java - How do I implement online/offline feature with REST based application? -

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i building chat feature(web application) in user has list of friends can chat with. friends can online or offline, in facebook. since, application built on rest apis, don't save session , of authentication stateless.

so question is, how know online or offline among list? best way implement this?

usually chat applications send message client server after set time period, if message not received, user offline.

i not java developer, suggest websockets chat. hope helps.


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