MQTT Response Pattern in Node.js

npm init -y

npm install mqtt

npm install @types/node ts-node typescript -D

tsc --init

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```

Creating utils/connectMqtt.ts

```ts
// utils/connectMqtt.ts file
import mqtt, { IClientOptions, MqttClient } from "mqtt";

const options: IClientOptions = {
  port: 1883,
  protocolVersion: 5,
  keepalive: 60,
  properties: {
    requestResponseInformation: true,
    requestProblemInformation: true,
  },
};

const mqttServerClient: MqttClient = mqtt.connect("mqtt:/127.0.0.1", options);

mqttServerClient.on("connect", () => {
  console.log(`connected to mqtt  ${new Date()}`);
});

mqttServerClient.on("error", (err) => {
  console.log(err);
});

export default mqttServerClient;
```

The properties object in option is supported by just MQTT 5.0. So we set the protocolVersion 5 to use properties object. At this point we have 2 necessary options, requestResponseInformation and requestProblemInformation. The [RequestResponseInformation](https://www.hivemq.com/blog/mqtt5-essentials-part9-request-response-pattern/#:~:text=sender%20and%20receiver.-,Response%20Information,-In%20the%20spirit) is used for the broker sending the clients the response information in the CONNACK packet. The requestProblemInformation is used by clients to indicate any failures.

Creating interfaces/relay.interface.ts

```typescript
export interface RelayResponseMessage {
  error: boolean;
  message: string;
}

export interface RelayRequestMessage {
  relayState: number;
}
```

Creating helpers/mqtt-async.helper.ts

```ts
import { IClientPublishOptions, MqttClient } from "mqtt";
import {
  RelayRequestMessage,
  RelayResponseMessage,
} from "../interfaces/relay.interface";

export function publishWithResponseBasic({
  client,
  message,
  publishOptions,
  requestTopic,
  responseTopic,
}: {
  client: MqttClient;
  message: 0 | 1;
  publishOptions: IClientPublishOptions;
  requestTopic: string;
  responseTopic: string;
}): Promise<string> {
  return new Promise((resolve, reject) => {
    const relayRequestMessage: RelayRequestMessage = {
      relayState: message,
    };
    client.subscribe(responseTopic);
    client.once("message", (topic, payload) => {
      client.unsubscribe(responseTopic);
      try {
        const relayResponseMessage: RelayResponseMessage = JSON.parse(
          payload.toString()
        );
        relayResponseMessage.error
          ? reject(relayResponseMessage.message)
          : resolve(relayResponseMessage.message);
      } catch (error) {
        resolve("JsonConvertError");
      }
    });
    client.publish(
      requestTopic,
      JSON.stringify(relayRequestMessage),
      publishOptions
    );
  });
}
```

mqtt-async.helpers.ts is a wrapper that turns the mqttClient callbacks into the async await function. It is required to subscribe to the response topic before publishing the message to the request topic. So that the publisher can receive the response data from the subscriber. In this situation the subscriber has to publish the response message to the response topic, otherwise, the publishWithResponseBasic function will not be closed and it will cause errors, memory leaks, etc. This is the weakness of this wrapper.

Creating requester_basic.ts

```ts
import { IClientPublishOptions } from "mqtt";
import { publishWithResponseBasic } from "./helpers/mqtt-async.helper";
import mqttServerClient from "./utils/connectMqtt";

const deviceName: string = "device_1";
const relayName: string = "relay_1";

setTimeout(() => {
  startSystem();
}, 1000);

const startSystem = () => {
  startResponsePatternExample();
};

const startResponsePatternExample = async () => {
  const responseTopic = `response/${deviceName}/${relayName}`;
  const requestTopic = `request/${deviceName}/${relayName}`;
  const publishOptions: IClientPublishOptions = {
    qos: 1,
    properties: {
      responseTopic,
      correlationData: Buffer.from("secret", "utf-8"),
    },
  };
  try {
    const responseMessage = await publishWithResponseBasic({
      client: mqttServerClient,
      publishOptions,
      requestTopic,
      responseTopic,
      message: 1,
    });
    console.log(responseMessage);
  } catch (error) {
    console.log(error);
  }
};
```

We have two optional parameters,[responseTopic](https://www.hivemq.com/blog/mqtt5-essentials-part9-request-response-pattern/#:~:text=a%20concrete%20example.-,Response%20Topic,-A%20response%20topic) and [correlationData](https://www.hivemq.com/blog/mqtt5-essentials-part9-request-response-pattern/#:~:text=out%20the%20request.-,Correlation%20Data,-Correlation%20data%20is). The responseTopic option represents the topic that the subscriber uses to publish the response message. The correlationData is a buffer data in nodeJS and used for additional information about the request message or a specific request. Also, it can be used for identifying by using a secret word as buffer data.

Creating subscriber_basic.ts

```ts
import { IClientSubscribeOptions } from "mqtt";
import mqttServerClient from "./utils/connectMqtt";

const opts: IClientSubscribeOptions = {
  qos: 1,
};
mqttServerClient.subscribe("request/+/+", opts);

mqttServerClient.on("message", (topic, payload, packet) => {
  console.log(packet);
  const { relayState } = JSON.parse(payload.toString());
  console.log(payload.toString());
  if (
    packet.properties &&
    packet.properties.responseTopic &&
    packet.properties.correlationData &&
    packet.properties.correlationData.toString() === "secret"
  ) {
    const responseData = {
      error: false,
      message: `${relayState === 1 ? "relay opened" : "relay closed"}`,
    };
    mqttServerClient.publish(
      packet.properties.responseTopic,
      JSON.stringify(responseData)
    );
  }
});
```

We focus on the packet parameter that contains the responseTopic and the correlationData. I use the correlationData as a secret code. The response message is published if the converted correlationData is 'secret'.

Compiling the typescript codes.

```bash
tsc
```

Compiled project is in build folder.

First of all we start the subscriber.

```bash
node build/subscriber_basic.js
```

Then we start the requester.

```bash
node build/requester_basic.js
```

The console log of the subscriber_basic.js.

```bash
Packet {
  cmd: 'publish',
  retain: false,
  qos: 1,
  dup: false,
  length: 82,
  topic: 'request/device_1/relay_1',
  payload: <Buffer 7b 22 72 65 6c 61 79 53 74 61 74 65 22 3a 31 7d>,
  messageId: 1,
  properties: {
    responseTopic: 'response/device_1/relay_1',
    correlationData: <Buffer 73 65 63 72 65 74>
  }
}
```

As you can see the subscriber receives the responseTopic and correlationData in the properties object in the packet. The subscriber can use these informations to publish the response message. This example works well within certain limits. We have one topic to request and response. What would happen if the publisher subscribed to multiple response topics or other topics ? We can not listen to the specific topics while using nodeJS MQTT package. The message coming from the subscribed topics can trigger the listener in the publishWithResponseBasic function. We need a solution more complex than before.

### Solution

For complex MQTT subscribe structures in nodeJS projects we take advanced of the nodeJS events. So we could pick out the specific topic by using event listeners with unique event names.

Creating relayResponseEvent.helper.ts

```ts
import EventEmitter from "events";
import { RelayResponseMessage } from "src/interfaces/relay.interface";

export function relayResponseEvent({
  eventEmitter,
  deviceName,
  relayName,
  payload,
}: {
  eventEmitter: EventEmitter;
  deviceName: string;
  relayName: string;
  payload: Buffer;
}) {
  const eventName = `responseEvent/${deviceName}/${relayName}`;
  const eventData: RelayResponseMessage = JSON.parse(payload.toString());
  return eventEmitter.emit(eventName, eventData);
}
```

The function that emits the eventData to the eventName.

Creating the publishWithResponse function in mqtt-async.helper.ts

```ts
export function publishWithResponse({
  client,
  data,
  publishOptions,
  responseEventName,
  requestTopic,
  eventEmitter,
}: {
  client: MqttClient;
  data: 0 | 1;
  publishOptions: IClientPublishOptions;
  requestTopic: string;
  responseEventName: string;
  eventEmitter: EventEmitter;
}): Promise<RelayResponseMessage> {
  return new Promise((resolve, reject) => {
    const checkTimeOut = setTimeout(() => {
      const relayResponseMessage: RelayResponseMessage = {
        error: true,
        message: "timeOut",
      };

      eventEmitter.emit(responseEventName, relayResponseMessage);
    }, 5000);
    eventEmitter.once(
      responseEventName,
      (relayResponseMessage: RelayResponseMessage) => {
        clearTimeout(checkTimeOut);
        relayResponseMessage.error
          ? reject(relayResponseMessage.message)
          : resolve(relayResponseMessage);
      }
    );

    const payload = { relayState: data };
    client.publish(requestTopic, JSON.stringify(payload), publishOptions);
  });
}
```

In this function we have 2 sections. The checkTimeOut is the setTimeout timer that will pass the relayResponseMessage to responseEventName after 5 seconds. The eventEmitter.once listens to the responseEventName. When the eventEmitter.once listener is invoked the checkTimeOut is cleared even if eventEmitter.once listener is called by the checkTimeOut. Why do we need the checkTimeOut timer? Actually the eventEmitter.once listener is invoked when the response message is published by the subscriber. The response message is passed to a specific responseEvent listener As soon as it is received by the publisher but somehow the subscriber may not publish the response message to the response topic. After waiting 5 seconds, if the eventEmitter.once listener with specific eventName is not invoked it means that something has been missing or gone wrong. So we can easily understand whether the subscriber publishes the response data or not.

Creating requester.ts

```ts
import EventEmitter from "events";
import { IClientPublishOptions, MqttClient } from "mqtt";
import { publishWithResponse } from "./helpers/mqtt-async.helper";
import { relayResponseEvent } from "./helpers/relayResponseEvent.helper";
import mqttServerClient from "./utils/connectMqtt";

const eventEmitter = new EventEmitter();

// ==> response/deviceName/relayName
mqttServerClient.subscribe("response/+/+");
mqttServerClient.subscribe("otherTopics/#");
mqttServerClient.on("message", (topic, payload) => {
  const topicArr = topic.split("/"); //spliting the topic ==> [response,deviceName,relayName]
  switch (topicArr[0]) {
    case "response":
      return relayResponseEvent({
        eventEmitter,
        deviceName: topicArr[1],
        relayName: topicArr[2],
        payload,
      });
    case "otherTopics":
      console.log("other topics");
      return;
    default:
      return console.log("can not find anything");
  }
});

const startSystem = () => {
  startResponsePatternExample({
    deviceName: "device_1",
    relayName: "relay_1",
    message: 1,
  });
  startResponsePatternExample({
    deviceName: "device_2",
    relayName: "relay_1",
    message: 1,
  });
};

const startResponsePatternExample = async ({
  deviceName,
  relayName,
  message,
}: {
  deviceName: string;
  relayName: string;
  message: 1 | 0;
}) => {
  try {
    const responseTopic = `response/${deviceName}/${relayName}`;
    const requestTopic = `request/${deviceName}/${relayName}`;
    const responseEventName = `responseEvent/${deviceName}/${relayName}`;
    const publishOptions: IClientPublishOptions = {
      qos: 1,
      properties: {
        responseTopic,
        correlationData: Buffer.from("secret", "utf-8"),
      },
    };
    const responseMessage = await publishWithResponse({
      client: mqttServerClient,
      data: message,
      publishOptions,
      requestTopic,
      responseEventName,
      eventEmitter,
    });
    console.log(`${deviceName}/${relayName} : ${responseMessage.message}`);
  } catch (error) {
    console.log(`${deviceName}/${relayName} : ${error}`);
  }
};
setTimeout(() => {
  startSystem();
}, 1000);
```

The requester subscribed to the response and other topics. When a message is received by the requester the topic gets split and converted to a string array(topicArr). If the first item of this string array is 'response' the payload is passed to the responseEventName listener by using the deviceName and relayName.

Compile and run

```bash
tsc
```

First of all we start the subscriber.

```bash
node build/subscriber_basic.js
```

Then we start the requester.

```bash
node build/requester.js
```

The console log of the requester.js.

```bash
device_1/relay_1 : relay opened
device_2/relay_1 : relay opened
```

We can pick out the response topics by using events. This example is not the best solution but I hope this tutorial gives you an idea about the MQTT response pattern.

Thanks for reading.

Contact me

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