Layers of Abstraction in Policy

Table of Contents

CFEngine offers a number of layers of abstraction. The most fundamental atom in CFEngine is the promise. Promises can be made about many system issues, and you described in what context promises are to be kept.

CFEngine is designed to handle high level simplicity (without sacrificing low level capability) by working with configuration patterns. After all, configuration is all about promising consistent patterns in the resources of the system. Lists, for instance, are a particularly common kind of pattern: for each of the following... make a similar promise. There are several ways to organize patterns, using containers, lists and associative arrays.

At this high level, a user selects from a set of pre-defined services (or bundles in CFEngine parlance). The selection is not made by every host, rather one places hosts into roles that will keep certain promises.

    bundle agent service_catalogue # menu
      any:: # selected by everyone
         "everyone" usebundle => time_management,
                    comment => "Ensure clocks are synchronized";
         "everyone" usebundle => garbage_collection,
                    comment => "Clear junk and rotate logs";

      mailservers:: # selected by hosts in class
        "mail server"  -> { "goal_3", "goal_1", "goal_2" }
                      usebundle => app_mail_postfix,
                        comment => "The mail delivery agent";
        "mail server"  -> goal_3,
                      usebundle => app_mail_imap,
                        comment => "The mail reading service";
        "mail server"  -> goal_3,
                      usebundle => app_mail_mailman,
                        comment => "The mailing list handler";

Bundle level

At this level, users can switch on and off predefined features, or re-use standard methods, e.g. for editing files:

    body common control
    bundlesequence => {

The set of bundles that can be selected from is extensible by the user.

Promise level

This is the most detailed level of configuration, and gives full convergent promise behavior to the user. At this promise level, you can specify every detail of promise-keeping behavior, and combine promises together, reusing bundles and methods from standard libraries, or creating your own.

    bundle agent addpasswd

      # want to set these values by the names of their array keys

      "pwd[mark]" string => "mark:x:1000:100:Mark B:/home/mark:/bin/bash";
      "pwd[fred]" string => "fred:x:1001:100:Right Said:/home/fred:/bin/bash";
      "pwd[jane]" string => "jane:x:1002:100:Jane Doe:/home/jane:/bin/bash";


      "/etc/passwd"           # Use standard library functions
            create => "true",
           comment => "Ensure listed users are present",
             perms => mog("644","root","root"),
         edit_line => append_users_starting("addpasswd.pwd");