|Developer(s)||Red Hat and the community|
|Stable release||1.10.0 / August 25, 2015|
|License||GNU GPL version 2 or later, or AFL 2.1|
D-Bus is an inter-process communication (IPC) and remote procedure call (RPC) mechanism that allows communication between multiple computer programs (that is, processes) concurrently running on the same machine. D-Bus was designed as part of the effort of the freedesktop.org project to standardize services provided by Linux desktop environments such as GNOME and KDE.
The freedesktop.org project also developed a free and open-source software library called libdbus, as a reference implementation of the specification. This library is often confused with the D-Bus itself. Other implementations of D-Bus also exist, such as GDBus (GNOME), QtDBus (Qt/KDE), dbus-java and sd-bus (part of systemd).
D-Bus is an IPC mechanism initially designed to replace the software component communications systems used by GNOME and KDE Linux desktop environments (CORBA and DCOP respectively). The components of these desktop environments are normally distributed in many processes, each one providing only a few —usually one— service. These services may be used by regular client applications or by other components of the desktop environment to perform their tasks.
Due to the large number of processes involved —adding up processes providing the services and clients accessing them— establishing one-to-one IPC communications between all of them is an inefficient and quite unreliable approach. Instead, D-Bus provides an abstraction called bus that gathers all the communications between a group of processes over a single shared virtual channel. Processes connected to a bus don't know how it is internally implemented, but D-Bus specification guarantees that every process connected to the bus is able to communicate with each other through it.
Linux desktop environments take advantage of the D-Bus facilities by instancing not one bus but many:
- a single system bus, available to all users and processes of the system, that provides access to system services (i.e. services provided by the operating system and also by any system daemons).
- a session bus for each user login session, that provides desktop services to user applications in the same desktop session, and allows the integration of the desktop session as a whole.
A process can connect to any number of buses, provided that it has granted access to them. In practice, this means that an user process can connect to the system bus and to its current session bus, but not to another users' session buses, or even to a different session bus owned by the same user. The latter restriction may change in the future if all user sessions are combined into a single one user bus.
D-Bus provides additional or simplifies existing functionality to the applications, including information sharing, modularity and privilege separation. For example, information on an incoming voice call received through Bluetooth or Skype can be propagated and interpreted by any currently running music player, which can react by muting the volume or pausing playback until the call is finished.
D-Bus can also be used as a framework to integrate different components of an user application. For instance, an office suite can communicate through the session bus to share data between the word processor and the spreadsheet.
- libdbus - a library that allows two applications to connect to each other and exchange messages
- dbus-daemon - a message-bus daemon executable, built on
libdbus, that multiple applications can connect to. The daemon can route messages from one application to zero or more applications, thereby implementing the publish/subscribe paradigm.
- wrapper libraries based on particular application frameworks
Messages received over a D-Bus connection get routed to a specific object, not to a process. It thus appears to clients as if they are interacting with an object whether or not there actually is an object on the other side.
D-Bus defines a name for each object which looks like (but is not actually) a POSIX filesystem path, e.g., /org/kde/kspread/sheets/3/cells/4/5. D-Bus objects' names are conventionally namespaced to help with independently developing code modules. Namespaces are generally prefixed with the developer's reserved domain name components (e.g. /org/kde).
Heavily influenced by the DCOP system used by versions 2 and 3 of KDE, D-Bus has replaced DCOP in the KDE 4 release. An implementation of D-Bus supports most POSIX operating systems, and a port for Windows exists. It is used by Qt 4 and GNOME. In GNOME it has gradually replaced most parts of the earlier Bonobo mechanism. It is also used by Xfce.
The usage of D-Bus is steadily expanding beyond the initial scope of desktop environments to cover an increasing amount of system services. For instance, NetworkManager network daemon, BlueZ bluetooth stack and Pulseaudio sound server use D-Bus to provide part or all of its services, and systemd is promoting traditional system daemons to D-Bus services, such as logind.
It is also used as the Wire protocol for the AllJoyn protocol for home automation, to this end AllJoyn adds discovery, session management, security, header compression, embedded device support and makes it transport agnostic.
- Although there are several implementations of D-Bus, the most widely used is the reference implementation libdbus, developed by the same freedesktop.org project that designed the specification. However, libdbus is a low-level implementation that was never meant to be used directly by application developers, but as a reference guide for other reimplementations of D-Bus (such as those included in standard libraries of desktop environments, or in programming language bindings). The freedesktop.org project itself recommends applications authors to "use one of the higher level bindings or implementations" instead.
- The predominance of libdbus as the most used D-Bus implementation caused the terms "D-Bus" and "libdbus" to be often used interchangeably, leading to confusion.
- GDBus is an implementation of D-Bus based on GIO streams included in GLib, aiming to be used by GTK+ and GNOME. GDBus is not a wrapper of libdbus, but a complete and independent reimplementation of the D-Bus specification and protocol.
- QtDBus is an implementation of D-Bus included in the Qt library since its version 4.2. This component is used by KDE applications, libraries and components to access the D-Bus services available in a system.
- In 2013, the systemd project rewrote libdbus in an effort to simplify the code, but it also resulted in a significant increase of the overall D-Bus performance. In preliminary benchmarks, BMW found that the systemd's D-Bus library increased performance by 360%. As of version 221 of systemd, the sd-bus API has been declared stable.
- There is an ongoing development project called kdbus that aims to reimplement D-Bus as a kernel-mediated peer-to-peer inter-process communication mechanism. Beside performance improvements, kdbus would have advantages arising from already existing Linux kernel features such as namespaces and auditing, security from the kernel mediating, closing race conditions, and allowing D-Bus to be used during boot and shutdown (as needed by systemd). kdbus inclusion in the Linux kernel has proven controversial, and As of September 2015[update] has not been merged yet.
- Language bindings
- Several programming language bindings for D-Bus has been developed, such as those for Java, C# and Ruby.
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we are working on moving things to a true user bus, of which there is only one per user on a system, regardless how many times that user happens to log in
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- Poettering, Lennart. "The new sd-bus API of systemd". Retrieved 5 October 2015.
Since systemd's inception it has been the IPC system it exposes its interfaces on.
- "What is D-Bus?". FreeDesktop.org. Retrieved 5 January 2015. "It should be noted that the low-level implementation is not primarily designed for application authors to use. Rather, it is a basis for binding authors and a reference for reimplementations. If you are able to do so it is recommended that you use one of the higher level bindings or implementations."
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- D-Bus home page at Freedesktop.org
- Introduction to D-Bus on the Freedesktop.org wiki
- It doesn't look like kdbus will make it for Linux kernel 4.1, Phoronix, April 26, 2015, by Michael Larabel