<article id="index">
<articleinfo>
- <title>D-BUS FAQ</title>
- <releaseinfo>Version 0.1</releaseinfo>
- <date>22 January 2005</date>
+ <title>D-Bus FAQ</title>
+ <releaseinfo>Version 0.3</releaseinfo>
+ <date>17 November 2006</date>
<authorgroup>
<author>
<firstname>Havoc</firstname>
</author>
<author>
<firstname>David</firstname>
+ <othername role="mi">A</othername>
<surname>Wheeler</surname>
</author>
</authorgroup>
<qandaentry>
<question>
<para>
- What is D-BUS?
+ What is D-Bus?
</para>
</question>
<answer>
<para>
- This is probably best answered by reading the D-BUS <ulink url="dbus-tutorial.html">tutorial</ulink>. In
+ This is probably best answered by reading the D-Bus <ulink url="dbus-tutorial.html">tutorial</ulink> or
+ the introduction to the <ulink url="dbus-specification.html">specification</ulink>. In
short, it is a system consisting of 1) a wire protocol for exposing a
typical object-oriented language/framework to other applications; and
2) a bus daemon that allows applications to find and monitor one another.
+ Phrased differently, D-Bus is 1) an interprocess communication (IPC) system and 2) some higher-level
+ structure (lifecycle tracking, service activation, security policy) provided by two bus daemons,
+ one systemwide and one per-user-session.
</para>
</answer>
</qandaentry>
<qandaentry>
<question>
<para>
- Is D-BUS stable/finished?
+ Is D-Bus stable/finished?
</para>
</question>
<answer>
<para>
- D-BUS has not yet reached 1.0. The <ulink url="README">README</ulink>
- file has a discussion of the API/ABI stability guarantees before and
- after 1.0. In short, there are no guarantees before 1.0, and stability
- of both protocol and reference library will be maintained after 1.0.
- As of January 2005 we don't expect major protocol or API changes prior
- to the 1.0 release, but anything is possible.
+ The low-level library "libdbus" and the protocol specification are considered
+ ABI stable. The <ulink url="README">README</ulink>
+ file has a discussion of the API/ABI stability guarantees.
+ Higher-level bindings (such as those for Qt, GLib, Python, Java, C#) each
+ have their own release schedules and degree of maturity, not linked to
+ the low-level library and bus daemon release. Check the project page for
+ the binding you're considering to understand that project's policies.
</para>
</answer>
</qandaentry>
as well. The AFL is an "X-style" or "BSD-style" license compatible
with proprietary licensing, but it does have some requirements; in
particular it prohibits you from filing a lawsuit alleging that the
- D-BUS software infringes your patents <emphasis>while you continue to
- use D-BUS</emphasis>. If you're going to sue, you have to stop using
+ D-Bus software infringes your patents <emphasis>while you continue to
+ use D-Bus</emphasis>. If you're going to sue, you have to stop using
the software. Read the licenses to determine their meaning, this FAQ
entry is not intended to change the meaning or terms of the licenses.
</para>
</question>
<answer>
<para>
- If you imagine a C++ program that implements a network
- service, then the bus name is the domain name
- of the computer running this C++ program, the object path
- is a C++ object instance pointer, and an interface is a C++
- class (a pure virtual or abstract class, to be exact).
+ If you imagine a C++ program that implements a network service, then
+ the bus name is the hostname of the computer running this C++ program,
+ the object path is a C++ object instance pointer, and an interface is
+ a C++ class (a pure virtual or abstract class, to be exact).
</para>
<para>
In Java terms, the object path is an object reference,
</para>
<para>
However, a text editor application could as easily own multiple bus
- names (for example, <literal>org.kde.KWrite</literal>), have multiple
- objects (maybe <literal>/org/kde/documents/4352</literal>),
- and each object could implement multiple interfaces,
- such as <literal>org.freedesktop.Introspectable</literal>,
- <literal>org.freedesktop.BasicTextField</literal>,
+ names (for example, <literal>org.kde.KWrite</literal> in addition to
+ generic <literal>TextEditor</literal>), have multiple objects (maybe
+ <literal>/org/kde/documents/4352</literal> where the number changes
+ according to the document), and each object could implement multiple
+ interfaces, such as <literal>org.freedesktop.DBus.Introspectable</literal>,
+ <literal>org.freedesktop.BasicTextField</literal>,
<literal>org.kde.RichTextDocument</literal>.
</para>
</answer>
are normally launched according to the bus name they will
have.
</para>
+ <para>
+ People often misuse the word "service" for any
+ bus name, but this tends to be ambiguous and confusing so is discouraged.
+ In the D-Bus docs we try to use "service" only when talking about
+ programs the bus knows how to launch, i.e. a service always has a
+ .service file.
+ </para>
</answer>
</qandaentry>
<qandaentry id="components">
<question>
<para>
- Is D-BUS a "component system"?
+ Is D-Bus a "component system"?
</para>
</question>
<answer>
<para>
- D-BUS is not a component system. "Component system" was originally
+ It helps to keep these concepts separate in your mind:
+ <orderedlist>
+ <listitem>
+ <para>
+ Object/component system
+ </para>
+ </listitem>
+ <listitem>
+ <para>
+ GUI control/widget embedding interfaces
+ </para>
+ </listitem>
+ <listitem>
+ <para>
+ Interprocess communication system or wire protocol
+ </para>
+ </listitem>
+ </orderedlist>
+ </para>
+ <para>
+ D-Bus is not a component system. "Component system" was originally
defined by COM, and was essentially a workaround for the limitations
of the C++ object system (adding introspection, runtime location of
objects, ABI guarantees, and so forth). With the C# language and CLR,
in modern languages all objects are effectively "components."
</para>
<para>
- A third, orthogonal feature is interprocess communication or IPC.
- D-BUS is an IPC system. Given an object (or "component" if you must),
+ So components are fancy objects, and some objects are GUI controls.
+ </para>
+ <para>
+ A third, unrelated feature is interprocess communication or IPC.
+ D-Bus is an IPC system. Given an object (or "component" if you must),
you can expose the functionality of that object over an IPC system.
- Examples of IPC systems are DCOM, CORBA, SOAP, XML-RPC, and D-BUS.
+ Examples of IPC systems are DCOM, CORBA, SOAP, XML-RPC, and D-Bus.
You can use any of these IPC systems with any object/component system,
though some of them are "tuned" for specific object systems.
You can think of an IPC system primarily as a wire protocol.
then you can have an out-of-process or dynamically-loaded GUI control.
</para>
<para>
- Summarizing, there are three orthogonal things:
- <orderedlist>
- <listitem>
- <para>
- Object/component system
- </para>
- </listitem>
- <listitem>
- <para>
- Control embedding interfaces
- </para>
- </listitem>
- <listitem>
- <para>
- Interprocess communication system or wire protocol
- </para>
- </listitem>
- </orderedlist>
- </para>
- <para>
Another related concept is the <firstterm>plugin</firstterm> or
<firstterm>extension</firstterm>. Generic plugin systems such as the
<ulink url="http://eclipse.org">Eclipse</ulink> system are not so different
<qandaentry id="speed">
<question>
<para>
- How fast is the D-BUS reference implementation?
+ How fast is the D-Bus reference implementation?
</para>
</question>
<answer>
Of course it depends a bit on what you're doing.
<ulink url="http://lists.freedesktop.org/pipermail/dbus/2004-November/001779.html">
This mail</ulink> contains some benchmarking. At the time of that
- benchmark, D-BUS one-to-one communication was about 2.5x slower than
+ benchmark, D-Bus one-to-one communication was about 2.5x slower than
simply pushing the data raw over a socket. After the recent rewrite of
- the marshaling code, D-BUS is slower than that because a lot of
+ the marshaling code, D-Bus is slower than that because a lot of
optimization work was lost. But it can probably be sped up again.
</para>
<para>
- D-BUS communication with the intermediate bus daemon should be
+ D-Bus communication with the intermediate bus daemon should be
(and as last profiled, was) about twice as slow as one-to-one
mode, because a round trip involves four socket reads/writes rather
than two socket reads/writes.
not a good idea all things considered.
</para>
<para>
- Raw bandwidth isn't the only concern; D-BUS is designed to
+ Raw bandwidth isn't the only concern; D-Bus is designed to
enable asynchronous communication and avoid round trips.
This is frequently a more important performance issue
than throughput.
<qandaentry id="size">
<question>
<para>
- How large is the D-BUS reference implementation?
+ How large is the D-Bus reference implementation?
</para>
</question>
<answer>
<qandaentry id="other-ipc">
<question>
<para>
- How does D-BUS differ from other interprocess communication
+ How does D-Bus differ from other interprocess communication
or networking protocols?
</para>
</question>
<answer>
<para>
- The best place to start is to read the D-BUS <ulink url="dbus-tutorial.html">tutorial</ulink>, so
- you have a concrete idea what D-BUS actually is. If you
+ Keep in mind, it is not only an IPC system; it also includes
+ lifecycle tracking, service activation, security policy, and other
+ higher-level structure and assumptions.
+ </para>
+ <para>
+ The best place to start is to read the D-Bus <ulink url="dbus-tutorial.html">tutorial</ulink>, so
+ you have a concrete idea what D-Bus actually is. If you
understand other protocols on a wire format level, you
- may also want to read the D-B
US <ulink url="dbus-specification.html">specification</ulink> to see what
- D-BUS looks like on a low level.
+ may also want to read the D-B
us <ulink url="dbus-specification.html">specification</ulink> to see what
+ D-Bus looks like on a low level.
</para>
<para>
- As the <ulink url="dbus-tutorial.html">tutorial</ulink> and <ulink url="dbus-specification.html">specification</ulink> both explain, D-BUS is tuned
+ As the <ulink url="dbus-tutorial.html">tutorial</ulink> and <ulink url="dbus-specification.html">specification</ulink> both explain, D-Bus is tuned
for some specific use cases. Thus, it probably isn't tuned
for what you want to do, unless you are doing the things
- D-BUS was designed for. Don't make the mistake of thinking
- that any system labeled "IPC" is the same thing.
+ D-Bus was designed for. Don't make the mistake of thinking
+ that any system involving "IPC" is the same thing.
</para>
<para>
- The D-BUS authors would not recommend using D-BUS
+ The D-Bus authors would not recommend using D-Bus
for applications where it doesn't make sense.
- The following questions compare D-BUS to some other
- protocols primarily to help you understand D-BUS
- and decide whether it's appropriate; D-BUS is neither intended
+ The following questions compare D-Bus to some other
+ protocols primarily to help you understand D-Bus
+ and decide whether it's appropriate; D-Bus is neither intended
nor claimed to be the right choice for every application.
</para>
<para>
- It should be possible to bridge D-BUS to other IPC systems,
- just as D-BUS can be bridged to object systems.
+ It should be possible to bridge D-Bus to other IPC systems,
+ just as D-Bus can be bridged to object systems.
</para>
<para>
- Note: the D-BUS mailing list subscribers are <emphasis>very much not
+ Note: the D-Bus mailing list subscribers are <emphasis>very much not
interested</emphasis> in debating which IPC system is the One True
System. So if you want to discuss that, please use another forum.
</para>
<qandaentry id="corba">
<question>
<para>
- How does D-BUS differ from CORBA?
+ How does D-Bus differ from CORBA?
</para>
</question>
<answer>
SOAP are the latest fad.
</para>
<para>
- Like D-BUS, CORBA uses a fast binary protocol (IIOP). Both systems
+ Like D-Bus, CORBA uses a fast binary protocol (IIOP). Both systems
work in terms of objects and methods, and have concepts such as
- "oneway" calls. Only D-BUS has direct support for "signals" as in
+ "oneway" calls. Only D-Bus has direct support for "signals" as in
GLib/Qt (or Java listeners, or C# delegates).
</para>
<para>
- D-BUS hardcodes and specifies a lot of things that CORBA leaves open-ended,
- because CORBA is more generic and D-BUS has two specific use-cases in mind.
- This makes D-BUS a bit simpler.
+ D-Bus hardcodes and specifies a lot of things that CORBA leaves open-ended,
+ because CORBA is more generic and D-Bus has two specific use-cases in mind.
+ This makes D-Bus a bit simpler.
</para>
<para>
- However, unlike CORBA D-BUS does <emphasis>not</emphasis> specify the
+ However, unlike CORBA D-Bus does <emphasis>not</emphasis> specify the
API for the language bindings. Instead, "native" bindings adapted
specifically to the conventions of a framework such as QObject,
GObject, C#, Java, Python, etc. are encouraged. The libdbus reference
</para>
<para>
Many CORBA implementations such as ORBit are faster than the libdbus
- reference implementation. One reason is that D-BUS considers data
+ reference implementation. One reason is that D-Bus considers data
from the other end of the connection to be untrusted and extensively
validates it. But generally speaking other priorities were placed
- ahead of raw speed in the libdbus implementation. A fast D-BUS
+ ahead of raw speed in the libdbus implementation. A fast D-Bus
implementation along the lines of ORBit should be possible, of course.
</para>
<para>
- On a more trivial note, D-BUS involves substantially fewer acronyms
+ On a more trivial note, D-Bus involves substantially fewer acronyms
than CORBA.
</para>
</answer>
<qandaentry id="xmlrpcsoap">
<question>
<para>
- How does D-BUS differ from XML-RPC and SOAP?
+ How does D-Bus differ from XML-RPC and SOAP?
</para>
</question>
<answer>
constant fine-grained IPC among applications in a desktop session.
</para>
<para>
- D-BUS offers persistent connections and with the bus daemon
+ D-Bus offers persistent connections and with the bus daemon
supports lifecycle tracking of other applications connected
to the bus. With XML-RPC and SOAP, typically each method call
exists in isolation and has its own HTTP connection.
<qandaentry id="dce">
<question>
<para>
- How does D-BUS differ from DCE?
+ How does D-Bus differ from DCE?
</para>
</question>
<answer>
has released an implementation as open source software</ulink>. DCE
is quite capable, and includes a vast amount of functionality such as
a distributed time service. As the name implies, DCE is intended for
- use in a large, multi-computer distributed application. D-BUS would
+ use in a large, multi-computer distributed application. D-Bus would
not be well-suited for this.
</para>
</answer>
<qandaentry id="dcom">
<question>
<para>
- How does D-BUS differ from DCOM and COM?
+ How does D-Bus differ from DCOM and COM?
</para>
</question>
<answer>
Start by reading <xref linkend="other-ipc"/>.
</para>
<para>
- Comparing D-BUS to COM is apples and oranges;
+ Comparing D-Bus to COM is apples and oranges;
see <xref linkend="components"/>.
</para>
<para>
<qandaentry id="internet-communications-engine">
<question>
<para>
- How does D-BUS differ from ZeroC's Internet Communications Engine (Ice)
+ How does D-Bus differ from ZeroC's Internet Communications Engine (Ice)
</para>
</question>
<answer>
<para>
The <ulink url="http://www.zeroc.com/ice.html"> Internet
Communications Engine (Ice)</ulink> is a powerful IPC mechanism more
- on the level of SOAP or CORBA than D-BUS. Ice has a "dual-license"
+ on the level of SOAP or CORBA than D-Bus. Ice has a "dual-license"
business around it; i.e. you can use it under the GPL, or pay for a
proprietary license.
</para>
<qandaentry id="inter-client-exchange">
<question>
<para>
- How does D-BUS differ from Inter-Client Exchange (ICE)?
+ How does D-Bus differ from Inter-Client Exchange (ICE)?
</para>
</question>
<answer>
</para>
<para>
ICE is a binary protocol designed for desktop use, and KDE's DCOP
- builds on ICE. ICE is substantially simpler than D-BUS (in contrast
+ builds on ICE. ICE is substantially simpler than D-Bus (in contrast
to most of the other IPC systems mentioned here, which are more
complex). ICE doesn't really define a mapping to objects and methods
(DCOP adds that layer). The reference implementation of ICE (libICE)
</para>
<para>
DCOP and XSMP are the only two widely-used applications of ICE,
- and both could in principle be replaced by D-BUS. (Though whether
+ and both could in principle be replaced by D-Bus. (Though whether
GNOME and KDE will bother is an open question.)
</para>
</answer>
<qandaentry id="dcop">
<question>
<para>
- How does D-BUS differ from DCOP?
+ How does D-Bus differ from DCOP?
</para>
</question>
<answer>
Start by reading <xref linkend="other-ipc"/>.
</para>
<para>
- D-B
US is intentionally pretty similar to <ulink
+ D-B
us is intentionally pretty similar to <ulink
url="http://developer.kde.org/documentation/library/kdeqt/dcop.html">DCOP</ulink>,
and can be thought of as a "DCOP the next generation" suitable for
sharing between the various open source desktop projects.
</para>
<para>
- D-BUS is a bit more complex than DCOP, though the Qt binding for D-BUS
+ D-Bus is a bit more complex than DCOP, though the Qt binding for D-Bus
should not be more complex for programmers. The additional complexity
- of D-BUS arises from its separation of object references vs. bus names
+ of D-Bus arises from its separation of object references vs. bus names
vs. interfaces as distinct concepts, and its support for one-to-one
connections in addition to connections over the bus. The libdbus
reference implementation has a lot of API to support multiple bindings
in order to support secure applications such as the systemwide bus.
</para>
<para>
- D-BUS is probably somewhat slower than DCOP due to data validation
+ D-Bus is probably somewhat slower than DCOP due to data validation
and more "layers" in the reference implementation. A comparison
hasn't been posted to the list though.
</para>
<para>
- At this time, KDE has not committed to using D-BUS, but there have
- been discussions of KDE bridging D-BUS and DCOP, or even changing
- DCOP's implementation to use D-BUS internally (so that GNOME and KDE
+ At this time, KDE has not committed to using D-Bus, but there have
+ been discussions of KDE bridging D-Bus and DCOP, or even changing
+ DCOP's implementation to use D-Bus internally (so that GNOME and KDE
would end up using exactly the same bus). See the KDE mailing list
archives for some of these discussions.
</para>
<qandaentry id="yet-more-ipc">
<question>
<para>
- How does D-BUS differ from [yet more IPC mechanisms]?
+ How does D-Bus differ from [yet more IPC mechanisms]?
</para>
</question>
<answer>
</para>
<para>
If you're writing a desktop application for UNIX,
- then D-BUS is of course our recommendation for
+ then D-Bus is of course our recommendation for
talking to other parts of the desktop session.
- (With the caveat that you should use a stable release
- of D-BUS; until we reach 1.0, there isn't a stable release.)
+ </para>
+ <para>
+ D-Bus is also designed for communications between system daemons and
+ communications between the desktop and system daemons.
</para>
<para>
If you're doing something complicated such as clustering,
distributed swarms, peer-to-peer, or whatever then
the authors of this FAQ don't have expertise in these
areas and you should ask someone else or try a search engine.
+ D-Bus is most likely a poor choice but could be appropriate
+ for some things.
</para>
<para>
- Note: the D-BUS mailing list is probably not the place to
+ Note: the D-Bus mailing list is probably not the place to
discuss which system is appropriate for your application,
though you are welcome to ask specific questions about
- D-BUS <emphasis>after reading this FAQ, the tutorial, and
+ D-Bus <emphasis>after reading this FAQ, the tutorial, and
searching the list archives</emphasis>. The best way
to search the list archives is probably to use
an Internet engine such as Google. On Google,
</question>
<answer>
<para>
- The D-B
US <ulink url="http://dbus.freedesktop.org">web site</ulink>
+ The D-B
us <ulink url="http://dbus.freedesktop.org">web site</ulink>
has a link to the bug tracker, which is the best place to store
patches. You can also post them to the list, especially if you want
to discuss the patch or get feedback.
projects / platform / upstream / dbus.git / blobdiff