If I want to make sure that a device receives a message, over the network, for an app used by sellers in a shop for example.  

I heard push notifications are not 100% reliable, sometimes some of the notifications don't arrive, or not on time. 

The app could be in a shop, where the staff communicate with one another, and there could be 10 devices connected. (ipad, iphones)

edit 1: I heard about sockets, is it the right direction to go?

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EDIT 2: 

I am not sure why a socket should be used, rather than a webserver for example, I found these 2 sentences (source raywenderlich) :

* You can send connected clients data whenever you want, rather than requiring the clients to poll.
* You can write socket servers without a dependency of a web server, and can write in the language of your choice : don't understand what the "dependency" is?

Does it also mean :
Sockets let 2 (or more) specific devices to connect one another in a private connection, compared to webservers where everybody could connect if there is no login/password?

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**EDIT 3** : Maybe a bluetooth solution with MultiPeerConnectivity would be better...