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2003-10-02 Havoc Pennington <hp@redhat.com>

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* doc/dbus-tutorial.xml: write some stuff
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@ -1,3 +1,7 @@
2003-10-02 Havoc Pennington <hp@redhat.com>
* doc/dbus-tutorial.xml: write some stuff
2003-09-29 Havoc Pennington <hp@pobox.com>
* configure.in: split checks for Doxygen from XML docs, check for

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doc/dbus-tutorial.xml
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<articleinfo>
<title>D-BUS Tutorial</title>
<releaseinfo>Version 0.1</releaseinfo>
<date>29 September 2003</date>
<date>02 October 2003</date>
<authorgroup>
<author>
<firstname>Havoc</firstname>
@ -23,28 +23,427 @@
</authorgroup>
</articleinfo>
<sect1 id="introduction">
<title>Introduction</title>
<sect1 id="whatis">
<title>What is D-BUS?</title>
<para>
D-BUS blah blah blah
D-BUS is a system for <firstterm>interprocess communication</firstterm>
(IPC). Architecturally, it has several layers:
<itemizedlist>
<listitem>
<para>
foo
A library, libdbus, that allows two applications to connect
to each other and exchange messages.
</para>
</listitem>
<listitem>
<para>
bar
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 other applications.
</para>
</listitem>
<listitem>
<para>
Wrapper libraries based on particular application frameworks.
For example, libdbus-glib and libdbus-qt. There are also
bindings to languages such as Python. These wrapper libraries
are the API most people should use, as they simplify the
details of D-BUS programming.
</para>
</listitem>
</itemizedlist>
</para>
<para>
blah blah blah
libdbus only supports one-to-one connections, just like a raw network
socket. However, rather than sending byte streams over the connection, you
send <firstterm>messages</firstterm>. Messages have a header identifying
the kind of message, and a body containing a data payload. libdbus also
abstracts the exact transport used (sockets vs. whatever else), and
handles details such as authentication.
</para>
<para>
blah blah blah
The message bus daemon has multiple instances on a typical computer. The
first instance is a machine-global singleton, that is, a system daemon
similar to sendmail or Apache. This instance has heavy security
restrictions on what messages it will accept, and is used for systemwide
communication. The other instances are created one per user login session.
These instances allow applications in the user's session to communicate
with one another.
</para>
<para>
The systemwide and per-user daemons are separate. Normal within-session
IPC does not involve the systemwide message bus process and vice versa.
</para>
<sect2 id="uses">
<title>D-BUS applications</title>
<para>
There are many, many technologies in the world that have "Inter-process
communication" or "networking" in their stated purpose: <ulink
url="http://www.mbus.org/">MBUS</ulink>, <ulink
url="http://www.omg.org">CORBA</ulink>, <ulink
url="http://www.xmlrpc.com">XML-RPC</ulink>, <ulink
url="http://www.w3.org/TR/SOAP/">SOAP</ulink>, and probably hundreds
more. Each of these is tailored for particular kinds of application.
D-BUS is designed for two specific cases:
<itemizedlist>
<listitem>
<para>
Communication between desktop applications in the same desktop
session; to allow integration of the desktop session as a whole,
and address issues of process lifecycle (when do desktop components
start and stop running).
</para>
</listitem>
<listitem>
<para>
Communication between the desktop session and the operating system,
where the operating system would typically include the kernel
and any system daemons or processes.
</para>
</listitem>
</itemizedlist>
</para>
<para>
For the within-desktop-session use case, the GNOME and KDE desktops
have significant previous experience with different IPC solutions
such as CORBA and DCOP. D-BUS is built on that experience and
carefully tailored to meet the needs of these desktop projects
in particular.
</para>
<para>
The problem solved by the systemwide or communication-with-the-OS case
is explained well by the following text from the Linux Hotplug project:
<blockquote>
<para>
A gap in current Linux support is that policies with any sort of
dynamic "interact with user" component aren't currently
supported. For example, that's often needed the first time a network
adapter or printer is connected, and to determine appropriate places
to mount disk drives. It would seem that such actions could be
supported for any case where a responsible human can be identified:
single user workstations, or any system which is remotely
administered.
</para>
<para>
This is a classic "remote sysadmin" problem, where in this case
hotplugging needs to deliver an event from one security domain
(operating system kernel, in this case) to another (desktop for
logged-in user, or remote sysadmin). Any effective response must go
the other way: the remote domain taking some action that lets the
kernel expose the desired device capabilities. (The action can often
be taken asynchronously, for example letting new hardware be idle
until a meeting finishes.) At this writing, Linux doesn't have
widely adopted solutions to such problems. However, the new D-Bus
work may begin to solve that problem.
</para>
</blockquote>
</para>
<para>
D-BUS may happen to be useful for purposes other than the one it was
designed for. Its general properties that distinguish it from
other forms of IPC are:
<itemizedlist>
<listitem>
<para>
Binary protocol designed to be used asynchronously
(similar in spirit to the X Window System protocol).
</para>
</listitem>
<listitem>
<para>
Stateful, reliable connections held open over time.
</para>
</listitem>
<listitem>
<para>
The message bus is a daemon, not a "swarm" or
distributed architecture.
</para>
</listitem>
<listitem>
<para>
Many implementation and deployment issues are specified rather
than left ambiguous.
</para>
</listitem>
<listitem>
<para>
Semantics are similar to the existing DCOP system, allowing
KDE to adopt it more easily.
</para>
</listitem>
<listitem>
<para>
Security features to support the systemwide mode of the
message bus.
</para>
</listitem>
</itemizedlist>
</para>
</sect2>
</sect1>
<sect1 id="concepts">
<title>Concepts</title>
<para>
Some basic concepts apply no matter what application framework you're
using to write a D-BUS application. The exact code you write will be
different for GLib vs. Qt vs. Python applications, however.
</para>
<sect2 id="objects">
<title>Objects and Object Paths</title>
<para>
Each application using D-BUS contains <firstterm>objects</firstterm>,
which generally map to GObject, QObject, C++ objects, or Python objects
(but need not). An object is an <emphasis>instance</emphasis> rather
than a type. When messages are received over a D-BUS connection, they
are sent to a specific object, not to the application as a whole.
</para>
<para>
To allow messages to specify their destination object, there has to be a
way to refer to an object. In your favorite programming language, this
is normally called a <firstterm>pointer</firstterm> or
<firstterm>reference</firstterm>. However, these references are
implemented as memory addresses relative to the address space of your
application, and thus can't be passed from one application to another.
</para>
<para>
To solve this, D-BUS introduces a name for each object. The name
looks like a filesystem path, for example an object could be
named <literal>/org/kde/kspread/sheets/3/cells/4/5</literal>.
Human-readable paths are nice, but you are free to create an
object named <literal>/com/mycompany/c5yo817y0c1y1c5b</literal>
if it makes sense for your application.
</para>
<para>
Namespacing object paths is smart, by starting them with the components
of a domain name you own (e.g. <literal>/org/kde</literal>). This
keeps different code modules in the same process from stepping
on one another's toes.
</para>
</sect2>
<sect2 id="interfaces">
<title>Interfaces</title>
<para>
Each object supports one or more <firstterm>interfaces</firstterm>.
Think of an interface as a named group of methods and signals,
just as it is in GLib or Qt. Interfaces define the
<emphasis>type</emphasis> of an object instance.
</para>
</sect2>
<sect2 id="messages">
<title>Message Types</title>
<para>
Messages are not all the same; in particular, D-BUS has
4 built-in message types:
<itemizedlist>
<listitem>
<para>
Method call messages ask to invoke a method
on an object.
</para>
</listitem>
<listitem>
<para>
Method return messages return the results
of invoking a method.
</para>
</listitem>
<listitem>
<para>
Error messages return an exception caused by
invoking a method.
</para>
</listitem>
<listitem>
<para>
Signal messages are notifications that a given signal
has been emitted (that an event has occurred).
You could also think of these as "event" messages.
</para>
</listitem>
</itemizedlist>
</para>
<para>
A method call maps very simply to messages, then: you send a method call
message, and receive either a method return message or an error message
in reply.
</para>
</sect2>
<sect2 id="services">
<title>Services</title>
<para>
Object paths, interfaces, and messages exist on the level of
libdbus and the D-BUS protocol; they are used even in the
1-to-1 case with no message bus involved.
</para>
<para>
Services, on the other hand, are a property of the message bus daemon.
A <firstterm>service</firstterm> is simply a name mapped to
some application connected to the message bus daemon.
These names are used to specify the origin and destination
of messages passing through the message bus. When a name is mapped
to a particular application, the application is said to
<firstterm>own</firstterm> that service.
</para>
<para>
On connecting to the bus daemon, each application immediately owns a
special name called the <firstterm>base service</firstterm>. A base
service begins with a ':' (colon) character; no other services are
allowed to begin with that character. Base services are special because
each one is unique. They are created dynamically, and are never re-used
during the lifetime of the same bus daemon. You know that a given
base service name will have the same owner at all times.
</para>
<para>
Applications may ask to own additional <firstterm>well-known
services</firstterm>. For example, you could write a specification to
define a service called <literal>com.mycompany.TextEditor</literal>.
Your definition could specify that to own this service, an application
should have an object at the path
<literal>/com/mycompany/TextFileManager</literal> supporting the
interface <literal>org.freedesktop.FileHandler</literal>.
</para>
<para>
Applications could then send messages to this service,
object, and interface to execute method calls.
</para>
<para>
Services have another important use, other than routing messages. They
are used to track lifecycle. When an application exits (or crashes), its
connection to the message bus will be closed by the operating system
kernel. The message bus then sends out notification messages telling
remaining applications that the application's services have lost their
owner. By tracking these notifications, your application can reliably
monitor the lifetime of other applications.
</para>
</sect2>
<sect2 id="addresses">
<title>Addresses</title>
<para>
Applications using D-BUS are either servers or clients. A server
listens for incoming connections; a client connects to a server. Once
the connection is established, it is a symmetric flow of messages; the
client-server distinction only matters when setting up the
connection.
</para>
<para>
A D-BUS <firstterm>address</firstterm> specifies where a server will
listen, and where a client will connect. For example, the address
<literal>unix:path=/tmp/abcdef</literal> specifies that the server will
listen on a UNIX domain socket at the path
<literal>/tmp/abcdef</literal> and the client will connect to that
socket. An address can also specify TCP/IP sockets, or any other
transport defined in future iterations of the D-BUS specification.
</para>
<para>
When using D-BUS with a message bus, the bus daemon is a server
and all other applications are clients of the bus daemon.
libdbus automatically discovers the address of the per-session bus
daemon by reading an environment variable. It discovers the
systemwide bus daemon by checking a well-known UNIX domain socket path
(though you can override this address with an environment variable).
</para>
<para>
If you're using D-BUS without a bus daemon, it's up to you to
define which application will be the server and which will be
the client, and specify a mechanism for them to agree on
the server's address.
</para>
</sect2>
<sect2 id="bigpicture">
<title>Big Conceptual Picture</title>
<para>
Pulling all these concepts together, to specify a particular
method call on a particular object instance, a number of
nested components have to be named:
<programlisting>
Address -> [Service] -> Path -> Interface -> Method
</programlisting>
The service is in brackets to indicate that it's optional -- you only
provide a service name to route the method call to the right application
when using the bus daemon. If you have a direct connection to another
application, services aren't used.
</para>
<para>
The interface is also optional, primarily for historical
reasons; DCOP does not require specifying the interface,
instead simply forbidding duplicate method names
on the same object instance. D-BUS will thus let you
omit the interface, but if your method name is ambiguous
it is undefined which method will be invoked.
</para>
</sect2>
</sect1>
<sect1 id="glib-client">
<title>GLib API: Using Remote Objects</title>
<para>
</para>
</sect1>
<sect1 id="glib-server">
<title>GLib API: Implementing Objects</title>
<para>
</para>
</sect1>
<sect1 id="qt-client">
<title>Qt API: Using Remote Objects</title>
<para>
</para>
</sect1>
<sect1 id="qt-server">
<title>Qt API: Implementing Objects</title>
<para>
</para>
</sect1>
<sect1 id="python-client">
<title>Python API: Using Remote Objects</title>
<para>
</para>
</sect1>
<sect1 id="python-server">
<title>Python API: Implementing Objects</title>
<para>
</para>
</sect1>
</article>