Table of Contents

Tags for variables, classes, and bundles

Table of Contents

Introduction

CFEngine 3.6.0 makes great use of the meta attribute to set tags on variables and classes. While that attribute was always in the language, only in 3.6.0 is it fully utilized

Furthermore, bundles can now be tagged and found by tag, so you can construct a fully dynamic bundle sequence.

Problem statement

We'd like to apply tags to variables and classes for many purposes, from stating their provenance (whence they came, why they exist, and how they can be used) to filtering them based on tags.

We'd also like to be able to include all the files in a directory and then run all the discovered bundles if they are tagged appropriately.

Syntax

Tagging variables and classes is easy with the meta attribute. Here's an example that sets the inventory tag on a variable and names the attribute that it represents. This one is actually built into the standard 3.6.0 inventory policy under masterfiles/inventory/any.cf, so you have it available out of the box in either Community or Enterprise.

bundle agent cfe_autorun_inventory_listening_ports
# @brief Inventory the listening ports
#
# This bundle uses `mon.listening_ports` and is always enabled by
# default, as it runs instantly and has no side effects.
{
  vars:
      "ports" slist => { @(mon.listening_ports) },
      meta => { "inventory", "attribute_name=Ports listening" };
}

In the Enterprise Mission Portal, you can then make a report for "Ports listening" across all your machines. For more details, see https://cfengine.com/docs/master/guide-enterprise-cfengine-guide-mission-portal-reporting.html

Class tags work exactly the same way, you just apply them to a classes promise with the meta attribute.

Tagging bundles is different because you have to use the meta promise type (different from the meta attribute).

An example is easiest:

bundle agent run_deprecated
{
  meta:
      "tags" slist => { "deprecated" };
}

This declares an agent bundle with a single tag.

Functions

Several new functions exist to give you access to variable and class tags, and to find classes and variables with tags.

classesmatching: this used to be somewhat available with the allclasses.txt file. You can now call a function to get all the defined classes, optionally filtering by name and tags. See https://cfengine.com/docs/master/reference-functions-classesmatching.html
getvariablemetatags: get the tags of a variable as an slist. See https://cfengine.com/docs/master/reference-functions-getvariablemetatags.html
variablesmatching: just like classesmatching but for variables. See https://cfengine.com/docs/master/reference-functions-variablesmatching.html
getclassmetatags: get the tags of a class as an slist. See https://cfengine.com/docs/master/reference-functions-getclassmetatags.html

There is also a new function to find bundles.

bundlesmatching: find the bundles matching some tags. See https://cfengine.com/docs/master/reference-functions-bundlesmatching.html (the example shows how you'd find a deprecated bundle like run_deprecated earlier).

Module protocol

The module protocol has been extended to support tags. You set the tags on a line and they persist for every subsequent variable or class.

^meta=inventory
+x
=a=100
^meta=report,attribute_name=My vars
+y
=n=100

This will create class x and variable a with tag inventory.

Then it will create class y and variable b with tags report and attribute_name=My vars.

Enterprise Reporting with tags

In CFEngine Enterprise 3.6.0, you can build reports based on tagged variables and classes.

Please see https://cfengine.com/docs/master/guide-enterprise-cfengine-guide-mission-portal-reporting.html for a full tutorial, including troubleshooting possible errors. In short, this is extremely easy as long as you:

allow classes and variables with the inventory and report tags to be reported in controls/cf_serverd.cf. This is already the case if you install the stock CFEngine 3.6.0 packages.
ensure there are no report collection issues due to firewalls (TCP port 5308 must be open from the hub to the agents, for instance)

Dynamic bundlesequence

Dynamic bundlesequences are extremely easy. You simply say

vars:
  "bundles" slist => bundlesmatching("regex", "tag1", "tag2", ...);

methods:
  "run $(bundles)" usebundle => $(bundles);

Then every bundle matching the regular expression regex and all the tags will be found and run.

Note that the discovered bundle names will have the namespace prefix, e.g. default:mybundle. The regular expression has to match that. So mybundle as the regular expression would not work. See https://cfengine.com/docs/master/reference-functions-bundlesmatching.html for another detailed example.

In fact we found this so useful we implemented services autorun in the masterfiles policy framework.

There is only one thing to beware. All the bundles have to have the same number of arguments (0 in the case shown). Otherwise you will get a runtime error and CFEngine will abort. We recommend only using 0-argument bundles in a dynamic sequence to reduce this risk.

Summary

Tagging variables and classes and bundles in CFEngine 3.6.0 is easy and allows more dynamic behavior than ever before. Try it out and see for yourself how it will change the way you use and think about system configuration policy and CFEngine.

tags
meta