why Magic Folders doesn't work for more than 2 clients

Ben Laurie ben at links.org
Wed Dec 2 08:53:05 UTC 2015

On Fri, 6 Nov 2015 at 17:55 Zooko Wilcox-O'Hearn <zookog at gmail.com> wrote:

> Dear folks:
> Context: “Magic Folders” is a new layer on top of Tahoe-LAFS which
> implements Dropbox-like "auto-sync" behavior. It's super exciting!
> Development of it was sponsored by Open Technology Fund. We've
> finished implementing it, but now we're noticing some bugs and
> limitations, and this is the biggest one I'm aware of right now.
> As David described in this comment:
> https://tahoe-lafs.org/trac/tahoe-lafs/ticket/2551#comment:22
> The current design and implementation of Magic Folders only works if
> you have no more than two clients attached to it. By "clients" I mean
> here either two devices owned by the same user or two users each with
> their own device — Tahoe-LAFS and Magic Folders are unaware of any
> distinction between a separate human user with their own device versus
> a separate client/node/process/device operated by the same human user.
> The problem is subtle, and I'm writing this letter mostly in order to
> cement my own understanding of *why* the current design fails in the
> 3-party (or more) case. Also in order to draw Daira's attention to it
> and see what sort of fix she would suggest.
> Here is the section of the design doc about the problem of
> distinguishing conflicts from overwrites pushed by your peer. We
> whimsically named this problem the "Fire Dragons" problem.
> https://tahoe-lafs.org/trac/tahoe-lafs/browser/docs/proposed/magic-folder/remote-to-local-sync.rst#fire-dragons-distinguishing-conflicts-from-overwrites
> The goal of the Fire Dragon slaying protocol is to make it so that you
> can reliably tell whether a given new version submitted to you by your
> peer was derived from the most recent version that *you* created, or
> if it was derived from a previous version than the most recent version
> that you created. If your peer says that it was derived from the most
> recent version that you created, then this means your peer is
> instructing you to *overwrite* your most recent version with their new
> version. If instead your peer says that it was derived from an earlier
> version, then this means there is a *conflict*, where you and your
> peer simultaneously made changes to the file.
> The way the current Fire Dragon slaying protocol attempts to track
> this distinction is by including a slot with each upload called the
> "last-downloaded record". When you receive a new version from someone
> else, you inspect the last-downloaded record that accompanies that
> version, and if it shows that the version they started from was the
> most recent version you sent them, then you know it is an overwrite
> [*], and if it isn't, then you know it is a conflict.
> Now here's the problem: this design assumes that you are the only
> source of downloadable versions that they could have started with! But
> if there's a *third* party, and they started with a download from that
> third party, then with this design you will assume that they are
> starting from an *earlier version from you*, when in fact they are
> starting from a version from the third party, which might in turn have
> been derived from the most recent version from you. (Or it might not
> have — it might have been derived from a previous version from you.)
> Bottom line:
> 1. Magic Folders exists, and is awesome.
> 2. It works fine if you have no more than 2 devices/users/endpoints
> attached. (Or if no more than two of them are editing the same
> document at the same time.)
> 3. The current version doesn't work if more than 2 endpoints edit the
> same file at the same time.
> I have an idea about how to fix this, but I'm not going to include it
> in this note, because I want Daira to have a chance to think about the
> problem and propose a fix before being biased by my proposed fix, and
> I want Leif to have a chance to reply with his proposed fix (that he
> attempted to explain to me at the Nuts & Bolts party today).

Git solves this problem all the time - worth looking at that?

> Regards,
> Zooko
> [*] Except if there turns out to be an Earth Dragon — see the design
> doc for details.
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