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Apply Pub/Sub Patterns

Tarant implements a basic, but powerful pubsub mechanism named topics. A topic is a stream of events that are sent to all subscribers of the topic. For example, if we send the event hello to the topic salutations, and there are three subscribers, all of them will receive a copy of hello.

Messages send through a topic are guaranteed to be processed only once by the topic and in order.

How to create a Topic

A Topic, [like an actor](/docs/how tos/how-to-create-an-actor), has a protocol. All subscribers to that topic need to satisfy also the protocol of the topic. For example, if we have a protocol:


All subscribers of the topic will need to implement foo() and bar().

So first, we need to define a protocol:

class UserEvents {
onUserRegistered(username) {}

And then we need to create an actor protocol that satisfies the UserEvents protocol.

class EmailSender extends Actor {
onUserRegistered(username) {


Now we can create a topic for that protocol and subscribe to it:

const topic = Topic.for(system, 'user-events-topic', UserEvents)

For sending a message through a topic, we need to call the method that represents the message:


And all subscribers of that topic will receive the message. Is worth to note that a topic is also an actor, so all properties of an actor also apply for topics.

Fore a example on how to use PubSub, you can see the showcase about pubsub.