IoT service，mqtt，publish topic different from subscribe topic

万万没想到，万万没想到~~

没想到的是IoT service不仅仅是一个MQTT Broker，”他“还”好心“帮你多做了一件事情，就是topic转换，哈哈，没想到吧，subscribe的topic竟然和publish的topic不一样！

神马？！是的，你没听错！当然，多加topic转换的一步是有一定原因的（至于为什么会有这样的顶层设计，我目前还无暇思考），也是容易实现的，因为app和device都是注册到”他“那里的，app和device的认证信息都在”他“那，做个topic转换还不简单！

至于”MQTT 连接成功订阅不成功“的难题也顺利解决了。因为IoT service不认识你publish的topic，所以MQTTListener不给你subscribe！所以有”一连一断（不断的循环往复connect和disconnect，就是不执行subscribe()方法）“。这个问题是不是socket closed by remote peer呢？好像还不一样。

MQTTGataway回调函数里subscribe的topic如下：

String topic = "iot-2/type/" + MqttUtil.DEFAULT_DEVICE_TYPE+ "/id/+/evt/"

                                             + MqttUtil.DEFAULT_EVENT_ID + "/fmt/json";

MQTTListener的监听的onPublish中（用来监听device或者sensor端的publish的topic）的topic如下：

Pattern pattern = Pattern.compile("iot-2/type/"
+ MqttUtil.DEFAULT_DEVICE_TYPE + "/id/(.+)/evt/"
+ MqttUtil.DEFAULT_EVENT_ID + "/fmt/json");

XXDevice或者XXSensor端publish的topic如下（重要的事情说三遍，是的，你没看错！）：

connection.publish("iot-2/evt/eid/fmt/json", data.getBytes(),
QoS.AT_LEAST_ONCE, false);

终于搞定了，监听端的console log：

debug: Connecting
debug: Transport connected
send: MQTTFrame { type: CONNECT, qos: AT_MOST_ONCE, dup:false }
debug: Logging in
recv: MQTTFrame { type: CONNACK, qos: AT_MOST_ONCE, dup:false }
debug: MQTT login accepted
subscribe topic: iot-2/type/devicecbs1/id/+/evt/eid/fmt/json
send: MQTTFrame { type: SUBSCRIBE, qos: AT_LEAST_ONCE, dup:false }
mqtt connected succeed
recv: MQTTFrame { type: SUBACK, qos: AT_MOST_ONCE, dup:false }
subscribe successfully[B@30ddb86
----------------------------------saveToDB--------------------------------------- = null
----------------------------------saveToDB--------------------------------------- = null
recv: MQTTFrame { type: PUBLISH, qos: AT_LEAST_ONCE, dup:false }
mqtt onPublic----------- :iot-2/type/devicecbs1/id/cbsdevicea/evt/eid/fmt/json
mqtt onPublic----------- :{"type" : "Feature", "geometry": {"coordinates": [0.4079238821522253, 51.58289257363429], "type":"Point"}} }
mqtt save data ....onPublish 
send: MQTTFrame { type: PUBACK, qos: AT_MOST_ONCE, dup:false }
----------------------------------saveToDB--------------------------------------- = {"type" : "Feature", "geometry": {"coordinates": [0.4079238821522253, 51.58289257363429], "type":"Point"}} }
save to cloudant DB succeeded!
----------------------------------saveToDB--------------------------------------- = null
----------------------------------saveToDB--------------------------------------- = null
----------------------------------saveToDB--------------------------------------- = null
send: MQTTFrame { type: PINGREQ, qos: AT_MOST_ONCE, dup:false }
recv: MQTTFrame { type: PINGRESP, qos: AT_MOST_ONCE, dup:false }
----------------------------------saveToDB--------------------------------------- = null
----------------------------------saveToDB--------------------------------------- = null
----------------------------------saveToDB--------------------------------------- = null
recv: MQTTFrame { type: PUBLISH, qos: AT_LEAST_ONCE, dup:false }
mqtt onPublic----------- :iot-2/type/devicecbs1/id/cbsdevicea/evt/eid/fmt/json
mqtt onPublic----------- :{"type" : "Feature", "geometry": {"coordinates": [0.4079238821522253, 51.58289257363429], "type":"Point"}} }
mqtt save data ....onPublish 
send: MQTTFrame { type: PUBACK, qos: AT_MOST_ONCE, dup:false }
----------------------------------saveToDB--------------------------------------- = {"type" : "Feature", "geometry": {"coordinates": [0.4079238821522253, 51.58289257363429], "type":"Point"}} }
save to cloudant DB succeeded!
----------------------------------saveToDB--------------------------------------- = null
----------------------------------saveToDB--------------------------------------- = null
----------------------------------saveToDB--------------------------------------- = null
----------------------------------saveToDB--------------------------------------- = null
----------------------------------saveToDB--------------------------------------- = null
send: MQTTFrame { type: PINGREQ, qos: AT_MOST_ONCE, dup:false }
recv: MQTTFrame { type: PINGRESP, qos: AT_MOST_ONCE, dup:false }
----------------------------------saveToDB--------------------------------------- = null
----------------------------------saveToDB--------------------------------------- = null
----------------------------------saveToDB--------------------------------------- = null
----------------------------------saveToDB--------------------------------------- = null
----------------------------------saveToDB--------------------------------------- = null
----------------------------------saveToDB--------------------------------------- = null
send: MQTTFrame { type: PINGREQ, qos: AT_MOST_ONCE, dup:false }
recv: MQTTFrame { type: PINGRESP, qos: AT_MOST_ONCE, dup:false }
----------------------------------saveToDB--------------------------------------- = null
----------------------------------saveToDB--------------------------------------- = null
----------------------------------saveToDB--------------------------------------- = null

补充一点：

一般情况下在app端subscribe（是个listener），在device或者sensor端publish。

但是在特殊情况下，可能需要在同一端需要subscribe和publish，这里也说一下。

1. 如果subscribe和publish都在device端：

//Subscribe the Command events
//iot-2/cmd/<cmd-type>/fmt/<format-id>
handler.subscribe("iot-2/cmd/" + MqttUtil.DEFAULT_CMD_ID + "/fmt/json",0);

//Publish device events to the app
//iot-2/evt/<event-id>/fmt/<format> 
handler.publish("iot-2/evt/" + MqttUtil.DEFAULT_EVENT_ID
+ "/fmt/json", jsonObj.toString(), false, 0);

2. 如果subscribe和publish都在app端：

//Subscribe Device Events
//iot-2/type/<type-id>/id/<device-id>/evt/<event-id>/fmt/<format-id>
handler.subscribe("iot-2/type/" + MqttUtil.DEFAULT_DEVICE_TYPE
+ "/id/+/evt/" + MqttUtil.DEFAULT_EVENT_ID + "/fmt/json", 0);

//Publish command to one specific device
//iot-2/type/<type-id>/id/<device-id>/cmd/<cmd-id>/fmt/<format-id>
handler.publish("iot-2/type/" + MqttUtil.DEFAULT_DEVICE_TYPE
+ "/id/" + deviceid + "/cmd/" + MqttUtil.DEFAULT_CMD_ID
+ "/fmt/json", jsonObj.toString(), false, 0);

Note：这里仅指IoT service（一种特殊的MQTT Broker）！