Get on the D-BUS
D-BUS is an interprocess communication (IPC) system, providing a simple yet powerful mechanism allowing applications to talk to one another, communicate information and request services. D-BUS was designed from scratch to fulfill the needs of a modern Linux system. D-BUS' initial goal is to be a replacement for CORBA and DCOP, the remote object systems used in GNOME and KDE, respectively. Ideally, D-BUS can become a unified and agnostic IPC mechanism used by both desktops, satisfying their needs and ushering in new features.
D-BUS, as a full-featured IPC and object system, has several intended uses. First, D-BUS can perform basic application IPC, allowing one process to shuttle data to another—think UNIX domain sockets on steroids. Second, D-BUS can facilitate sending events, or signals, through the system, allowing different components in the system to communicate and ultimately to integrate better. For example, a Bluetooth dæmon can send an incoming call signal that your music player can intercept, muting the volume until the call ends. Finally, D-BUS implements a remote object system, letting one application request services and invoke methods from a different object—think CORBA without the complications.
D-BUS is unique from other IPC mechanisms in several ways. First, the basic unit of IPC in D-BUS is a message, not a byte stream. In this manner, D-BUS breaks up IPC into discrete messages, complete with headers (metadata) and a payload (the data). The message format is binary, typed, fully aligned and simple. It is an inherent part of the wire protocol. This approach contrasts with other IPC mechanisms where the lingua franca is a random stream of bytes, not a discrete message.
Second, D-BUS is bus-based. The simplest form of communication is process to process. D-BUS, however, provides a dæmon, known as the message bus dæmon, that routes messages between processes on a specific bus. In this fashion, a bus topology is formed, allowing processes to speak to one or more applications at the same time. Applications can send to or listen for various events on the bus.
A final unique feature is the creation of not one but two of these buses, the system bus and the session bus. The system bus is global, system-wide and runs at the system level. All users of the system can communicate over this bus with the proper permissions, allowing the concept of system-wide events. The session bus, however, is created during user login and runs at the user, or session, level. This bus is used solely by a particular user, in a particular login session, as an IPC and remote object system for the user's applications.
Messages are sent to objects. Objects are addressed using path names, such as /org/cups/printers/queue. Processes on the message bus are associated with objects and implemented interfaces on that object.
D-BUS supports multiple message types, such as signals, method calls, method returns and error messages. Signals are notification that a specific event has occurred. They are simple, asynchronous, one-way heads-up messages. Method call messages allow an application to request the invocation of a method on a remote object. Method return messages provide the return value resulting from a method invocation. Error messages provide exceptions in response to a method invocation.
D-BUS is fully typed and type-safe. Both a message's header and payload are fully typed. Valid types include byte, Boolean, 32-bit integer, 32-bit unsigned integer, 64-bit integer, 64-bit unsigned integer, double-precision floating point and string. A special array type allows for the grouping of types. A DICT type allows for dictionary-style key/value pairs.
D-BUS is secure. It implements a simple protocol based on SASL profiles for authenticating one-to-one connections. On a bus-wide level, the reading of and the writing to messages from a specific interface are controlled by a security system. An administrator can control access to any interface on the bus. The D-BUS dæmon was written from the ground up with security in mind.
These concepts make nice talk, but what is the benefit? First, the system-wide message bus is a new concept. A single bus shared by the entire system allows for propagation of events, from the kernel (see The Kernel Event Layer sidebar) to the uppermost applications on the system. Linux, with its well-defined interfaces and clear separation of layers, is not very integrated. D-BUS' system message bus improves integration without compromising fine engineering practices. Now, events such as disk full and printer queue empty or even battery power low can bubble up the system stack, available for whatever application cares, allowing the system to respond and react. The events are sent asynchronously, and without polling.
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