RFC: expose erlang pg's scope ability for Phoenix.PubSub.PG2

[xiaoqinglee]

Process groups can be organised into multiple scopes. Scopes are completely independent of each other. A process may join any number of groups in any number of scopes. Scopes are designed to decouple single mesh into a set of overlay networks, reducing amount of traffic required to propagate group membership information. Default scope pg is started automatically when kernel(6) is configured to do so.

start_link/1

pg was contributed by Maxim Fedorov from WhatsApp. Maxim’s work is focused on performance and scalability of the server side.

pg

WhatsApp deprecated and removed wandist. With pg it is possible to create any number of disjoint sub-clusters (by running several pg scopes).

[josevalim]

We don't plan to expose this in our default implementation but you can provide your own adapters to PubSub that relies on scope. Thank you!