This actor manages a registry of actor references and replicates the entries to peer actors among all cluster nodes or a group of nodes tagged with a specific role.
The DistributedPubSubMediator is supposed to be started on all nodes, or all nodes with specified role, in the cluster. The mediator can be started with the DistributedPubSub extension or as an ordinary actor.
Changes are only performed in the own part of the registry and those changes are versioned. Deltas are disseminated in a scalable way to other nodes with a gossip protocol. The registry is eventually consistent, i.e. changes are not immediately visible at other nodes, but typically they will be fully replicated to all other nodes after a few seconds.
You can send messages via the mediator on any node to registered actors on any other node. There is three modes of message delivery.
1. Send - The message will be delivered to one recipient with a matching path, if any such exists in the registry. If several entries match the path the message will be sent via the supplied `routingLogic` (default random) to one destination. The sender of the message can specify that local affinity is preferred, i.e. the message is sent to an actor in the same local actor system as the used mediator actor, if any such exists, otherwise route to any other matching entry. A typical usage of this mode is private chat to one other user in an instant messaging application. It can also be used for distributing tasks to registered workers, like a cluster aware router where the routees dynamically can register themselves.
2. SendToAll - The message will be delivered to all recipients with a matching path. Actors with the same path, without address information, can be registered on different nodes. On each node there can only be one such actor, since the path is unique within one local actor system. Typical usage of this mode is to broadcast messages to all replicas with the same path, e.g. 3 actors on different nodes that all perform the same actions, for redundancy.
3. Publish - Actors may be registered to a named topic instead of path. This enables many subscribers on each node. The message will be delivered to all subscribers of the topic. For efficiency the message is sent over the wire only once per node (that has a matching topic), and then delivered to all subscribers of the local topic representation. This is the true pub/sub mode. A typical usage of this mode is a chat room in an instant messaging application.
4. Publish with sendOneMessageToEachGroup - Actors may be subscribed to a named topic with an optional property `group`. If subscribing with a group name, each message published to a topic with the `sendOneMessageToEachGroup` flag is delivered via the supplied `routingLogic` (default random) to one actor within each subscribing group. If all the subscribed actors have the same group name, then this works just like Send and all messages are delivered to one subscribe. If all the subscribed actors have different group names, then this works like normal Publish and all messages are broadcast to all subscribers.
You register actors to the local mediator with Put or Subscribe. `Put` is used together with `Send` and `SendToAll` message delivery modes. The `ActorRef` in `Put` must belong to the same local actor system as the mediator. `Subscribe` is used together with `Publish`. Actors are automatically removed from the registry when they are terminated, or you can explicitly remove entries with Remove or Unsubscribe.
Successful `Subscribe` and `Unsubscribe` is acknowledged with SubscribeAck and UnsubscribeAck replies.
