I do, with Dropbox, and it mostly works... but it's very easy to get the database corrupted even if you are careful not to have more than one instance of Calibre open.

I haven't tried using a source control system, that might work better. Hmmm.

And of course that only works for computers, There isn't a full version of Calibre for Android.

Out of curiosity, how does your DB on Dropbox get corrupted if not by having more than one instance of Calibre open?

I'm asking because I use Dropbox to store my Keepass DB, and so far (2+ years) I haven't had any problems.

I don't know. But I occasionally find files with names like foo(username's conflicted copy).db.

I do the Keepass thing too, and this has happened with it's files as well, but that doesn't seem to cause any problems.

Those aren't corrupted databases, it just can't merge so it leaves you to pick the version you want. Just pick one and keep using it, at most you'll lose some recent changes from the other computer.

I should have known not to use language imprecisely on HN. :)

To the layman, any situation that results in having Calibre show books which are not in fact there, or having books in the library which are not shown in Calibre, or having Calibre's check library function report that there are "invalid titles", "missing formats" etc., could be casually described as "corruption".

Sure, but the worst case is that you don't see books which are in your filesystem, and then all you need to do to fix that is click "Add books".

There is calibre companion an app that lets you sync all or part of your library. To sync you would merely connect wirelessly and then search for ondevice:false and send those to the device.

Exactly. The problem is that you lose searchability. On the filesystem, Calibre organizes your library in a series of subdirectories named by author. If you want to find a book by title, or some other criteria... or even when it's a book with multiple co-authors and don't know which one Calibre selected as the primary... then you'll have a bad time.

Don't people learn how to use their OS's file searching capabilities anymore?

I do with Dropbox too, along with a portable version of Calibre. For syncing to my andoid phone, I use [Calibre Companion](https://play.google.com/store/apps/details?id=com.multipie.c...).

Yup, that's exactly what I do. I sync over the database and library over to Nextcloud to have it available on both of the operating systems I'm using.

It's also synced automatically to an external hard drive connected to the Raspberry Pi, so I have two backups (one online, one offline) in case something goes wrong.

Wouldn't that trigger copyright defender algorithms and cause your Dropbox account to be suspended?

AFAIK they only run the DMCA checksum when the file/folder is shared. Keeping it inside your own account is OK.