RPC daemon
In OpenWrt we commonly use ubus for all kinds of communication. It can provide info from various software as well as request various actions. Nevertheless, not every part of OpenWrt has a daemon that can register itself using ubus. For an example uci and opkg are command-line tools without any background process running all the time.
It would be not efficient to write a daemon for every software like this and run them independently. This is why rpcd was developed. It’s a tiny daemon with support for plugins using trivial API. It loads library .so files and calls init function of each of them.
The code is published under ISC license and can be found via git at https://git.openwrt.org/project/rpcd.git.
Default plugins
There are few small plugins distributed with the rpcd sources. Two of them (session and uci) are built-in, others are optional and have to be build as separated .so libraries. Apart from that there are other projects providing their own plugins.
See details here: rpcd: OpenWrt ubus RPC daemon for backend server