[jdev] Semantic question

Dave Cridland dave at cridland.net
Thu Apr 30 10:54:39 UTC 2015

On 30 April 2015 at 11:23, Goffi <goffi at goffi.org> wrote:

> G'day,
> for years I have used decentralised, distributed, and federated with, in
> my head, the following meaning:
>         - decentralised: the ability to have several servers communicating
> together, the servers can be under the same domain (example.net can have
> several servers)

By "decentralised", I really just mean it has no centre - there aren't any
special, or especially privileged, servers.

XMPP fits this description, but not all services built on XMPP do - for
example, if we had a single user directory (and I think one such did exist
at one point) that would be a centralised service.

Matrix has identity servers, which are a centralised trusted naming
service, DNS is similarly centralised (due to the root service).

In neither case are the "centralised" servers a single entity; they're just
a privileged set, providing a distributed service.

Of course, these all assume actual servers, and a service defined in terms
of a simple protocol. For a particularly odd example, consider PKIX, where
we have multiple, fully independent, Certification Authorities forming a
heterogeneous privileged set of providers - PKIX is clearly not
decentralised, but has multiple central points...

>         - distributed: 1 server = 1 user, no intermediate (not even DNS,
> so XMPP is not distributed according to this definition, but something like
> retroshare is)
Your definition fits what people tend to mean by "peer to peer", although

Almost anything can be described as distributed.

It might mean a service which is available equivalently at multiple points
on the network. Which really doesn't say anything useful.

It might mean a service which is provided equivalently by multiple points
on the network; that's a slightly tighter definition, but covers clustered
XMPP servers, for example.

I suspect people usually intend to mean a service which is provided
equivalently by a decentralized set of providers, and in extremis they can
mean peer to peer.

Well, no.

People usually seem to mean "whatever it is that we do", and use it
liberally on marketing brochures.

>         - federated: the ability from servers of different domains (
> example.net and capulet.lit) to talk together, in both directions.
Yes, or more generally, the ability for multiple disparate administrative
domains to intercommunicate on an equal basis.

> But after a talk I realise that the definitions accepted is not the same
> everywhere, e.g. Diaspora people talk about federation for what I call
> decentralisation. Actualy it's a bit tricky, because if one entity has 1000
> servers but everybody is using the same domain, the data are centralised in
> the hands of the same entity.
> So, what meaning do you put behind these words ? Are decentralisation and
> federation more or less synonyms ?
> Sorry to put this on jdev@, I was not sure of which mailing list to use :)
> Goffi
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