ipc-pubsub

Simplified IPC over POSIX shared memory, with a publisher/subscriber interface.

Design Notes

Hub maintains a list of topology events that it sends to each new client.
Hub is responsible for minting new Topology Events when a Client sends updates about themselves
New hubs may be missing history (for instance if a just-started client wins the race to create a hub) or create history that conflicts with the spokes ( for instance if a new client with no history attachs and registers itself befure the rest of the clients send the full history).
Upon connection clients will send their full version of history which the hub will integrate into a complete history. The hub is the only source of truth on history so if it sends updates that contradict clients, the clients must accept the new history and trigger events for them. This should be rare though
The hub 'mints' new history events by giving them a sequence (seq) and unique ID (uid) number. It then sends these events to all clients.

The purpose of the Topology Client is to call callbacks for topology update events. In general duplicate callbacks for events should be avoided, however during Hub transfer (the hub dies and is restarted) there may occasionaly be a duplicate.
New clients will have no history and be sent the complete history upon connecting to the hub.
Its possible through various fail conditions that a client could have conflicting history with the hub. In this case all conflicts are removed and any events sent by the Hub should trigger additional Topology Events
During Hub transfer the Client will reconnect to the new hub and send 1. the entire history 2. and information about itself

Messages consist of a shared memory segment, the associated file descriptor is then sent over Unix Domain Socket to all subscribers, then the segment (and fd) is closed at the sender