CFEngine 3.17.0 introduced custom promise types, which enable CFEngine users to extend core functionality and policy language in a simple way. As an example of the power and simplicity of this new feature, I will show a promise type that helps to observe a website’s status. The module which implements this promise type was written in a couple of hours.
Creating a promise type for whether a site is up We will use Python and the CFEngine library to implement a promise module.
This tutorial focuses on how to write a promise module, implementing a new CFEngine promise type. It assumes you already know how to install promise modules and use custom promise types, as shown in the previous blog post.
Why Python? Promise modules can be written in any programming language, but there are some advantages of using python:
Readable and beginner friendly language / syntax Popular and familiar to a lot of people, also used in some CFEngine package modules Big standard library, allowing you to reuse data structures, parsers, etc.
In CFEngine 3.17, custom promise types were introduced. This allows you to extend policy language, managing resources which don’t have built in promise types. The implementation of custom promise types is open source, and available in both CFEngine Enterprise and CFEngine Community. To implement a new custom promise type, you need a promise module. (The promise type is what you use in policy language (the concept), while the module is the underlying implementation - can be a python script, compiled executable or similar).
