android multi player game over network

Question

I'm programming an Android multi-player game, which basically consist of a server where the clients connect and exchange messages. When the player connects to a server, a player list is return to him/her. A player can then select a user to challenge - of course he must select a player from the player list, which only contains connected users.

When a player1 challenges player2, a message needs to be transmitted from player1 to the server, which in turn must send a message to the player2, notifying him about the challenge. The player2 can then accept/decline the challenge.

I can use the following techniques to make this happen:

1. Use custom server/client with Java socket programming. The server basically accepts a connection from the client, spawning a new thread for each connected client. The problem with this are:

   • There needs to be a persistent connection open from client to server wasting battery life of the android phone. This is not really big limitation since the battery isn't consumed that much.
   • When I'll want to develop another game I'll have to rewrite the client/server code from the scratch - also choosing another port to listen for incoming connections - the whole concept gets rather difficult to maintain.
   • I'm also worried if this is the way to do it. Spawning another thread for each clients sound quite a lot if thousands clients are connecting at the same time. But I'm guessing the PC games do it like this. Not sure about android.

2. Use Java REST jersey to build the client-server on top of HTTP. This would be a perfect solution if the server could easily send notifications to clients. There are actually multiple design decisions here:

   • the client pulls the server for any new data/notifications every few seconds - this is really bad, since we're stuck with non responsiveness, delay, etc.
   • the client can send a waiting request to server, so the client receives the response only after some data becomes available. This is better, but can still produce a delay when two notifications one after another need to be sent to the user. The first notification is sent instantly, since the client already has a connection open, waiting for data to receive. But we would have to wait for the client to initiate another long http request to receive the second notification. The problem gets bigger as there are multiple notifications that need to be send in a row to a specific client.
   • the client can initiate a http streaming, where the communication is left open when the request is handled, so the server can also send multiple messages to client whenever it wishes. The problem here is that I don't know how well this works on Android. I've looked at several implementations:
     • Java jersey + atmosphere: didn't succeed in actually making it work. This seems the most promising, but I don't want to spend too much time on it, since I'm not even sure if it does what I want.
     • Deacon: seems pretty neat, but after seen the video tutorial on their official web page, I'm not sure that it can do what I need. When a player1 challenges player2, can it send a notification to player2 letting it know about the match request?

3. I would be glad to know how other multi-player games handle the network communications, if the two players are playing the game over the network.

4. I'm also open to a totally new suggestion how to achieve what I want. I can pretty much code anything, so don't hesitate to let me know of some more difficult way to achieve the network communication.

Let me also mention that I'll be glad to implement a totally specific method to work in my case, so it can be anything that will do the job done, but I'm also looking at more general way for communication between clients and server. So that I can program an interface/whatever and reuse the code in other android games, android applications.

I hope I presented the problem allright and that I'll receive some valuable answers.

Thank you

Answer 1

You should take a look at XMPP. It's a protocol (originally created for chat programs) that allows sending of xml data between users.
It has a separated client-server relationship, so that you can focus on developing a client application fit for phones, and a different server depending on your needs.

There are loads of information available on the protocol (I should know, I wrote a thesis about using the protocol in game applications), but you can start by looking it up on wikipedia to see if it is what you want.

aSmack is a library for creating android xmpp-clients. It takes some tweaking to set it up and get everything to work, but once you do, it's neat.

EDIT: relating to the answer suggesting using the C2DM:
from the c2dm docs "Sending large numbers of C2DM messages":

Are you sending C2DM messages too frequently? If you need to communicate with your application frequently over a short period of time, C2DM is probably not the best solution. Instead, consider implemeting XMPP or your own protocol to exchange messages, and use C2DM only to send the initial notification.