Digging into CSN diffs for CDS models

| 2 min read

I wanted to increase my understanding of CSN a little by noting changes in it as I built up my CDS model, and ended up with a script csndiff to help me with that.

To grok something complex, I try to get into the habit of looking at that something sideways and only through the corner of my eye, a piece at a time. Each piece that I look at is less overwhelming and more likely to stay in my brain for longer.

I was also curious as to how "robust" and "canonical" Core Schema Notation (CSN) was, or rather how "stable" it was regarding differences in expression, in the CDS model1, of what amounts to the same thing, definition-wise.

CSN has a couple of regular representations - JSON and YAML2. For this experiment I used the JSON representation, mostly because at the outset I wasn't sure how I might process the CSN and thought I might use jq, but in fact I ended up discovering and using the excellent JSON diff and patch tool called jd.

I wrote a small script csndiff that:

  • expects, via STDIN, a CSN definition (in JSON)
  • compares it to any previous version of that CSN definition that it has*
  • emits that comparison as a JSON diff thanks to jd
  • saves this latest CSN definition as the previous version for the next comparison

*at the very start, this simply defaults to the empty object {}

Used in a tmux split window, it's great for observing how your CDS model changes and additions are actually "reified" in CSN. Here's an example session, where I'm editing a file services.cds in the left pane, and in the right pane I'm running csndiff as soon as this file changes, using entr of course, like this:

ls services.cds \
| entr -c bash -c 'cds compile services.cds | csndiff'

Here's what the session looks like:


I'd love to hear about any investigations you've done into CSN too!


  1. I'm tempted to write "CDL" for "CDS model" because that's the human-oriented language that the CDS model is actually written in - Core Definition Language. What we call "CDS" is actually "CDL". But that's a discussion for another time.
  2. For more on the YAML representation of CSN, see Simple script for previewing CDS models in CSN - cdsray