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!

Footnotes

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