Distributed Operating Multi Access Interactive Network
Operating System (DOMAIN/OS) was the operating system for the DN series workstations made by Apollo Computer Inc. ( acquired by HP in 1989).
The OS was written in Pascal and based on the Multics operating system. On top of the OS were command and API layers that allowed the user the choice of interacting & programming in either System V, BSD 4.2, or Apollo's Aegis environments.
Underneath the OS offered a fully integrated network layer in which you could view the entire filesystem of any node on the network by cd'ing to //machinename. User accounts were maintained in an account registry database which was also distributed over the network.
The Apollo filesystem supported full ACL's on all objects and every object in the filesystem had a "type".
For each type of object, the OS would run a module for
accessing the type - for instance, /etc/passwd was not a text file, but a file of type passwd - any attempt to read
from the file would retrieve data from the account registry instead. (Linux & some Unix'es have similar
concepts now, but mostly limit them to certain filesystems such as /proc or Solaris' /etc/mnttab.)
After the HP buyout of Apollo, some of the DOMAIN concepts were used in HP-UX, OSF/1, and DCE, but DOMAIN/OS itself slowly and quietly died as HP converted customers to HP-UX workstations.