undefined | Better HN

0 pointswilltim6y ago0 comments

> there's a million fiddly things to get right, and Unison hasn't had that much usage

Unison is the only bidirectional sync tool that I trust to get the details right. It is backed by a formal model with various proofs of correctness. Such models are also easily representated in OCaml; which I can assure you is more than a fine language, especially if one cares about correctness. Dropbox has struggled to get these details correct in the past (see the paper "Mysteries of DropBox", by Unison's author Professor Benjamin Pierce). TLDR, Pierce teaches these Python hackers how to fix their broken code.

0 comments

4 comments · 1 top-level

coldtea6y ago· 3 in thread

>Unison is the only bidirectional sync tool that I trust to get the details right. It is backed by a formal model with various proofs of correctness.

That's just about the sync process/stages, the easy part that can actually be formalized.

The "million fiddly things" are about OS and filesystem issues, incompatibilities, and so on, and Dropbox has a hugely larger test base for those things...

willtimOP6y ago

I disagree. POSIX, although somewhat dated, has provided a good enough abstraction layer for filesystems and OSs. My proof of this is the number of different and successful filesystems for Unix/Linux. If the abstraction didn't work, everyone would be forced to use the same filesystem.

The issues and subtleties are with bidirectional sync. It is not "the easy part". Dropbox didn't get it right in the past, we have no proof they have it right now.

coldtea6y ago

>POSIX, although somewhat dated, has provided a good enough abstraction layer for filesystems and OSs.

First of all, POSIX semantics are not what Windows support. Second, even where available, POSIX is a tiny part of the possible issues. Adequate for naive apps that need to open or write some files, not for a reliable sync tool.

https://danluu.com/deconstruct-files/

1 more reply

robocat6y ago

> POSIX, although somewhat dated, has provided a good enough abstraction layer for filesystems and OSs

I know nothing about POSIX, but I presume we all want stronger guarantees to avoid the risk of syncing a corrupted file.

https://danluu.com/file-consistency/

1 more reply

j / k navigate · click thread line to collapse