[gruez]

What you're describing is significantly harder than what's described in the blog post. Not only do you have to validate a file looks like a .jpg/.json/.zip file, you also need to validate that it looks semantically correct (ie. the file format is valid but a chunk of it is missing).

Most people solve this issue by keeping multiple versions, not by trying to "validate" the backups somehow.

[tonyarkles]

Maybe I'm misinterpreting the output from their backup tool, but isn't that exactly what it's doing?

    Metrics for todoist-fullsync:
    
    name   1 days ago  8 days ago
    ------------------------------
    files  1           1
    bytes  82363       86661
    items  85          87

The "items" line there seems like it's actually parsing the file and counting the number of entries in it? It's also captured in point #2: "Can be intuitively evaluated as plausible or suspicious. If the number of tasks in my Todoist export dropped from dozens to 1, that would be cause for concern."

[jaw]

Right. Aside from 'files' and 'bytes', the metrics are just the result of running shell commands specified in a config file. In this case it's `jq '.items | length' $PARCEL_PATH`, i.e., parse the file and print the length of the attribute named "items".

Obviously, that won't catch all potential problems in the file, but it's a low-effort way to catch some.

[jaw]

I keep multiple versions as well, and also use third-party backup software on all these files. These techniques are meant to be part of something analogous to a 'defense in depth' against errors in the backup process, not thorough or foolproof.

> Not only do you have to validate a file looks like a .jpg/.json/.zip file, you also need to validate that it looks semantically correct (ie. the file format is valid but a chunk of it is missing).

But you don't have to do that perfectly to get value out of it; for example:

- If the .json file parses as json, then at least you probably didn't truncate the download mid-stream.

- If it also contains a particular attribute, then you probably didn't save a structured error response instead of the actual data, or save something from a radically-nonpassively-changed endpoint that might no longer be adequate.

- If it also has roughly the number of elements you expect, you probably didn't miss entire pages of the response.