如果服务器崩溃，如何跟踪用户的“在线”状态？

I have multiple heroku dynos and a chat app. When a user logs in, their status is set to "online" in MongoDB. However, if a server crashes, their status will still be set as online. How can I update the user status to be "offline" when a server crashes?I have multiple heroku dynos and a chat app. Wh