How does Trello show users online/offline/idle state accurately?

Question

Trello shows the information of online/offline/idle status very accurately as I personally saw. There are several solutions for tracking users online statuses, and to do so accurately requires a serious overhead. There are also the traditional problems of not knowing if a user closed the browser, disconnected etc. that cause inaccuracy in showing the offline/idle status. For a platform that does not have chat functionality (which, I believe, which would have taken the overhead of tracking users' status and therefore makes the task finally easy) how is Trello achieving this, albeit without much overhead?

Also Trello is able to detect disconnects and resume connection (not so smoothly though).

The Pushing and Polling layer of Trello is described here

Answer 1

Each browser tab is considered to be a session, associated with one user. Each session's activity level is tracked separately, with a user being shown with the highest activity level of all of its sessions (where active is highest, then idle, then disconnected).

Each tab periodically sends its current status to the server - every minute if it has a working WebSocket, every 5 minutes otherwise. Current status is reset to active every time the tab gains focus, or there is a keydown or mousedown event. It goes idle if it has been more than 5 minutes since the most recent of those events. The tab sends a status of disconnected in the onunload event, but we don't depend on this.

Every time a session sends its status to the server, the server looks at all of that user's sessions to determine whether that indicates a change to the user's status. If it does, it broadcasts that new status to every person watching every board that user is on (this goes through the same framework that the rest of Trello's instant updates go through, and is too complicated to explain here).

The server also checks each session to see if its been too long since we've heard from that session (2 minutes if its a WebSocket session, 10 minutes otherwise) and removes the session if it has been (removing a session indicates it is disconnected).

There are a couple of optimizations hiding in there, but that's the main story.

Answer 2

Trello using socket.io.. so when ever a user login it broadcast a message that user has logged in , then they show him as online. Once he disconnects they send a message on socket again to show him offline.

Many realtime apps using socket.io for showing realtime activities.