A consensus algorithm (Raft, Paxos) picks one node as the writer/coordinator.
Pick exactly one node to coordinate, and re-pick safely when it dies, without ever ending up with two leaders.
Each partition has an elected leader replica that handles all reads/writes; on broker failure a new leader is elected from the in-sync replicas
ZooKeeper/etcd elect a coordinator for ring membership and config; cache nodes follow the elected leader's view
The single active matching engine per symbol is the leader; a hot standby is promoted via election on failure to keep one authoritative order book