Mqtt: Persist message on server side - mqtt

Question

We decided to use mqtt protocol for chat module in our mobile application. I want to save messages of topic in server side also. But i saw,mqtt client is global here,so one way is i have to subscribe single instance of mqtt client to all topics and save messages in database. but is it right approach to do it. i am just worring about it.
private void buildClient(){
log.debug("Connecting... "+CLIENT_ID);
try {
mqttClient = new MqttClient(envConfiguration.getBrokerUrl(), CLIENT_ID);
} catch (MqttException e) {
log.debug("build client stopped due to "+e.getCause());
}
chatCallback = new ChatCallback();
mqttClient.setCallback(chatCallback);
mqttConnectOptions = new MqttConnectOptions();
mqttConnectOptions.setCleanSession(false);
}
#Override
public void connect() {
if(mqttClient == null || !mqttClient.getClientId().equals(CLIENT_ID)){
buildClient();
}
boolean tryConnecting = true;
while(tryConnecting){
try {
mqttClient.connect(mqttConnectOptions);
} catch (Exception e) {
log.debug("connection attempt failed "+ e.getCause() + " trying...");
}
if(mqttClient.isConnected()){
tryConnecting = false;
}else{
pause();
}
}
}
#Override
public void publish() {
boolean publishCallCompletedErrorFree = false;
while (!publishCallCompletedErrorFree) {
try {
mqttClient.publish(TOPIC, "hello".getBytes(), 1, true);
publishCallCompletedErrorFree = true;
} catch (Exception e) {
log.debug("error occured while publishing "+e.getCause());
}finally{
pause();
}
}
}
#Override
public void subscribe() {
if(mqttClient != null && mqttClient.isConnected()){
try {
mqttClient.subscribe(TOPIC, 2);
} catch (MqttException e) {
log.debug("subscribing error.."+e.getCause());
}
}
}
#Override
public void disconnect() {
System.out.println(this.mqttClient.isConnected());
try {
mqttClient.disconnect();
log.debug("disconnected..");
} catch (MqttException e) {
log.debug("erro occured while disconneting.."+e.getCause());
}
}

Answer 1

There are two possibilities how to solve this issue:
Write a MQTT client that subscribes to all topics using a wildcard (# in MQTT)
Write a broker plugin that does the job for you, depending on the broker implementation you're using
There is a good description of how to implement both options at the HiveMQ website, also describing limitations of the first option.