Distributed systems

Distributed systems are useful for various purposes, but the common/achievable niceties are:

These are mostly shared with federated systems, but take it a bit further.

Existing systems

There is quite a few of them; I am going to write mostly about those which work over Internet, though mesh networks which are based on lower levels are interesting, too. Grid computing systems, like BOINC, are nice as well, but it is not about them. There's also a related Wikipedia category.

Generic networks

Tor and I2P: both support "hidden services", on top of which regular protocols could be used, but it is more about privacy (and a bit about routing) than about decentralisation: they provide NAT traversal and static addresses, but that's it. Tor documentation is relatively nice, and there is plenty of I2P docs. Tor provides a nice C client, I2P uses Java – what makes Tor considerably easier to install.

Mesh networks

Some mesh networks, like Telehash, provide routing as well, though advantages for decentralisation seem to be similar to those of Tor and I2P; just better in that they extend it beyond internet. Telehash documentation is also pretty nice and full of references.

Cjdns (or its name, at least) seems to be relatively well-known, but it relies on node.js.

Netsukuku and B.A.T.M.A.N. are two more protocols the names of which are known.

Those would be nice to get someday, but they would require quite a lot of users to function, and various government restrictions seem to complicate their usage (this varies from jurisdiction to jurisdiction and from year to year, but seems to be pretty bad in Russia in 2018).

IM and other social services

File sharing and websites

Search

Web crawling

YaCy and a few more (some of which are dead by now) distributed search engines exist. I have only tried YaCy, and it works, though haven't managed to find its technical documentation – so it's not clear how it works.

Other information

These networks include search for files, but by their names, not content-addressable (so they can't be easily verified, which brings additional challenges).

Related papers:

Cryptocurrencies

Plenty of those popped up recently. Looks like quite a waste of electricity and hardware to me, yet the idea itself is interesting.

GNUnet

Not sure how to classify it, but here are some links: gnunet.org, wiki://GNUnet, A Secure and Resilent Communication Infrastructure for Decentralized Networking Applications. Seems promising, but tricky to build, to figure how it all works, and to do anything with it now (a lack of documentation seems to be the primary issue, though probably there's more).

Taler and secushare (using PSYC) are getting built on top of it, but it's not clear how's it going, how abandoned or alive it is, etc. Their documentation also seems to be obsolete/outdated/abandoned/incomplete. Update (January 2018): apparently secushare prototype won't be released this year.

Generic protocols

There are more or less generic network protocols that may be used on top of e.g. Tor, to get usable distributed services.

Plan 9's 9P

9P looks nice: it's simple, documented, and generic. Security in Plan 9 is also nice. It's somewhat future-proof, though has certain limitations (though it's a rabbit hole to try to make something distant-future-proof). There's not much of software that supports it, and hacking authentication usable for distributed services into it might be tricky.

SSH

SSH is quite nice and layered. But apparently its authentication is not designed for distributed systems (such as distributed IMs or file sharing), its connection layer looks rather bloated, and generally it's not particularly simple. Those are small bits of a large protocol, but they seem to make it not quite usable for peer-to-peer communication.

TLS

TLS may provide mutual authentication, and there is plenty of tools to work with it.

Ad hoc messaging

Pretty much every distributed IM tries to reinvent everything, and virtually none are satisfactory, but at least some of the problems are already solved separately, and there are:

Combinations such as Tor + TLS + rlwrap may serve as ad hoc IMs rather easily. Well, if participants are willing and able to use those, which often isn't the case. In an attempt to try and facilitate things like that, I wrote TLSd and a P2P IM with a libpurple plugin as one of the examples.

And my another attempt to get distributed messaging with existing tools is XMPP over Tor.

Users

Distributed systems, particularly when used for social activities, require users – so that there would be somebody to send messages to in case of an IM. That's quite a problem, since even by sticking to federated networks it is easy to lose or decrease contact with most people one knows, even if those are tech savvy; apparently it's even easier when moving to distributed and less common ones.

In my experience so far, for personal communication it works like this:

Apart from not being backed by a huge company for marketing and not having a lot of users using them already, probably the primary issue for non-centralised systems is that there's usually little focus on finding new contacts and establishing new relationships.

Though XMPP MUCs and that microblogging fediverse thing (see OStatus above) may be suitable for it; not distributed, but at least closer to it. A nicer approach would be to use distributed search, but I'm not aware of generic/suitable systems of that kind: they tend to either be an easy target for spammers, or require existing social connections in a system (although some may be suitable still).

Search, FOAF, and the rest of RDF

Some kind of a distributed search/directory may connect small peer-to-peer islands into a usable network. While it is hard to decide on an algorithm, lists of known and/or somewhat trusted nodes are common for both structured and unstructured networks, as well as for use of social graphs: if those would be provided by peers, a client may decide by itself what algorithm to apply. This reduces the task to just including known nodes into local directory entries, which can be shipped over any other protocols (e.g., HTTP over Tor).

Knowledge representation, which is needed for a generic directory structure, is tricky, but there is RDF (resource description framework) already. And there even is FOAF (friend of a friend ontology), specifically for describing persons, their relationships (including linking the persons they know), and other social things. There's a FOAF search engine, too: it's centralised and searches mostly in large centralised places, but such an engine can relatively easily be set locally, searching for users and information they provide in a distributed, peer-to-peer network. See also: Semantic Web.