How to detect online users in a web application in Google App Engine

I am currently working on an application for Google App Engine, and I need some advice to detect the number of online users in the application. How can I do this?

I am using a session library. Do I need to overwrite the session methods (create_session, destr开发者_运维百科oy_session increment/and decrement a value in datastore) or is there another method that I can use?

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HTTP is stateless, so there's no inherent definition of "online user". You could count the number of non-destroyed sessions you've created, but unless you've got a cron job that destroys old sessions, this won't give an accurate picture.

You basically need to decide how much time without a new page request counts as "online" and query for the sessions that have been updated in that range of time.

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You may use channel api to maintain a connection with the client. "The Channel API creates a persistent connection between your application and Google servers, allowing your application to send messages to JavaScript clients in real time without the use of polling. " http://code.google.com/appengine/docs/java/channel/overview.html