I think it doesn't have to be a message bus per-se, that design decision is mostly just because it's convenient. Even D-Bus can actually be used without the bus, if you really want to.
D-Bus is just a bus so it can solve a bunch of different IPC problems at the same time. e.g. D-Bus can handle ACLs/permissions on RPC methods, D-Bus can handle multiple producers and multiple consumers, and so on. I think ultimately all that's really needed is IPC, and having a unified bus is just to allow for some additional use cases that are harder if you're only using UNIX domain sockets directly.
If there are going to be systemd components that have IPC, then I'd argue they should probably use something standard rather than something bespoke. It's good to not re-invent the wheel.
Not that I think Varlink is any better. It seems like at best it's probably a lateral move. I hope this does not go forward.
D-Bus is just a bus so it can solve a bunch of different IPC problems at the same time. e.g. D-Bus can handle ACLs/permissions on RPC methods, D-Bus can handle multiple producers and multiple consumers, and so on. I think ultimately all that's really needed is IPC, and having a unified bus is just to allow for some additional use cases that are harder if you're only using UNIX domain sockets directly.
If there are going to be systemd components that have IPC, then I'd argue they should probably use something standard rather than something bespoke. It's good to not re-invent the wheel.
Not that I think Varlink is any better. It seems like at best it's probably a lateral move. I hope this does not go forward.