This blog post will focus on the bash programming part of implementing a promise type. To understand what custom promise types are, and how to use them, you should read the introduction first.
To implement a custom promise type in CFEngine, you need a promise module. The module is an executable, and can be written in any language. It’s possible to write one from scratch, but to make it as easy as possible, we decided to provide libraries for common programming languages.
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.
Introduction In the CFEngine Core team, we have recently been working on a fix for our WaitForCriticalSection() function. In short, the function checks a timestamp in a chunk of (lock) data stored in a local LMDB database and if the timestamp is too old, it writes a new chunk of (lock) data with the new timestamp. However, this used to be done in separate steps - read the data from the DB and close DB, check the data and potentially write the new data into the DB.
Announcing CF4! (or is it CF-FORTH?!) I imagine you didn’t expect such a big release so soon after our most recent release of 3.12.4 and 3.15.1 on March 26, but here it is: our alpha-release. Thus the reason for the .-4 in the version number. Of course choosing -4 has something to do with the fun of spelling FORTH without the ‘U’. Also, it’s nearly a palindrome and I imagine we’ll have a few alphas/betas before the final release is finished.
Problem to solve When working on the new Federated Reporting feature for CFEngine we had to solve the problem of collecting data from multiple CFEngine hubs (feeders) on a single hub (superhub). CFEngine hubs are using PostgreSQL to store data, so, more specifically, the problem was how to collect data from multiple PostgreSQL databases in one PostgreSQL database. And because we are talking about ~1 GiB of SQL data per feeder hub and for example 10 feeders connected to a superhub here, the initial and trivial solution using basically this ETL (Extract Transform Load) pipeline - pg_dump | gz | ssh | gunzip | psql - provided really poor performance.
In some performance critical situations, it makes sense to limit management software to a single CPU (core). We can do this using systemd and cgroups. CFEngine already provides systemd units on relevant platforms, we just need to tweak them. I’m using CFEngine Enterprise 3.12 on CentOS 7, but the steps should be very similar on other platforms/versions. This post is based on an excellent article from Red Hat: https://access.redhat.com/solutions/1445073
Using ps to check what CPU core is utilized Listing all processes and their core We can use ps to check CPU core for desired processes:
