DBus
DBus stands for Desktop Bus. It's a message bus system provided by Linux to allow communication between different processes.
Built on top of Unix Domain Sockets
DBus daemon uses Unix Domain Sockets to allow communication between different processes. The producer and the consumer processes connect to the DBus daemon using the sockets.
For every connection set to the DBus daemon socket, there are two file descriptors created - one for reading and one for writing.
As we learnt in file descriptors, every connection to a file creates a file descriptor. That's how every connection becomes a full duplex connection.
See sockets for more details.
Use cases
For example, when you connect to a new WiFi network, the Network Manager daemon needs to notify the user about it. This is done via DBus. The network manager sends if the connection was successful or not to the DBus daemon and the DBus daemon then forwards the message to the Desktop application which then shows the notification to the user.
Another use case, hardware controls such as volume control, brightness control, etc. are also done via DBus.
Types of DBus
- System DBus - This is used for system level services such as Network Manager, Bluetooth, etc.
- Session DBus - This is used for user level services such as desktop notifications, media control, etc.
For each user session, there is a separate session DBus daemon running. The daemon is started when the user logs in and is stopped when the user logs out.