108 lines
4.5 KiB
Markdown
108 lines
4.5 KiB
Markdown
Among all possible [[todo/Bittorrent-like_features]] implementations,
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i think [Syncthing][] is one of the most interesting ones.
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First off, it is already [packaged for Debian][] with an [ITP
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underway][]. Second, it seems to use a fairly simple protocol, the
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[Block Exchange Protocol][]. It doesn't try to do everything under the
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sun and keeps things simple: NAT transversal, reuse TLS primitives and
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TCP, etc. It also seems to scale pretty well, if we are to believe the
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[usage statistics][].
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[Syncthing]: https://syncthing.net/
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[packaged for Debian]: http://apt.syncthing.net/
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[ITP underway]: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=749887
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[Block Exchange Protocol]: https://github.com/syncthing/specs/blob/master/BEPv1.md
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[usage statistics]: https://data.syncthing.net/
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It does require the syncthing daemon to be running in order to
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transfer files so it could have similar problems than the
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[[special_remotes/ipfs]] remote which is that files get locally copied
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between the git-annex repository and the special remote.
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Furthermore, one of the main problems with this remote is that [public
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shares are not supported][], that is, in order to share with another
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remote, both remotes need to explicitely add each other, in syncthing!
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That makes pairing a little more difficult that it needs to.
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[public shares are not supported]: https://forum.syncthing.net/t/implementing-public-shares/1186
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[[!toc levels=2]]
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Possible implementations
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========================
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I can think of a few different ways of implementing such a remote:
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1. share the `.git/annex/objects` directory through syncthing
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2. copy objects to the `~/Sync` directory (or elsewhere)
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3. interoperate with syncthing through the API
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4. reimplement the [Block Exchange Protocol][] natively
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Sharing the objects
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-------------------
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This is the easiest, but maybe the most dangerous: start syncthing and
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expose the `.git/annex/objects` directory to other peers.
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This of course has the downside that syncthing could technically start
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destroying objects without git-annex's knowledge, which is really
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bad. Hopefully, the readonly permissions on files could keep that from
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happening, but it still seems pretty unsafe.
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There is a way to mark a folder as "master" which makes it ignore
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changes from other nodes, but then that breaks the peer to peer nature
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of the protocol, which is hardly what we want. Marking the repo as
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untrusted would also be an important requirement here.
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Copying objects
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---------------
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Copying objects is the safest and easiest way to implement this. Add a
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new key? You just copy it to the sync directory. Remove a key? Just
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remove the file, and syncthing picks up the change.
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The main problem with this approach is of course the duplication of
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data, doubling the disk usage of all objects stored in the syncthing
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remote locally.
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There's also the problem that we do not reflect the fact that the
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git-annex objects are (potentially) in multiple syncthing remotes, and
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thus changing the number of copies. Even worse, once a file is dropped
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on one syncthing remote, it gets dropped everywhere. The solution for
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this of course is simply treat syncthing as a single copy of the
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objects. Note that this also applies to the shared objects method
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above.
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This can be easily implemented with the [[special_remotes/directory]]
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special remote:
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git annex initremote syncthing type=directory directory=$HOME/Sync/ encryption=none
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git annex describe syncthing "default syncthing directory"
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git annex untrust syncthing
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Note that the last step isn't necessary if the syncthing folder is
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marked as "master".
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Communicate with the API
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------------------------
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Another way would be to talk directly to the [REST API][] (there's
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also a separate [event API][] for GUIs). Currently, this doesn't seem
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to hold much promise because the APIs are mostly read-only and don't
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allow adding objects at all, for example.
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[REST API]: http://docs.syncthing.net/dev/rest.html
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[event API]: http://docs.syncthing.net/dev/events.html
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Reimplement the protocol
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------------------------
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This would involve writing a syncthing client using the
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[Block Exchange Protocol][] specification. This would allow more
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complete control over the distribution of objects and so on,
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respecting git-annex's wanted/required content policies while at the
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same time sharing the data with other syncthing endpoints. It would
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also allow for tracking the number of copies of the objects and so on.
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Of course, this is a major undertaking and probably the hardest
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approach, but also the one potentially giving the most benefits.
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