What is Pub/Sub?
Ably Pub/Sub enables you to implement the publish-subscribe (pub-sub) pattern. Any number of publishers can send messages to a channel, and any number of subscribers can receive those messages. Publishers and subscribers are completely decoupled from one another. Channels are used to separate messages into different topics. Messages contain the data that a client is communicating, such as the contents of an individual chat message, or an event that has occurred, such as updated financial information. Whilst billions of messages may be delivered by Ably, clients receive only the messages on the channels they subscribe to.Use cases
Pub/Sub is ideal for building realtime applications that require:- Live updates: Push data updates to multiple clients in realtime, such as stock prices, sports scores, or news feeds
- Chat and messaging: Build chat rooms, direct messaging, or group conversations
- Notifications: Send alerts and notifications to users across devices
- Collaborative apps: Enable multiple users to work together in realtime
- IoT and telemetry: Stream sensor data and device updates
- Live dashboards: Display realtime metrics and analytics
Key features
The following features are some of the basics that you need to develop realtime applications:Publish and subscribe
Publishers send messages to channels, and subscribers receive those messages. This decoupled architecture allows you to build scalable realtime applications without tight coupling between components.Message history
Messages received by Ably are stored in memory for 2 minutes in every location that the channel is active in. This enables Pub/Sub SDKs to automatically retrieve them in the event of network connectivity issues, or a lost connection, as long as the connection is re-established within 2 minutes. Messages can be stored for much longer on disk by Ably through additional configuration. The history feature can be used to retrieve previously sent messages as a paginated list. History can retrieve messages from as far back as the message persistence configured for a channel.Presence
The presence feature enables clients to be aware of other clients that are currently present on a channel. Subscribers receive three types of updates from presence members. These are when a client joins the presence set, when they leave the presence set, and when they update an optional payload associated with each member. The payload can be used to describe their status, or attributes associated with them, such as setting their status to ‘out for lunch’. Presence is most commonly used as an online indicator to create an avatar stack for an application, or to notify occupants of a chat room that a member has joined or left.Occupancy
Occupancy provides high level metrics about the clients attached to a channel. This includes the number of connections currently attached to a channel, and the number of connections attached that are permitted to publish and subscribe to the channel.Push notifications
Push notifications notify user devices whether or not an application is open and running. They deliver information, such as app updates, social media alerts, or promotional offers, directly to the user’s screen. Ably sends push notifications to devices using Google’s Firebase Cloud Messaging service (FCM) and Apple’s Push Notification Service (APNs). Push notifications don’t require a device to stay connected to Ably. Instead, a device’s operating system maintains its own battery-efficient transport to receive notifications. You can publish push notifications to user devices directly or via channels.Realtime and REST
Pub/Sub SDKs provide a consistent and idiomatic API across a variety of supported platforms and are the most feature-rich method of integrating Ably into an application. Ably SDKs contain a realtime and a REST interface, each of which can be used to satisfy different use cases.REST interface
The REST interface communicates with Ably using the HTTP protocol and is effectively stateless. The REST interface provides a convenient way to access the REST HTTP API and is intended to be used by clients that don’t require realtime updates. It is more commonly used server-side. It is used to:- publish messages
- publish messages on behalf of other clients
- issue tokens on behalf of other realtime clients
- retrieve persisted messages, presence history and application usage statistics
Realtime interface
The realtime interface is implemented using an Ably-defined protocol, primarily over WebSockets. It enables clients to establish and maintain a persistent connection to Ably. The realtime interface extends the functionality of the REST interface and is most commonly used client-side. It is used to:- maintain a persistent connection to Ably
- attach to one or more channels, and publish and subscribe to messages to them
- register their presence on a channel, or listen for others present in realtime
- publish at very high message rates, or with the lowest possible realtime latencies
Next steps
- Follow the quickstart guide to start building with Pub/Sub
- Learn about authentication to secure your applications
- Explore channels to organize your messages