diff options
author | Frederik Gladhorn <frederik.gladhorn@digia.com> | 2012-11-21 15:39:03 +0100 |
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committer | The Qt Project <gerrit-noreply@qt-project.org> | 2012-11-21 16:36:50 +0100 |
commit | 045b15191150103bd8290ced4d6f9b53a382adc4 (patch) | |
tree | 094761f0748e395b80176896ca43b1f56f3652fb /src/dbus/doc/src/qtdbus-index.qdoc | |
parent | 79fcb97be516203dd47a4ea5be1b71afd78bf3f3 (diff) |
Rename DBus landing page according to rules.
Change-Id: Idf26f89f41940c4883cbbe99b3cdd868d0d36b06
Reviewed-by: Jerome Pasion <jerome.pasion@digia.com>
Diffstat (limited to 'src/dbus/doc/src/qtdbus-index.qdoc')
-rw-r--r-- | src/dbus/doc/src/qtdbus-index.qdoc | 215 |
1 files changed, 215 insertions, 0 deletions
diff --git a/src/dbus/doc/src/qtdbus-index.qdoc b/src/dbus/doc/src/qtdbus-index.qdoc new file mode 100644 index 0000000000..ab9635da6d --- /dev/null +++ b/src/dbus/doc/src/qtdbus-index.qdoc @@ -0,0 +1,215 @@ +/**************************************************************************** +** +** Copyright (C) 2012 Digia Plc and/or its subsidiary(-ies). +** Contact: http://www.qt-project.org/legal +** +** This file is part of the documentation of the Qt Toolkit. +** +** $QT_BEGIN_LICENSE:FDL$ +** Commercial License Usage +** Licensees holding valid commercial Qt licenses may use this file in +** accordance with the commercial license agreement provided with the +** Software or, alternatively, in accordance with the terms contained in +** a written agreement between you and Digia. For licensing terms and +** conditions see http://qt.digia.com/licensing. For further information +** use the contact form at http://qt.digia.com/contact-us. +** +** GNU Free Documentation License Usage +** Alternatively, this file may be used under the terms of the GNU Free +** Documentation License version 1.3 as published by the Free Software +** Foundation and appearing in the file included in the packaging of +** this file. Please review the following information to ensure +** the GNU Free Documentation License version 1.3 requirements +** will be met: http://www.gnu.org/copyleft/fdl.html. +** $QT_END_LICENSE$ +** +****************************************************************************/ + +/*! + \page qtdbus-index.html + \title Qt 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 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: + + \image 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 \li D-Bus Concept \li Analogy \li Name format + \row \li Service name \li Network hostnames \li Dot-separated + ("looks like a hostname") + \row \li Object path \li URL path component \li Slash-separated + ("looks like a path") + \row \li Interface \li Plugin identifier \li 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 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 + \li \l{Using QtDBus Adaptors} + \li \l{The QtDBus Type System} + \li \l{QtDBus XML compiler (qdbusxml2cpp)} + \endlist +*/ |