Version 0.1 22 January 2003 Havoc Pennington

hp@pobox.com

D-BUS is a system for low-latency, low-overhead, easy to use interprocess communication (IPC). In more detail: D-BUS is low-latency because it is designed to avoid round trips and allow asynchronous operation, much like the X protocol. D-BUS is low-overhead because it is uses a binary protocol, and does not have to convert to and from a text format such as XML. Because D-BUS is intended for potentially high-resolution same-machine IPC, not primarily for Internet IPC, this is an interesting optimization. D-BUS is easy to use because it works in terms of messages rather than byte streams, and does not require users to understand any complex concepts such as a new type system or elaborate APIs. Libraries implementing D-BUS may choose to abstract messages as "method calls" (see ). The base D-BUS protocol is a peer-to-peer protocol, specified in . That is, it is a system for one application to talk to a single other application. However, the primary intended application of D-BUS is the D-BUS message bus, specified in . The message bus is a special application that accepts connections from multiple other applications, and forwards messages among them. A message consists of a header and a body. If you think of a message as a package, the header is the address, and the body contains the package contents. The message delivery system uses the header information to figure out where to send the message and how to interpret it; the recipient inteprets the body of the message. The body of the message is made up of zero or more arguments, which are typed values, such as an integer or a byte array. [document the required header fields and how they are encoded] In addition to the required header information mentioned in , the header may contain zero or more named header fields. These fields are named to allow future versions of this protocol specification to add new fields; implementations must ignore fields they do not understand. Implementations must not invent their own header fields; only changes to this specification may introduce new header fields. The message body is made up of arguments. Each argument is a type code, followed by the value of the argument in a type-dependent format. The types are: The types are encoded as follows: Before the flow of messages begins, two applications must authenticate. A simple text protocol is used for authentication; this protocol is a SASL profile, and maps fairly directly from the SASL specification. [move the dbus-sasl-profile.txt stuff into here and clean it up] [document the string format of an address, and how it maps to unix domain sockets, tcp, or whatever] [document how something that looks like a method call is conventionally represented in terms of messages, for method-call-style API bindings] org.freedesktop.Peer.Ping generates org.freedesktop.Peer.PingReply The message bus accepts connections from one or more applications. Once connected, applications can send and receive messages from the message bus, as in the peer-to-peer case. The message bus keeps track of a set of services. A service is simply a name, such as com.yoyodyne.Screensaver, which can be owned by one of the connected applications. The message bus itself always owns the special service org.freedesktop.DBus. Messages may have a srvc field (see ). When the message bus receives a message, if the srvc field is absent, the message is taken to be a standard peer-to-peer message and interpreted by the message bus itself. For example, sending an org.freedesktop.Peer.Ping message with no srvc will cause the message bus itself to reply to the ping immediately; the message bus would never make this message visible to other applications. If the srvc field is present, then it indicates a request for the message bus to route the message. In the usual case, messages are routed to the owner of the named service. Messages may also be broadcast by sending them to the special service org.freedesktop.Broadcast. Broadcast messages are sent to all applications with message matching rules that match the message. Continuing the org.freedesktop.Peer.Ping example, if the ping message were sent with a srvc name of com.yoyodyne.Screensaver, then the ping would be forwarded, and the Yoyodyne Corporation screensaver application would be expected to reply to the ping. If org.freedesktop.Peer.Ping were sent to org.freedesktop.Broadcast, then multiple applications might receive the ping, and all would normally reply to it. The special message bus service org.freedesktop.DBus responds to a number of messages, allowing applications to interact with the message bus. [document the messages here] [document file format, filesystem locations, etc. for activation] Two standard message bus instances are defined here, along with how to locate them. Each time a user logs in, a desktop session message bus may be started. All applications in the user's login session may interact with one another using this message bus. [specify how to find the address of the desktop session message bus via environment variable and/or X property] A computer may have a system message bus, accessible to all applications on the system. This message bus may be used to broadcast system events, such as adding new hardware devices. [specify how to find the address of the system message bus] This glossary defines some of the terms used in this specification. Broadcast A message sent to the special org.freedesktop.Broadcast service; the message bus will forward the broadcast message to all clients that have expressed interest in it. Message A message is the atomic unit of communication via the D-BUS protocol. It consists of a header and a body; the body is made up of arguments. Message Bus The message bus is a special application that forwards or broadcasts messages between a group of applications connected to the message bus. It also manages services. Service A service is simply a named application that other applications can refer to. For example, the hypothetical com.yoyodyne.Screensaver service might accept messages that affect a screensaver from Yoyodyne Corporation. An application is said to own a service if the message bus has associated the application with the service name.