Publisher-Subscriber

TLDR "Publisher" and "subscriber" form a pattern which enables objects to communicate asynchronously.

Problem/context

If we have a single concern that needs to trigger multiple other concerns, how can we have a setup that allows us to change or scale the concerns that need to be notified?

Concept

There are two roles:

1. A publisher which gets triggered by a single event that needs to be distributed.
2. One or more subscribers which are registered with the publisher and which get notified by the publisher when it is triggered.

Key characteristics

• One-way communication from publisher to subscriber; the publisher shouldn't care about what the subscribers do; this is not a request-response interaction.
• Subscribers can register or deregister with/from the publisher whenever they want.

Examples

A "general" channel in a chat-app that sends messages to all active members.

// Note: some class definitions and implementation details have been left out for brevity

/**
 * Usage
 */
const generalChat = new GeneralChat()

// user 1 logs on
const user1 = new User()
generalChat.register(user1)

// user 2 logs on
const user2 = new User()
generalChat.register(user1)

// someone types in the chat
generalChat.newMessage("I fear for the calendar. It's days are numbered.")

// user 2 logs off
generalChat.deregister(user2)

/**
 * Definitions
 */
class GeneralChat {
  private connectedUsers = new Set<User>()

  register(user: User[]) {
    connectedUsers.add(user)

    // notify general chat when a new user enters the chat
    this.newMessage(`${user.username} has entered the chat`)
  }

  deregister(user: User[]) {
    connectedUsers.delete(user)

    // notify general chat when a new user leaves the chat
    this.newMessage(`${user.username} has left the chat`)
  }

  newMessage(message: string) {
    for (let user of this.connectedUsers) {
      if (user.isActive()) {
        user.notifyGeneralChat(message)
      } else {
        // remove inactive users
        deregister(user)
      }
    }
  }
}

class User {
  notifyGeneralChat(message: string) {
    // show the message in the user's general chat
  }

  isActive() {
    // perform check to determine if the user is still active
  }
}

A real-word newsletter which sends new issues when they are published.