Introduction
D-BUS是一种进程间通信(IPC)机制，一般主要用于基于AF_UNIX套接字的本地进程间通信(local IPC)(当然也可以基于TCP/IP)实现跨主机的通信。
Linux上已经存在很多local IPC机制，比如FIFO、UNIX套接字等，为什么还要搞一个D-BUS呢？实际上，D-BUS采用了RPC(Remote Procedure Calling)的思想，它相当于本机的RPC，RPC相对于原始的FIFO、unix socket，是更加现代的通信方式，RPC框架本身会负责消息的编解码、安全验证等。这些会大大简化应用程序的开发。
D-Bus包含下面一些内容：
(1) libdbus: a low-level library
(2) dbus-daemon: a daemon based on libdbus. Handles and controls data transfers
between DBus peers
(3) two types of busses: a system and a session one. Each bus instance is managed
by a dbus-daemon
(4) a security mechanism using policy files
值得一提的是systemd没有使用libdbus，而是使用自己实现的library。
D-Bus Concepts
D-BUS协议是一个端到端（peer-to-peer or client-server）的通信协议，它包含一些基本的概念：
- 总线(bus)
相当于D-BUS的通信链路，应用之间通过总线进行通信。应用在总线上寻找service。有两种总线：
A “system bus” for notifications from the system to user sessions, and to allow the system to request input from user sessions.
A “session bus” used to implement desktop environments such as GNOME and KDE.
- 服务(service)
服务是提供IPC API的程序，每个服务都有一个reverse domain name结构的标识名称。比如org.freedesktop.NetworkManager对应系统总线上的NetworkManager；org.freedesktop.login1对应系统总线上的systemd-logind。
- 对象(object)
相当于通信的地址，每个service的object都通过object path来标识，object path类似文件系统的路径。比如/org/freedesktop/login1是服务org.freedesktop.login1的manager对象的路径。
- 接口(interfaces)
每个object包含一个或者多个interfaces。D-Bus interfaces define the methods and signals supported by D-Bus objects。
- 方法(method)
D-Bus methods may accept any number of arguments and may return any number of values, including none.
- signal
D-Bus signals provide a 1-to-many, publish-subscribe mechanism. Similar to method return values, D-Bus signals may contain an arbitrary ammount of data. Unlike methods however, signals are entirely asynchronous and may be emitted by D-Bus objects at any time.
- signature
signature用于描述参数的数据类型，比如s代表UTF-8字符串。
更多概念参考DBus Overview。
From shell
有一些D-BUS相关的工具，比如busctl、gdbus、dbus-send等，通过这些工具可以进行一些D-BUS相关的操作。
列出所有连接系统总线的端点：
# busctl
NAME PID PROCESS USER CONNECTION UNIT SESSION DESCR
:1.0 1 systemd root :1.0 init.scope - -
:1.1 516 polkitd polkitd :1.1 polkit.service - -
:1.10 1305 busctl root :1.10 sshd.service - -
:1.2 719 tuned root :1.2 tuned.service - -
com.redhat.tuned 719 tuned root :1.2 tuned.service - -
fi.epitest.hostap.WPASupplicant - - - (activatable) - -
fi.w1.wpa_supplicant1 - - - (activatable) - -
org.freedesktop.DBus - - - - - - -
org.freedesktop.PolicyKit1 516 polkitd polkitd :1.1 polkit.service - -
org.freedesktop.hostname1 - - - (activatable) - -
org.freedesktop.import1 - - - (activatable) - -
org.freedesktop.locale1 - - - (activatable) - -
org.freedesktop.login1 - - - (activatable) - -
org.freedesktop.machine1 - - - (activatable) - -
org.freedesktop.network1 - - - (activatable) - -
org.freedesktop.resolve1 - - - (activatable) - -
org.freedesktop.systemd1 1 systemd root :1.0 init.scope - -
org.freedesktop.timedate1 - - - (activatable) - -
D-Bus API of systemd
systemd提供了一些D-BUS API，通过API，我们可以进行systemd的各种操作，比如启动、停止服务等。
查看所有的API:
# gdbus introspect --system --dest org.freedesktop.systemd1 --object-path /org/freedesktop/systemd1
node /org/freedesktop/systemd1 {
interface org.freedesktop.DBus.Peer {
methods:
Ping();
GetMachineId(out s machine_uuid);
signals:
properties:
};
interface org.freedesktop.DBus.Introspectable {
methods:
Introspect(out s data);
signals:
properties:
};
interface org.freedesktop.DBus.Properties {
methods:
Get(in s interface,
in s property,
out v value);
GetAll(in s interface,
out a{sv} properties);
Set(in s interface,
in s property,
in v value);
signals:
PropertiesChanged(s interface,
a{sv} changed_properties,
as invalidated_properties);
properties:
};
interface org.freedesktop.systemd1.Manager {
methods:
GetUnit(in s arg_0,
out o arg_1);
GetUnitByPID(in u arg_0,
out o arg_1);
LoadUnit(in s arg_0,
out o arg_1);
StartUnit(in s arg_0,
in s arg_1,
out o arg_2);
StartUnitReplace(in s arg_0,
in s arg_1,
in s arg_2,
out o arg_3);
StopUnit(in s arg_0,
in s arg_1,
out o arg_2);
ReloadUnit(in s arg_0,
in s arg_1,
out o arg_2);
RestartUnit(in s arg_0,
in s arg_1,
out o arg_2);
...
接口org.freedesktop.systemd1.Manager包含了systemd提供的主要操作方法。
- StartUnit/StopUnit
# busctl --system call org.freedesktop.systemd1 /org/freedesktop/systemd1 org.freedesktop.systemd1.Manager GetUnit s crond.service
o "/org/freedesktop/systemd1/unit/crond_2eservice"
# busctl --system call org.freedesktop.systemd1 /org/freedesktop/systemd1 org.freedesktop.systemd1.Manager StopUnit ss crond.service replace
o "/org/freedesktop/systemd1/job/904"
# systemctl status crond.service
● crond.service - Command Scheduler
Loaded: loaded (/usr/lib/systemd/system/crond.service; enabled; vendor preset: enabled)
Active: inactive (dead) since 四 2017-01-05 03:13:29 CST; 4s ago
Process: 656 ExecStart=/usr/sbin/crond -n $CRONDARGS (code=exited, status=0/SUCCESS)
Main PID: 656 (code=exited, status=0/SUCCESS)
...
# busctl --system call org.freedesktop.systemd1 /org/freedesktop/systemd1 org.freedesktop.systemd1.Manager StartUnit ss crond.service replace
o "/org/freedesktop/systemd1/job/905"
# systemctl status crond.service
● crond.service - Command Scheduler
Loaded: loaded (/usr/lib/systemd/system/crond.service; enabled; vendor preset: enabled)
Active: active (running) since 四 2017-01-05 03:13:53 CST; 1s ago
Main PID: 12277 (crond)
Memory: 624.0K
CGroup: /system.slice/crond.service
└─12277 /usr/sbin/crond -n
...