A spreadsheet file on the machine this is running on.
Writes go to a temporary file beside the real one and are moved into place with a rename, which on every filesystem Erlang runs on is atomic: a reader opening the file while it is being written sees the old one whole or the new one whole, never half of either.
What it cannot promise
A conditional write. The version here is the file's size and modification time, which is checked immediately before the rename, so a write that would clobber somebody is usually refused rather than silently carried out. But "immediately before" is not "at the same instant as", and the modification time a filesystem keeps has one-second resolution, so two writes of the same size in the same second are indistinguishable.
It is a guard, not a guarantee, and Sheetshow.Backend.supports?(workbook, :conditional_write) answers false for it accordingly. A store that can do
better, one that speaks If-Match, is what closes that gap.