A pub sub topic is an actor that handles subscribing to a topic and publishing messages to all subscribed actors.
It is mostly useful in a clustered setting, where it is intended to be started once on every node that want to
house subscribers or publish messages to the topic, but it also works in a local setting without cluster.
In a clustered context messages are deduplicated so that there is at most one message sent to each node for
each publish and if there are no subscribers on a node, no message is sent to it. Note that the list of subscribers
is eventually consistent and there are no delivery guarantees built in.
Each topic results in a akka.actor.typed.receptionist.ServiceKey in the akka.actor.typed.receptionist.Receptionist
so the same scaling recommendation holds for topics, see docs: