基于MQTT实现Android消息推送(Push…
在Android手机平台上，Google提供了C2DM（Cloudto Device Messaging）服务，起初我就是准备采用这个服务来实现自己手机上的推送功能。
Android Cloud to Device Messaging (C2DM)是一个用来帮助开发者从服务器向Android应用程序发送数据的服务。该服务提供了一个简单的、轻量级的机制，允许服务器可以通知移动应用程序直接与服务器进行通信，以便于从服务器获取应用程序更新和用户数据。C2DM服务负责处理诸如消息排队等事务并向运行于目标设备上的应用程序分发这些消息。
但是经过一番研究发现，这个服务存在很大的问题：
1）C2DM内置于Android的2.2系统上，无法兼容老的1.6到2.1系统;
2）C2DM需要依赖于Google官方提供的C2DM服务器，由于国内的网络环境，这个服务经常不可用，如果想要很好的使用，我们的App Server必须也在国外，这个恐怕不是每个开发者都能够实现的;
有了上述两个使用上的制约，导致我最终放弃了这个方案，不过我想利用另外一篇文章来详细的介绍C2DM的框架以及客户端和App Server的相应设置方法，可以作为学习与参考之用。
即然C2DM无法满足我们的要求，那么我们就需要自己来实现Android手机客户端与App Server之间的通信协议，保证在App Server想向指定的Android设备发送消息时，Android设备能够及时的收到。下面我来介绍几种常见的方案：
1）轮询：应用程序应当阶段性的与服务器进行连接并查询是否有新的消息到达，你必须自己实现与服务器之间的通信，例如消息排队等。而且你还要考虑轮询的频率，如果太慢可能导致某些消息的延迟，如果太快，则会大量消耗网络带宽和电池。
2）SMS：在Android平台上，你可以通过拦截SMS消息并且解析消息内容来了解服务器的意图。这是一个不错的想法，我就见过采用这个方案的应用程序。这个方案的好处是，可以实现完全的实时操作。但是问题是这个方案的成本相对比较高，你很难找到免费的短消息发送网关，关于这个方案的实现，可以参考如下链接：https://labs.ericsson.com/apis/mobile-java-push/。
3）持久连接：这个方案可以解决由轮询带来的性能问题，但是还是会消耗手机的电池。Apple的推送服务之所以工作的很好，是因为每一台手机仅仅保持一个与服务器之间的连接，事实上C2DM也是这么工作的。不过这个方案也存在不足，就是我们很难在手机上实现一个可靠的服务。Android操作系统允许在低内存情况下杀死系统服务，所以你的通知服务很可能被操作系统Kill掉了。
前两个方案存在明显的不足，第三个方案也有不足，不过我们可以通过良好的设计来弥补，以便于让该方案可以有效的工作。毕竟，我们要知道GMail，GTalk以及GoogleVoice都可以实现实时更新的。
Ø 采用MQTT协议实现Android推送
MQTT是一个轻量级的消息发布/订阅协议，它是实现基于手机客户端的消息推送服务器的理想解决方案。
我们可以从这里下载该项目的实例代码，并且可以找到一个采用PHP书写的服务器端实现
Hello everyone. In this post I will try to provide you with a quick example on how to implement push notifications for your Android app using M
QTT protocol. I will NOT discuss here why an application might need push notifications or the advantages of Push over Pull. I assume that you know exactly
what I mean by push notifications are and why you might need them. However, before jumping in straight to the good stuff, let’s go over how it all started.
Introduction
It’s been around 4 months now since I’ve started developing apps on the Android platform. It began with me scoring a free Nexus One phone at one of the Android Developer Labs. Obviously, I couldn’t resist trying to hack around with some code, so I downloaded the SDK and dove in. I guess in some sense, that’s exactly what Google was hoping for when they starting giving out free phones. While it might sound like I got lucky, in the end Google is the one who won.
Anyway, developing for the Android platform turned out to the a pleasure. The SDK was easy to setup, easy to use and and easy to understand. Putting together your first app was a breeze. I was very impressed.
Unfortunately, I soon realized that Android is not perfect. One of the things that really disappointed me was the lack of a native method for performing push notifications. Over the past year push notifications became almost a standard in the mobile space thanks to Apple. Even though BlackBerry utlilized Push since god knows when, it was Apple that really brought Push mainstream. Obviously, lack of native Push on Android seems like a huge
drawback. Naturally, I started looking around for a solution. After Googling through dozens and dozens of blogs and message boards, I’ve realized that there are 3 generally accepted ways to implement push notifications for your Android app. All of which are non-trivial, hacky and have their own disadvantages. Let’s go over the list:
- Poll? The name obviously tells you that it’s really not even push. The idea here is to periodically poll the server for new messages from
a background local or remote service. The more often you poll the closer you get to the real-time push.
- SMS Android allows you to intercept SMS messages. Your server sends a specially encoded SMS to your phone, whenever there is something new. Your app intercepts all messages, looks for the ones from the server, then pops up a notification.
- Persistent TCP/IP The phone initiates a long-lived mostly idle TCP/IP connection with the server and maintains it by occasionally sending keepalive messages. Whenever there is something new on the server, it sends a messages to the phone over the TCP connection.
The first two methods have significant disadvantages that we cannot do anything about. However, the third method’s drawbacks are not as severe. It seems like with enough work and a good design, the persistent TCP/IP method can work. After all, that’s how GMail, GTalk and Google Voice implement their real-time updates. In fact, many developers out there agree that it is probably the best way to go until Google actually takes the matter in their own hands.
Persistent TCP/IP
After more Googling around I was able to come across three reasonable efforts to implement push notifications using a persistent TCP/IP connection:
- Josh Guilfoyle talks about how to create a most-idle TCP/IP connection with a long keep-alive timer based on the AlarmManager. He provides some really cool sample code with a service that runs in the background and makes connections. http://devtcg.blogspot.com/2009/01/push-services-implementing-persistent.html
- Dave Rea recently started the Deacon project, which is aimed at developing a 3rd party library for Android push notifications using comet technology based on the Meteor server. The project is still in a very early stage (at the time of this post), but looks quite promising. http://deacon.daverea.com/
- Dale Lane had done a number of presentations, where he talked about using the IBM developed MQTT protocol for implementing Android push notifications. He also provides some really useful sample Android code. http://dalelane.co.uk/blog/?p=938
While all of the work done by these guys is incredible, none of their results are quite ready for drop-in use by other developers. In my effort to implement push notifications, I decided to put the pieces of the puzzle together and combine their results to produce a relatively stable way of implementing push. The example that I provide you with further, is a combination of Josh Guilfoyle’s TestKeepAlive project and Dale Lane’s MQTT work. I borrow quite a bit of code from those guys, so they should get most of the credit. Anyways, enough for the introduction, let’s get to the good stuff.
My Idea
The problem with the TestKeepAlive project is that it creates a raw TCP connection, which means that you need write your own server to take care of push on the other side. While it’s, without a question, doable, it is exactly why TestKeepAlive is far from a working solution. On the other hand, the MQTT example shown by Dale Lane uses the IBM’s MQTT broker to handle the server work. To backup a little, MQTT stands for MQ Telemetry Transport, which is a protocol developed by IBM. Let’s take a quick look at the man page:
mqtt is a publish/subscribe messaging protocol intended that is designed to be lightweight. It is useful for use with low power sensors, but is applicable to many scenarios.
Did you see the part about ‘low power’? So did I. Basically, the reason why one might consider using MQTT is that it was designed to be very lightweight, so that it doesn’t consume much power. This is ideal for a mobile push solution as it addresses many battery life related concerns about persistent TCP/IP connections. Obviously, MQTT also has some disadvantages such as privacy, but we can talk about that later.
So, my idea consists of taking a KeepAliveService and replacing the raw TCP/IP connection with an MQTT connection. In this case, each device can simply subscribe to a unique topic which is based on its device ID. Now, assuming that your server knows the device ID, it can push data to the device over MQTT by publishing to that unique topic.
Architecture
In my example, I utilize a PHP script as a server. This uses the Simple Asynchronous Messaging library (see project SAM http://project-sam.awardspace.com/) to publish MQTT messages to the broker on which I host on my server. Let’s have a look at the overall system diagram:
wmqtt.jar is a simple drop-in implementation of MQTT protocol provided by IBM. It can be downloaded from http://www-01.ibm.com/support/docview.wss?rs=171&uid=swg24006006. The file that you download has a bunch of different stuff. Just look for the right jar file. You can include this jar as a part of your Android app.
Really Small Message Broker (RSMB) is a simple MQTT broker also provided by IBM http://www.alphaworks.ibm.com/tech/rsmb. It runs on port 1883 by default. In our architecture it accepts messages from the server and passes them on to the right devices. RSMB can also be replaced by the Mosquitto server http://mosquitto.atchoo.org/.
SAM is a drop-in PHP library for MQTT and other stuff. You can either get it as PECL extension or download the source online http://pecl.php.net/package/sam/download/0.2.0.
send_mqtt.php is a simple PHP script that accepts messages over POST and uses SAM to pass-on messages to the broker.
Sample Code and Demo
The goal of my work on push notifications was to develop a working demo, which is what all other examples out there lack. I’m happy to say that I accomplished my objective. You can download the sample android app on GitHub.
This app (shown on the left) has a TextView and two buttons. The TextView contains your device ID and the buttons are used to start and stop the push notifications service. Once you have the app on your phone, start the service. Then go to http://tokudu.com/demo/android-push/ and enter the device ID in the first text box and enter the message text in the textarea below. Press “Send Push Message” and you should get a notification on your phone. It’s as easy as that.
You can see the source code for andoid-push inthis GitHub project. It contains the aforementioned send_mqtt.php script.
If you didn’t get a notification, make sure you have network connectivity. It can also be that the broker is down on my server (see server status on the page). If that’s the case, please post a comment and I will look into it bringing the broker back up.
Final Thoughts and Comments
MQTT is definitely not the best way to implement push for Android, but it does work. One of the main drawbacks of MQTT is that anyone who knows the IP and the PORT at which the broker is running can connect and intercept your Push messages. So it’s probably a good idea to encrypt them. Alternatively, you could write your own broker and introduce some sort of authentication to MQTT.
The code I provide here for the push service still needs more testing. Reliability is definitely the main question. I think the code can definitely be improved to better handle connectivity loss and other erroneous situations. You are welcome to post your comments here regarding how it can be improved.
Also let me know if you find any bad bugs. Good luck testing!
Anton Lopyrev
Follow me on twitter @tokudu
简要总结：
1）client端
作者给出了android上面的client端程序见： AndroidPushNotificationsDemo
2）server端
首先需要自己安装Mosquitto http://mosquitto.org/download/, 在MQTT Wiki上可以找到更多关于API，关于如何使用MQTT的介绍。
3）发送消息
使用PHP发送消息的示例程序，见：PhpMQTTClient。
用上面给出的例子代码，就可以自己搭建一个环境进行测试。
上面就是对上面文章的简要总结。
上述步骤需要用PHP，然后搭建webserver来发送消息。但是，有时候我们并不方便搭建webserver，或者我们只想通过程序发消息，应该怎么办呢？可以用python脚本发送。
(Above tutorials need setup webserver, however, sometimes it is not so convenient for us to set up one web server and config corresponding environgments, we just need to send message to our android client, so I use python to solve this problem. a) install mosquitto. b) write a python script, send message to mosquitto, mosquitto will send message to android client. )
4）python脚本发送消息的程序
可以考虑使用python。下面给出使用python的例子程序。（参见：http://mqtt.org/wiki/doku.php/python_examples）
我安装了python27，并且安装了Mosquitto目录下面python目录下面的库（python setup.py install）。
给出代码：
#!/usr/bin/env python
#coding=utf-8
import mosquitto
import os
import time
broker = "127.0.0.1"
port = 1883
mypid = os.getpid()
client_uniq = "pubclient_"+str(mypid)
mqttc = mosquitto.Mosquitto(client_uniq)
#connect to broker
mqttc.connect(broker, port, 60, True)
#remain connected and publish
while mqttc.loop() == 0:
msg = "test message "+time.ctime()
mqttc.publish("topic", msg)
print "message published"
time.sleep(1)
pass
在执行上面脚本时，会提示“TypeError: expected string or Unicode object, NoneType found”
解决方法：把mosquitto.dll所在目录加入path环境变量。参见：https://lists.launchpad.net/mqtt-users/msg00043.html
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