Pubsubclient prefix topic

Projects Programming
1

Is it possible to have a prefix for a publish topic? For example the client id? Esphome does this by default. Such as: randomclientid/sensor/humidity

Reason for this is if there are multiple devices which all have a unique client id but the same topic of sensor/humidity they will all publish to the same topic.

I plan on deploying multiple Seeed XIAO esp32c3 devices, all with the same code. The code will extract the mac address to create a unique hostid so every single one will be unique. These will be deployed to multiple server rooms across the US for temp/humidity to a single mqtt host.

If it is possible, what is a good way to do it?

2

Of course it is possible. The topic name is just a string so you can concatenate it with another string to create a unique topic name

Where are you stuck ?

3

publish topic + clientID.

4

Made a little progress this morning.

What I am stuck with is how to concatenate it. I am ignorant on the how. I am admittingly not a programmer. Have just cobbled together code from various places. I have about 30 different things I want to publish(floats, doubles, etc) that already exist in the code so I want to simplify the publish lines as much as possible.

Here is the code that I think would be helpful to look at to start.

#include <PubSubClient.h>
WiFiClient espClient;
PubSubClient client(espClient);

const char* mqtt_username = "mqtt";
const char* mqtt_password = "password";

String temp_str;
String hum_str;
String client_str;
char APssid[25];
char temp[50];
char hum[50];
char clientid[50];
int32_t chipid=ESP.getEfuseMac();
snprintf(APssid,25, "Sensor-%08X",chipid);

const char broker[] = "192.168.80.21";
int        port     = 1883;

//set interval for sending messages (milliseconds)
const long interval = 2000;
unsigned long prevMills = 0;

void initMQTT() 
{
  Serial.println("MQTT Starting...");
  client.setServer(broker, 1883);
}

void mqttreconnect() 
{
  // Loop until we're reconnected
  while (!client.connected()) 
  {
    Serial.print("Attempting MQTT connection...");
    if (client.connect(APssid))
    //if (client.connect(APssid, mqtt_username, mqtt_password))
      {
      Serial.println("connected");
      } 
    else 
      {
      Serial.print("failed, rc=");
      Serial.println(client.state());
      // Wait 5 seconds before retrying
      delay(1000);
      }
  }
}

void mqttLoop() 
{
    if (!client.connected()) 
    {
    mqttreconnect();
    }
  client.loop();

  unsigned long mqttMills = millis();

  if (mqttMills - prevMills >= interval) {
    // save the last time a message was sent
    prevMills = mqttMills;
    
    // send message, the Print interface can be used to set the message contents
    
    client_str = String(APssid);
    client_str.toCharArray(clientid, client_str.length() + 1);
    //Serial.println("Sending MQTT Data");
    hum_str = String(Humidity);
    hum_str.toCharArray(hum, hum_str.length() + 1);
    client.publish(clientid "/sensor/Humidity", hum);
  }
}
5

Suppose that you have a topic named Humidity and a device named DHT11 then take a look at this

char topic[] = {"/Humidity"};
char device[] = {"DHT11"};
char fullTopic[30];

void setup()
{
  Serial.begin(115200);
  strcpy(fullTopic, device);  //copy device name to full topic
  strcat(fullTopic, topic);   //concatenate the topic name
  Serial.println(fullTopic);
}

void loop()
{
}
1 Like
6

ok, but in the above "dht11" is static. I wouldn't be able to use it unless I gave every single board a static name instead of it being based on the chipid.

Looking for a way to have the same sketch be used on hundreds of devices. Such as:
client.publish(clientID + "/sensor/humidity", hum);

7

Can you get the chipid into a variable ?

8

Yes. If you look at the code its APssid. It is what the wifi hostname is and is also used as the clientid for the client.connect

9

The code that I posted was an example of the use of the strcpy() and strcat() functions. Both will happily accept a variable as their arguments so you can ise APssid as the argument to strcpy() or you could create the full topic name in snprintf()

snprintf(APssid,25, "Sensor-%08X/Humidity",chipid);

I hope that it goes without saying that "/Humidity" could be replaced with a variable

10

Apricate the help, and appreciate "Teaching a man to fish".

Getting back to this now that I have got the circuit board design done and sent off.

11

Good luck with the fishing :grinning:

12

Thank you again.

Got it working. it was the strcpy/strcat that did it. Have all the sensors now reporting and each device shows up as I expect it to in mqtt explorer.

Now on to building out the web interface for this & the rest of the device.

Thanks again for leading the way to the answer.

13

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