I'm looking for Flux combined with React and I read a lot's about Dispatcher, Actions and Store. But I didn't understand if Flux handle (natively) data update on server side by broadcasting sockets as 'Actions' ? (like MeteorJS do with publish/subscribe)
Or do I have to handle an external API and sockets call alone ?
No, this is not handled by the library at all. Any implementation of this functionality would need to happen in user code (or in some other library).