User presence at scale
Blog post from Ably
User presence represents a user's online or offline status and is used in various applications such as chat systems, audience engagement platforms, medical apps, education software, and online collaboration tools. It helps build a sense of immediacy between participants by indicating who else is online and can be customized to display the user's status. Ably, an event-driven system that uses pub/sub messaging, offers feature-complete messaging, including presence features. However, as the number of people subscribed to a presence set increases, there is quadratic growth in presence messages sent, which can lead to high costs and message limits. To prevent this, Ably imposes a limit on the number of clients that can enter the presence set of any channel.