While it is useful to inspect values like
app-db, you are sometimes interested to see how a value has changed. What are the
diffs? During this Epoch, what got added or removed, and what was modified?
This page explains what you are seeing in sections labelled "Only Before" and "Only After".
Assume that a value is
X before the start of an epoch, and ends up being modified to
X'. To show such differences,
re-frame-10x chooses to do a calculation best explained by this pseudo code:
(let [[only-before only-after _] (clojure.data/diff X X')]
By looking at the docs for
clojure.data/diff, you'll see that it calculates how two values differ, and returns a triple of values.
re-frame-10x captures and displays the first two elements of this triple as "only before" and "only after" respectively. The 3rd element is ignored because it's what hasn't changed, which isn't interesting.
So, to correctly interpret "Only Before" and "Only after", you'll need to spend time properly familiarising yourself with how
clojure.data/diff works, but this will be a worthwhile investment.