Publish & Subscribe

Redis’ Publish and Subscribe model is really useful for notifications.

Use cases

Pub/sub is used for notifying subscribers of an event. A simple and common event for example would be a chat message.

A channel consists of a name and group of listeners. Think of it as being [String: [Listener]]. When you send a notification to a channel you need to provide a payload. Each listener will get a notification consisting of this payload.

Channels must be a string. For chat groups, for example, you could use the database identifier.

Publishing

You cannot get a list of listeners, but sending a payload will emit the amount of listeners that received the notification. Sending (publishing) an event is done like so:

// Any redis data
let notification: RedisData = "My-Notification"
client.publish(notification, to: "my-channel")

If you want access to the listener count:

let notifiedCount = client.publish(notification, to: "my-channel") // Future<Int>

Subscribing

To subscribe for notifications you’re rendering an entire Redis Client useless in exchange for listening to events.

A single client can listen to one or more channels, which is provided using a set of unique channel names. The result of subscribing is a SubscriptionStream.

let notifications = client.subscribe(to: ["some-notification-channel", "other-notification-channel"])

If you try to use the client after subscribing, all operations will fail. These errors are usually emitted through the Future.

This stream will receive messages asynchronously from the point of draining. This works like any other async stream.

Notifications consist of the channel and payload.

notifications.drain { notification in
  print(notification.channel)
  let payload = notification.payload
  // TODO: Process the payload
}