Send value of a variable from phone app to another phone real time

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http://stackoverflow.com – Im a little new to the android development, I believe I have the basics down but I am wondering the best way to communicate between two phones running the same app. I am looking for something that would be close to instant. For an example, if you sent a message or somekind of variable or string it would appear on the other phones app providing the app was open on both phones. Would be great if I could be pointed in the correct direction here, Thank you! (HowTos)