Best approach to send data from a server to an Android device

Question

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

Answer 1

This is what XMPP excels at. :)

See the Smack Java library, or asmack for an Android-customised version.

And for an Android app that is using asmack for long-lived bidirectional communications, see BuddyDroid.

Edit, in reply to comment below:
The advantage of this approach in your case is that you can push unlimited amounts of to the client immediately, without the latency (or non-guaranteed delivery) of SMS. XMPP also gives you connection security, compression and the ability to layer your own extensions on top of it.
In addition — and I don't know who you're targeting or whether this applies — this way you don't need to rely on the user having an unlimited text plan (but of course you need a data plan).

See Prosody or Tigase if you're interested and looking for a server (there are several open source ones).

However, having re-read your question, you seem to imply a low rate of activity so perhaps it's not worth maintaining (login, detecting connection loss, reconnecting) an XMPP session for whatever your application is.

Finally.. while you can intercept incoming SMS messages on an Android device, you cannot prevent them from showing up in the user's messaging inbox. So you can process the incoming SMS, but you can't then tell the system to discard the message.
I recently discovered this isn't true: if you define an android:priority attribute on your SMS-listening intent-filter, you will then receive an "ordered broadcast" which allows you to cancel the incoming SMS broadcast before it's propagated to other apps, like the SMS inbox. Note that this only works on Android 1.6 and above.