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improved gossip topology (#3270)

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* gossip-support: gossip topology

* some fixes

* handle view update for newly added gossip peers

* fix neighbors calculation

* fix test

* resolve TODOs

* typo

* guide updates

* spaces in the guide

* sneaky spaces

* hash randomness

* address some review nits

* use unbounded in bridge for subsystem msg
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Andronik Ordian
2021-06-18 21:30:35 +02:00
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commit ad9c02886d
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polkadot/roadmap/implementers-guide/src/node/utility/gossip-support.md
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# Gossip Support
The Gossip Support Subsystem is responsible for keeping track of session changes
and issuing a connection request to all validators in the next, current and a few past sessions
if we are a validator in these sessions.
The request will add all validators to a reserved PeerSet, meaning we will not reject a connection request
from any validator in that set.
and issuing a connection request to all validators in the next, current and
a few past sessions if we are a validator in these sessions.
The request will add all validators to a reserved PeerSet, meaning we will not
reject a connection request from any validator in that set.
Gossiping subsystems will be notified when a new peer connects or disconnects by network bridge.
It is their responsibility to limit the amount of outgoing gossip messages.
At the moment we enforce a cap of `max(sqrt(peers.len()), 25)` message recipients at a time in each gossiping subsystem.
In addition to that, it creates a gossip overlay topology per session which
limits the amount of messages sent and received to be an order of sqrt of the
validators. Our neighbors in this graph will be forwarded to the network bridge
with the `NetworkBridgeMessage::NewGossipTopology` message.
We also flip a coin with the same probability when handling peer view updates in the distribution subsystems.
Over time the probability of not handling a peer view update converges to zero, so it shouldn't be cause much trouble.
This should be considered as a temporary measure until we implement a more robust solution for gossiping.
See https://github.com/paritytech/polkadot/issues/3239 for more details.
The gossip topology is used by parachain distribution subsystems,
such as Bitfield Distrubution, (small) Statement Distributuion and
Approval Distibution to limit the amount of peers we send messages to
and handle view updates.