[android] Does Android support near real time push notification?

They have their listeners which has to be used by you by using their library classes in your code. You need not to bother about pushing. You have to send the message to server server will push the message to the device. They use OAuth. Regarding Protocols, there are two methods using CCS and XMPP. CCS just uses XMPP as an authenticated transport layer, so you can use most XMPP libraries to manage the connection. To send notifications to device you can write code in android app to send as well as your server code. the message sending will be done only by your code. Rest will be taken care by Google Server in GCM case. You can check detail at this link

http://developer.android.com/google/gcm/server.html

Also, for security issues

google cloud messaging security https://groups.google.com/forum/#!topic/android-gcm/M-EevBitbhQ

In case your app is not running then also devices can recieve notification because you have to write code for broadcast listeners. In background it will be listening to server and whenever any message packet will be there it will recieve the message as notification. Android has service you need to not to bother about it. You have only to use those resources using the library class that makes your work easier and let them write if your app is not running then also it recieve notification. Obviously, there would be some listener whick make the app to recieve.Check "Recieve the message" section in this link

http://developer.android.com/google/gcm/client.html

It will acccept request from users also. For GCM it will do. Please check "Send a message"

http://developer.android.com/google/gcm/client.html