Reliability of C2DM

问题:

I am having issues with C2DM. Sometimes works perfectly, sometimes my messages simply do not get pushed. Is there reliable way to enforce this connection? To pull messages. I read somewhere that what google do is keep low bandwidth TCP connection to their server at all time. So I assume that when switching between network types TCP connection falls down and Android tries to reestablish connection to C2DM servers. So that might fail on WiFi with restricted network. Is that wrong assumption?

I have noticed with WhatsApp that on WiFi sometimes I do not get messages. When I switch to 3G I usually get them at the moment of the switch. What tips from your experience with C2DM would you offer?

回答1:

C2DM is not suitable for critical parts of your application, since Google currently does not offer an SLA or paid tiers that will guarantee you reliable service and throughput.

I've considered several alternatives myself: XMPP via asmack, Parse, Deacon, Urban Airship, and MQTT.

After some reading and experimenting I decided to go with MQTT. It is a very lightweight telemetry protocol invented at IBM that fits quite nicely in the Android push notifications scenario. I recommend you give it a try, here's a nice blog post to guide you: Using MQTT in Android mobile applications.

Hope this helps.

回答2:

C2DM does not guarantee that your message will be delivered, and your application should not assume that in order to work correctly. Therefore, your C2DM message should never contain the data itself but, rather, a notification that there is data available. In other words, the loss of a C2DM message should never cause your application to lose data; it should, at most, cause it to take longer to notice that a certain piece of data is available on your server.

A typical app should connect to its server once in a while (a long while) to retrieve messages, even when using C2DM, to cover the case where C2DM messages might not be delivered.

Depending on network configuration, the device might not be able to receive C2DM messages; restrictive firewalls or other strange WiFi configs might do that.

回答3:

• With C2DM reliability is not a guarantee. So best to have a ACK message or some way in which you (the sender) the realize the message was received successfully.
• Also make it a point to override onRegister class properly because the device Reg ID keep on shuffling.
• Lastly, if you are planning to send updates regularly, I'd prefer polling to C2DM just because of the amount of requirements to get it functioning while reliability and ultimate control is still not assured.

回答4:

I've been struggling with the same problem myself. The behaviour you describe is accurate. I'm developing an app that uses c2dm mainly with Wifi connection, and I had to implement an AsyncTask to periodically (minute and a half) call WifiManager.reassociate() (turning wifi off and on again triggers the arrival of all pending notifications, that's what inspired this solution) so I can keep the notification arrival as accurate as possible. Not so sure about the correctness of this practice, though.

回答5:

Have you tested it every 15 minute connection? I created a schedule task to send the message. I use NotifyMyAndroid to push it. C2DM sometime pushes the message about 10 mninutes after not instantly. But, sometimes you get it in around a second.

回答6:

The best way you can do this is by testing. I have a mechanism in my app that when I enable debugging I receive a HTTP request from the client saying that they received the message.

I find that this number is about 80%. Fortunately that's enough for the scope of my application.

Wifi shouldn't interfere in the C2DM ability of receive messages. At least while the phone is active.

What happens is that android turns the wifi OFF after the phone is in standby for a while. The messages won't be available at that period of time, simply because there's no internet connection available. Right after the user wakes the phone up, they should receive the messages.

回答7:

After a long time research pretty much "all the internet" for an answer, I've found it. As I posted before, I was struggling with the problem myself and found out it was not a C2DM problem, or even a implementation problem. It was simply a router or firewall misconfiguration. Android uses a persistent TCP connection with a heartbeat keep-alive mecanism to ensures that the connection stays up. Google uses the state of the connection to determine if your device is idle or not. But if your router has a protection policy that checks for "unused" connections and terminates them, that won't work. Android notifications should be delivered (close to) instantly. I've tested this in my school network and home network, with two diferrent behaviour.

To resume: be sure to check your network configurations.

回答8:

Some APNs work better than others with C2DM. Google "gtalk apn", for example, to find forums about the impact of the APN on C2DM.