I don't want to have a running process just for this, I want to be able to take a "snapshot"/"index", store some data on Glacier, and the index on S3, and later, given the index, be able to compute deltas without accessing the full archive store on Glacier.
I haven't thought about the filesize/hash to reduce collision, but I chose to stick with the last-modified time in the article, because it can takes hours computing hashes for a big directory tree.
Tools like rsync relies on last-modified time by default, and since I want to use this to track my own files, I won't fake it, so I think it's not a big deal?
The service is not continually running, I use this method to make incremental backups with archives stored on AWS Glacier and meta-data stored on S3 (the index is stored on S3, and I can't access files on Glacier to compute deltas).
I see the confusion, I tweaked the README to remove that claim. But I indeed meant "no external dependencies like postgres/redis..".