to avoid infinite loops. Elektra guarantees consistency of the key database even in such cases.
Mount this plugin globally with default settings to use it as sending transport plugin for Elektra's notification feature:
kdb global-mount dbus announce=once
This plugin uses D-Bus signal messages as notifications. Notifications have the path /org/libelektra/configuration
and the D-Bus interface org.libelektra
. The following signal names are used:
The first argument contains the name of the changed key. The system bus is used if the affected keys is below the system
namespace. If the key is below the user
namespace the session bus is used.
Example output from dbus-monitor
:
Key names that are not valid utf-8 cause a warning within the D-Bus library: