Matrix mitigates the threat of a hub & spoke model by not being hub & spoke (particularly in P2P!) :)
> I am not sure that spanning trees or gossip protocols solve the problem I'm describing, but, if they do, I'd appreciate if you could elucidate further.
Perhaps the confusion here is the expression "hub and spoke" which sounds to me like a static centralised star topology routing model.
My point is that if you have 1M nodes trying to share data (e.g. follow a celebrity's personal pubsub topic), you clearly need a smarter routing algorithm than full mesh (where the celeb's node would have to do 1M parallel pokes to send its messages). You're completely right that one solution is a hub-and-spoke model (where the celeb's node would poke a big centralised hub somewhere, which would then relay the poke out to 1M followers on spokes) - but as you point out, the hub becomes a centralised chokepoint of failure/control/privacy-violation etc.
So, the main other options I'm aware of are either to arrange your 1M nodes into a spanning tree (or overlapping spanning trees) of some kind, as Yggdrasil does (their current one looks like https://yggdrasil-map.cwinfo.org/#, although only ~300 nodes are live atm)... or you let the nodes self-organise into some kind of hierarchy based on gossiping and fan out the messages that way (as per https://blog.ipfs.io/2020-05-20-gossipsub-v1.1, and some of the experimental routing stuff we've been doing with Matrix).
> I am not sure that spanning trees or gossip protocols solve the problem I'm describing, but, if they do, I'd appreciate if you could elucidate further.
Perhaps the confusion here is the expression "hub and spoke" which sounds to me like a static centralised star topology routing model.
My point is that if you have 1M nodes trying to share data (e.g. follow a celebrity's personal pubsub topic), you clearly need a smarter routing algorithm than full mesh (where the celeb's node would have to do 1M parallel pokes to send its messages). You're completely right that one solution is a hub-and-spoke model (where the celeb's node would poke a big centralised hub somewhere, which would then relay the poke out to 1M followers on spokes) - but as you point out, the hub becomes a centralised chokepoint of failure/control/privacy-violation etc.
So, the main other options I'm aware of are either to arrange your 1M nodes into a spanning tree (or overlapping spanning trees) of some kind, as Yggdrasil does (their current one looks like https://yggdrasil-map.cwinfo.org/#, although only ~300 nodes are live atm)... or you let the nodes self-organise into some kind of hierarchy based on gossiping and fan out the messages that way (as per https://blog.ipfs.io/2020-05-20-gossipsub-v1.1, and some of the experimental routing stuff we've been doing with Matrix).