Even though in most scenarios there is only one single machine, it is important to distinguish between the installation source machine and the installation target machine:

All communication with the installation target is handled via the System Configuration Repository (SCR) to guarantee the network abstraction design goal. This is much easier said than done, however: YaST2 module developers always have to keep in mind that it is strictly forbidden to access system files (or any other system resources, for that matter) directly, even if there may be very convenient CPAN Perl modules to do that. Rather, SCR is to be used instead - always. Otherwise everything might run fine if installation source and target are the same machine, but break horribly if they are not.

SCR in itself is also modularized: All calls are handled by "agents" that each know how to handle a particular configuration "path" like "/etc/fstab" or "/etc/passwd". That may be a simple file, but it may also be a directory hierarchy like "probe" - this particular agent handles all kinds of hardware probing, from mouse and display adapters to storage device controllers (like SCSI or IDE controllers), disks attached to each individual controller or partitions on those disks. Paths are denoted like ".etc.fstab" for SCR. YCP even has a special data type "path" for just this case (a special kind of string with some special operations).

SCR agents handle no more than three calls:

The first argument is always the path to handle, but there may be any number of additional parameters, depending on the agent.

While Read() and Write() are obvious, Execute() may not be: This is intended for some kinds of agents that actually run a program on the installation target. In particular, the ".target.bash" agent does that - it runs a "bash" shell on the target machine and accepts a shell command as an argument. This is the tool of choice for tasks such as creating backup copies of configuration files or running any special command on the target machine - and again, the distinction between installation source and installation target machine becomes very important: You want run these commands on the (possibly remote) target machine, not on the machine that happens to hold the installation media.

SCR agents can easily added when needed. There are frameworks available to write SCR agents in C++, in Perl, or as Bash shell scripts as well as several ready-made parsers for different file formats like the ".ini" file parser that can handle files with "key = value" pairs or the "anyagent" that generalizes that concept even more using regular expressions. Those parsers return YCP lists and maps ready for further processing.

Typically, a YaST2 module for a specific installation or administration task includes a set of YCP or Perl scripts as well as some SCR agents to handle its particular configuration files.