Does anybody know why dbus exists? I've been wracking my brain trying to come up with a usecase for dbus that isn't already covered by Unix sockets.
You want to remotely control a daemon? Use sockets. You want the daemon to respond to the client? Sockets. Want to exchange information in json? plaintext? binary data? Sockets can do it. Want to restrict access to a socket? Go ahead, change the socket's permissions. Want to prevent unauthorized programs from pretending to be someone they're not? Change the permissions of the directory containing the socket. Want network transparency? That's why we have abstract sockets.
Plenty of well-established software uses sockets. Music player daemon uses sockets. BSPWM uses sockets. Tmux uses sockets. Pipewire uses sockets. Dhcpcd uses sockets. Heck, dbus itself relies on sockets!
For developers, using sockets is easy. I once wrote a program that interfaced with BSPWM, and it was a breeze. Dbus, on the other hand, not so much. I tried writing a Python script that would contact Network Manager and check the WiFi signal strength. Right off the bat I'm using some obscure undocumented package for interfacing with dbus. What is an introspection? What is a proxy object? What is an interface? Why do I need 60 lines of (Python!) code for a seemingly trivial operation?
So why do some developers decide to use dbus when they could just use unix sockets and save a lot of hassle for themselves and others?
With pipes/sockets, each program has to coordinate the establishment of the connection with the other program. This is especially problematic if you want to have modular daemons, e.g. to support drop-in replacements with alternative implementations, or if you have multiple programs that you need to communicate with (each with a potentially different protocol).
To solve this problem, you want to standardize the connection establishment and message delivery, which is what dbus does.
With dbus, you just write your message to the bus. Dbus will handle delivering the message to the right program. It can even start the receiving daemon if it is not yet running.
It's a bit similar to the role of an intermediate representation in compilers.
A message bus won't magically remove the need for developers to sit down together and agree on how some API would work. And not having a message bus also doesn't magically prevent you from allowing for alternative implementations. Pipewire is an alternative implementation of pulseaudio, and neither of those rely on dbus (pulse can optionally use dbus, but not for its core features). When using dbus, developers have to agree on which path the service owns and which methods it exposes. When using unix sockets, they have to agree where the socket lives and what data format it uses. It's all the same.
We have a tool for that, it's called an init system. Init systems offer a large degree of control over daemons (centralized logging? making sure things are started in the correct order? letting the user disable and enable different daemons?). Dbus' autostart mechanism is a poor substitute. Want to run daemons per-user instead of as root? Many init systems let you do that too (I know systemd and runit do).
"Bro just use sockets lol" completely misses the point. When you decide you want message based IPC, you need to then design and implement:
And before you know it you've reimplemented dbus, but your solution is undocumented, full of bugs, has no library, no introspection, no debugging tools, can only be used from one language, and in general is most likely pure and complete garbage.
Well said. There are so many details to making code work that can so often be avoided by using the right tooling. OP said it was harder to get started, which implies they did not handle the details and have code not actually robust to all kinds of edge cases. Maybe they don't need it but they probably do.