source: trunk/doc/src/frameworks-technologies/dbus-intro.qdoc@ 846

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/*!
    \page intro-to-dbus.html
    \title D-Bus
    \brief An introduction to Inter-Process Communication and Remote Procedure Calling with D-Bus.

    \keyword QtDBus
    \ingroup technology-apis

    \section1 Introduction

    D-Bus is an Inter-Process Communication (IPC) and Remote Procedure
    Calling (RPC) mechanism originally developed for Linux to replace
    existing and competing IPC solutions with one unified protocol. It
    has also been designed to allow communication between system-level
    processes (such as printer and hardware driver services) and
    normal user processes.

    It uses a fast, binary message-passing protocol, which is suitable
    for same-machine communication due to its low latency and low
    overhead. Its specification is currently defined by the
    \tt{freedesktop.org} project, and is available to all parties.

    Communication in general happens through a central server
    application, called the "bus" (hence the name), but direct
    application-to-application communication is also possible. When
    communicating on a bus, applications can query which other
    applications and services are available, as well as activate one
    on demand.

    \section1 The Buses

    D-Bus buses are used to when many-to-many communication is
    desired. In order to achieve that, a central server is launched
    before any applications can connect to the bus: this server is
    responsible for keeping track of the applications that are
    connected and for properly routing messages from their source to
    their destination.

    In addition, D-Bus defines two well-known buses, called the
    system bus and the session bus. These buses are special in the
    sense that they have well-defined semantics: some services are
    defined to be found in one or both of these buses.

    For example, an application wishing to query the list of hardware
    devices attached to the computer will probably communicate to a
    service available on the system bus, while the service providing
    opening of the user's web browser will be probably found on the
    session bus.

    On the system bus, one can also expect to find restrictions on
    what services each application is allowed to offer. Therefore, one
    can be reasonably certain that, if a certain service is present,
    it is being offered by a trusted application.

    \section1 Concepts

    \section2 Messages

    On the low level, applications communicate over D-Bus by sending
    messages to one another. Messages are used to relay the remote
    procedure calls as well as the replies and errors associated
    with them. When used over a bus, messages have a destination,
    which means they are routed only to the interested parties,
    avoiding congestion due to "swarming" or broadcasting.

    A special kind of message called a "signal message"
    (a concept based on Qt's \l {Signals and Slots} mechanism),
    however, does not have a pre-defined destination. Since its
    purpose is to be used in a one-to-many context, signal messages
    are designed to work over an "opt-in" mechanism.

    The QtDBus module fully encapsulates the low-level concept of
    messages into a simpler, object-oriented approach familiar to Qt
    developers. In most cases, the developer need not worry about
    sending or receiving messages.

    \section2 Service Names

    When communicating over a bus, applications obtain what is
    called a "service name": it is how that application chooses to be
    known by other applications on the same bus. The service names
    are brokered by the D-Bus bus daemon and are used to
    route messages from one application to another. An analogous
    concept to service names are IP addresses and hostnames: a
    computer normally has one IP address and may have one or more
    hostnames associated with it, according to the services that it
    provides to the network.

    On the other hand, if a bus is not used, service names are also
    not used. If we compare this to a computer network again, this
    would equate to a point-to-point network: since the peer is
    known, there is no need to use hostnames to find it or its IP
    address.

    The format of a D-Bus service name is in fact very similar to a
    host name: it is a dot-separated sequence of letters and
    digits. The common practice is even to name one's service name
    according to the domain name of the organization that defined
    that service.

    For example, the D-Bus service is defined by
    \tt{freedesktop.org} and can be found on the bus under the
    service name:

    \snippet doc/src/snippets/code/doc_src_introtodbus.qdoc 0

    \section2 Object Paths

    Like network hosts, applications provide specific services to
    other applications by exporting objects. Those objects are
    hierarchically organised, much like the parent-child
    relationship that classes derived from QObject possess. One
    difference, however, is that there is the concept of "root
    object", that all objects have as ultimate parent.

    If we continue our analogy with Web services, object paths
    equate to the path part of a URL:

    \img qurl-ftppath.png

    Like them, object paths in D-Bus are formed resembling path
    names on the filesystem: they are slash-separated labels, each
    consisting of letters, digits and the underscore character
    ("_"). They must always start with a slash and must not end with
    one.

    \section2 Interfaces

    Interfaces are similar to C++ abstract classes and Java's
    \c interface keyword and declare the "contract" that is
    established between caller and callee. That is, they establish
    the names of the methods, signals and properties that are
    available as well as the behavior that is expected from either
    side when communication is established.

    Qt uses a very similar mechanism in its \l {How to Create Qt
    Plugins}{Plugin system}: Base classes in C++ are associated
    with a unique identifier by way of the Q_DECLARE_INTERFACE()
    macro.

    D-Bus interface names are, in fact, named in a manner similar to
    what is suggested by the Qt Plugin System: an identifier usually
    constructed from the domain name of the entity that defined that
    interface.

    \section2 Cheat Sheet

    To facilitate remembering of the naming formats and their
    purposes, the following table can be used:

    \table 90%
    \header \o D-Bus Concept  \o Analogy            \o Name format
    \row    \o Service name   \o Network hostnames  \o Dot-separated
                                                       ("looks like a hostname")
    \row    \o Object path    \o URL path component \o Slash-separated
                                                       ("looks like a path")
    \row    \o Interface      \o Plugin identifier  \o Dot-separated
    \endtable

    \section1 Debugging

    When developing applications that use D-Bus, it is sometimes useful to be able
    to see information about the messages that are sent and received across the
    bus by each application.

    This feature can be enabled on a per-application basis by setting the
    \c QDBUS_DEBUG environment variable before running each application.
    For example, we can enable debugging only for the car in the
    \l{D-Bus Remote Controlled Car Example} by running the controller and the
    car in the following way:

    \snippet doc/src/snippets/code/doc_src_introtodbus.qdoc QDBUS_DEBUG

    Information about the messages will be written to the console the application
    was launched from.

    \section1 Further Reading

    The following documents contain information about Qt's D-Bus integration
    features, and provide details about the mechanisms used to send and receive
    type information over the bus:

    \list
    \o \l{Using QtDBus Adaptors}
    \o \l{The QtDBus Type System}
    \o \l{QtDBus XML compiler (qdbusxml2cpp)}
    \endlist
*/