systemd-userdbd is a system service that multiplexes user/group lookups to all local services that provide JSON user/group record definitions to the system. In addition it synthesizes JSON user/group records from classic UNIX/glibc NSS user/group records in order to provide full backwards compatibility.
Most of systemd-userdbd's functionality is accessible through the userdbctl(1) command.
The user and group records this service provides access to follow the m[blue]JSON User Recordm and m[blue]JSON Group Recordm definitions. This service implements the m[blue]User/Group Record Lookup API via Varlinkm, and multiplexes access other services implementing this API, too. It is thus both server and client of this API.
This service provides two distinct m[blue]Varlinkm services: io.systemd.Multiplexer provides a single, unified API for querying JSON user and group records. Internally it talks to all other user/group record services running on the system in parallel and forwards any information discovered. This simplifies clients substantially since they need to talk to a single service only instead of all of them in parallel. io.systemd.NameServiceSwitch provides compatibility with classic UNIX/glibc NSS user records, i.e. converts struct passwd and struct group records as acquired with APIs such as getpwnam(1) to JSON user/group records, thus hiding the differences between the services as much as possible.