Not receiving any data by subscribing(mqtt) from arduino client | ThingSpeak Projects | Forum

Thank you for the reply

Please share your code. I tried everything. I used the pubsub client and only the publishing part was working. This is my code.

#include <PubSubClient.h>
#include <SPI.h>
#include <Ethernet.h>
byte mac[] = {0xDE, 0xAD, 0xBE, 0xEF, 0xFE, 0xED};
EthernetClient client;
unsigned long myChannelNumber = 409468;
char mqttUserName[] = "MQTTtrial";
char mqttPass[] = " MQTTAPIKEY";
const char* server = "mqtt.thingspeak.com";
char readAPIKey[] = "channelreadAPIkey";
unsigned long lastConnectionTime = 0;
const unsigned long postingInterval = 20L * 1000L;
static const char alphanum[] ="0123456789"
"ABCDEFGHIJKLMNOPQRSTUVWXYZ"
"abcdefghijklmnopqrstuvwxyz";
PubSubClient mqttClient(client);

void callback(char* topic, byte* payload, unsigned int length) {
Serial.print("Message arrived [");
Serial.print(topic);
Serial.print("] ");
for (int i=0;i<length;i++) {
Serial.print((char)payload[i]);
}
}

void reconnect()
{
char clientID[10];
while (!mqttClient.connected())
{
Serial.print("Attempting MQTT connection...");
for (int i = 0; i < 8; i++) {
clientID[i] = alphanum[random(51)];
delay(50);
}

// Connect to the MQTT broker
if (mqttClient.connect(clientID,mqttUserName,mqttPass))
{
Serial.print("Connected with Client");
mqttClient.subscribe( "channels/409468/subscribe/fields/field1/READAPIKEY");

} else
{
Serial.print("failed, rc=");
Serial.print(mqttClient.state());
Serial.println(" try again in 5 seconds");
delay(5000);
}
}
}

void setup()
{
Ethernet.begin(mac);
Serial.begin(9600);
mqttClient.setServer(server, 1883);
mqttClient.setCallback(callback);
delay(1500);
}

void loop()
{
if (!mqttClient.connected())
{
reconnect();
}
mqttClient.loop();
}

 I am grateful. Thanks

Thank you csatples, but I am still not receiving any reply. The mqtt connection is getting established. The mqtt state is '0' which says the client is connected. 

'mqttClient.subscribe(charBuf)' is also returning true which states that my subscription is successful. 

The only problem is with the 'callback'. This function not even getting invoked. The program is not entering this function. Please help me out with this.

And after every subscription, the client is disconnecting and then connecting again using 'reconnect' . Is that normal?

#include
#include
#include
byte mac[] = {0xDE, 0xAD, 0xBE, 0xEF, 0xFE, 0xED};
EthernetClient client;
unsigned long myChannelNumber = 409468;
char mqttUserName[] = "MQTTtrial";
char mqttPass[] = " mqttpass 
const char* server = "mqtt.thingspeak.com";
char readAPIKey[] = "read api key";
unsigned long lastConnectionTime = 0;
const unsigned long postingInterval = 20L * 1000L;
static const char alphanum[] ="0123456789"
"ABCDEFGHIJKLMNOPQRSTUVWXYZ"
"abcdefghijklmnopqrstuvwxyz";
PubSubClient mqttClient(client);

void setup()
{
Ethernet.begin(mac);
Serial.begin(9600);
mqttClient.setServer(server, 1883);
mqttClient.setCallback(callback);
delay(1500);
}

void loop()
{
if (!mqttClient.connected())
{
reconnect();
}
mqttClient.loop();
}

void reconnect()
{
char clientID[10];
while (!mqttClient.connected())
{
Serial.print("Attempting MQTT connection...");
for (int i = 0; i < 8; i++) {
clientID[i] = alphanum[random(51)];
delay(50);
}

// Connect to the MQTT broker
if (mqttClient.connect(clientID,mqttUserName,mqttPass))
{
Serial.print("Connected with Client");
int a = mqttSubscribe(myChannelNumber,1,readAPIKey);
Serial.print(" value :");
Serial.println(a);

} else
{
Serial.print("failed, rc=");
Serial.print(mqttClient.state());
Serial.println(" try again in 5 seconds");
delay(5000);
}
}
}

void callback(char* topic, byte* payload, unsigned int length) {
Serial.print("Message arrived [");
Serial.print(topic);
Serial.print("] ");
for (int i=0;i<length;i++) {
Serial.print((char)payload[i]);
}
}

int mqttSubscribe(long subChannelID,int field,char* readKey){

String myTopic="channels/"+String(subChannelID)+"/subscribe/fields/field"+String(field)+"/"+String(readKey);
Serial.println("Subscribed to " +myTopic);
Serial.print("State= " + String(mqttClient.state()));
char charBuf[myTopic.length()+1];
myTopic.toCharArray(charBuf,myTopic.length()+1);
return mqttClient.subscribe(charBuf);
}

Thanks for your patience with me.