How the Pusher team built subscription counting at scale

The Pusher development team recently introduced Subscription Count events to manage large presence views within their Channels apps, addressing previous limitations of the Presence channels that were effective for small groups but not scalable for larger audiences. The traditional method of using Presence channels allowed for a maximum of 100 subscribers, beyond which it required frequent API polling to update subscriber counts, which was inefficient for channels with thousands of users. To solve this, Pusher devised an event-driven model that uses Redis to manage connection counts and employs a locking mechanism to broadcast updates efficiently in large channels, minimizing message overload. The implementation distinguishes between small and large channels, broadcasting updates immediately for the former and employing a 5-second broadcast lock for the latter to manage high-frequency updates. While this approach streamlines scalability, it poses a trade-off by potentially delaying accurate count updates during high join activity within the lock period, making it less suitable for use cases requiring precise real-time updates. The Pusher team continues to explore more complex solutions to further optimize this feature for diverse applications.

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