iot-protocol-patterns

iot-protocol-patterns

Knowledge Base

IoT protocol patterns for constrained MCUs and embedded Linux.

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Pattern 1: MQTT Topic Structure

Well-structured topic hierarchy enables flexible filtering and routing:

device/{device_id}/telemetry     ← sensor data (QoS 0, no retain)
device/{device_id}/status        ← online/offline (QoS 1, retained)
device/{device_id}/cmd           ← commands to device (QoS 1)
device/{device_id}/cmd/ack       ← command acknowledgment
device/{device_id}/ota/cmd       ← OTA start/control
device/{device_id}/ota/data      ← firmware chunks
fleet/+/telemetry                ← subscribe to all device telemetry

LWT (Last Will and Testament) configuration:

esp_mqtt_client_config_t cfg = {
    .session.last_will = {
        .topic  = "device/" DEVICE_ID "/status",
        .msg    = "{\"online\":false}",
        .qos    = 1,
        .retain = true,   /* Retained: new subscribers see last status immediately */
    },
};

On connect, publish: device/{id}/status = {"online":true} retained.

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Pattern 2: Paho MQTT C Client (Linux/RTOS)

#include "MQTTClient.h"

#define BROKER_URI    "ssl://broker.example.com:8883"
#define DEVICE_ID     "device-001"
#define KEEPALIVE_SEC 60

static MQTTClient     s_client;
static MQTTClient_connectOptions s_conn_opts = MQTTClient_connectOptions_initializer;

void mqtt_message_arrived(void *ctx, char *topic, int topic_len,
                           MQTTClient_message *msg)
{
    /* Process command from broker */
    handle_command(topic, msg->payload, msg->payloadlen);
    MQTTClient_freeMessage(&msg);
    MQTTClient_free(topic);
}

int mqtt_init(void)
{
    MQTTClient_create(&s_client, BROKER_URI, DEVICE_ID,
                      MQTTCLIENT_PERSISTENCE_NONE, NULL);

    s_conn_opts.keepAliveInterval = KEEPALIVE_SEC;
    s_conn_opts.cleansession      = 1;
    s_conn_opts.username          = "devices";
    s_conn_opts.password          = "secret";

    MQTTClient_SSLOptions ssl = MQTTClient_SSLOptions_initializer;
    ssl.trustStore = "/etc/ssl/mqtt-ca.pem";
    s_conn_opts.ssl = &ssl;

    MQTTClient_setCallbacks(s_client, NULL, NULL, mqtt_message_arrived, NULL);

    int rc = MQTTClient_connect(s_client, &s_conn_opts);
    if (rc != MQTTCLIENT_SUCCESS) { return rc; }

    MQTTClient_subscribe(s_client, "device/" DEVICE_ID "/cmd", 1);
    return 0;
}