> store and query data as I have described, but I am wondering if I am
> reinventing the wheel. Does a tool such as this already exist?
You are describing a feature of Save/Compare/Restore applications.
SCR are also know as epics-backup application (i.e take a snapshot of a running machine, compare 2 snapshots, compare a snapshot with the running machine)
You next question is where to get such a piece of software. Well...
I am using SDDS save/compare/restore (very old)
SCORE may have the described feature as well (used at SNS and SLAC)
<Shameless plug for MASAR>
I am currently looking at MASAR part of epics-4, which is straight forward to install, but still in development.
For example, comparisons of 2 snapshots using the pyQt GUI is missing.
A CSS GUI is also a work-in-progress.
At this time, most of the development is at the GUI level
So if you don't mind a text output (i.e. in a terminal), you can use MASAR with its python API and have something working by tonight.
</Shameless plug for MASAR>