将数据从服务器发送到Android设备的最佳方法

I am developing an Android app that needs to communicate bi-directionally with a server. By that, I mean either the server or the device can send a message at any time, with an arbitrary amount of time in between messages. Sending data from the device to the server is a common and I think well understood task, but I'm not as sure what the best approach is to go in the opposite direction from the server to the device.I am developing an Android app that needs to co