Divergent, Convergent, and Congruent Infrastructures

There’s a very powerful concept that informs most of my thinking about system configuration management: the concept of divergent vs convergent vs congruent infrastructures. I first encountered this concept in Steve Traugott’s seminal paper Why Order Matters: Turing Equivalence in Systems Administration. I highly recommend reading the entire paper if you haven’t before – or even reviewing it if it’s been a while. Steve and Mark Burgess (of cfengine fame) are the two people who IMO brought the first real rigor to analysis of systems configuration management – their stuff is required reading for anyone in this space. Here’s the idea:

Divergent Infrastructure

When multiple systems are managed by hand by individual (well-meaning) sysadmins in a “classical” IT shop, they invariably become out of sync over time. This is known as “configuration drift” and is impossible to prevent if any changes to the systems are allowed at all. Many IT installations go through a divergent phase after their installation until it is recognized as a problem, which then leads to an attempt to create:

Convergent Infrastructure

After configuration drift is recognized as a problem, people attempt to bring the systems back to a common standard. passwd files may be unified, syslog configs may be made identical, etc. During this period, the configurations “converge” towards a common state – however it is very difficult (maybe even impossible) to reach a point where once-divergent systems are identical – what you hope for is an asymptotic approach to identity. Contrast this with

Congruent Infrastructure

A different way of managing configurations is to enforce that all systems are configured identically, so that configuration state remains the same between systems over time. This can be done in multiple ways and in my mind is one of the core goals of any system configuration management system.

This is illustrated in the following graph¹.

In the beginning, the state of the systems is getting more and more different – they are diverging. Then someone starts applying standards, templates, etc and the configurations begin to converge. Finally a congruent mechanism is implemented and the systems are enforced to identical state during the congruent phase.

Much more on this to come – this is just setting the stage….

1.That graph was generated with the neat-o stuff over at Google Charts. Mostly anyway, I had to load it into Skitch to put the text annotations and arrows on – Google Charts allows annotations of datapoints but I couldn’t find a good way to get it to identify a particular range of points and annotate those as a whole.