How would you design a communication protocol for C++ IoT devices to ensure synchronization and handle device failures?

This question is asking the interviewee to showcase their understanding of communication protocols in C++ and apply that knowledge in designing a protocol for Internet-of-Things (IoT) devices. The main aim is to ensure synchronization (devices are in sync with each other) and handle device failures (ensuring that the system keeps running even if a device fails).

Guidelines to consider:

• Breakdown the question components: Communication Protocol, C++, IoT Devices, Synchronization, Handling device failures.
• Mention the specific protocol you would use.
• Explain the mechanism of synchronization.
• Explain how the protocol would handle device failures.

One way to handle this would be to use a Publish/Subscribe model using MQTT protocol. IoT devices would publish data to their specific topic that all other devices are subscribed to.

Synchronization: Could be achieved through timestamps associated with each message. Devices would synchronize their states based on the timestamp of the received message.

Handling Device Failures: Retained messages feature of MQTT could be used.

Here is a sample code:

// Establish a MQTT client
MQTTClient client;
MQTTClient_create(&client, "tcp://mqtt.eclipse.org:1883", "publisher-id", MQTTCLIENT_PERSISTENCE_NONE, NULL);

// Connect to MQTT server
MQTTClient_connectOptions conn_opts = MQTTClient_connectOptions_initializer;
MQTTClient_connect(client, &conn_opts);

// Publish message for synchronization
MQTTClient_message pubmsg = MQTTClient_message_initializer;
pubmsg.payload = "SYNCHRONIZATION_MESSAGE";
pubmsg.payloadlen = strlen("SYNCHRONIZATION_MESSAGE");
pubmsg.retained = 1;
MQTTClient_publishMessage(client, "iot/device/synchronization", &pubmsg, NULL);

// Handle failures
// By setting the retained message, even if a device goes offline, it will receive the synchronization message when it comes back online. This ensures all devices are in a synchronized state even during failures.

A good solution could be a time-driven synchronization protocol using Simple Network Time Protocol (SNTP).

Synchronization: All devices would listen to the NTP server and synchronize their internal clocks at set intervals.

Handling Device Failures: Redundancy can be incorporated, where each IoT device can communicate with multiple NTP servers.

Here is a sample code:

// Initialize SNTP
sntp_setservername(0, "pool.ntp.org"); 
sntp_init();

// Sync time
time_t now = 0;
struct tm timeinfo = { 0 };
time(&now);
localtime_r(&now, &timeinfo);


// If device is disconnected, retry until reconnection
while(timeinfo.tm_year < (2016 - 1900)) 
{
    vTaskDelay(5000 / portTICK_RATE_MS);
    time(&now);
    localtime_r(&now, &timeinfo);
}

// Set timezone
setenv("TZ", "EST5EDT,M3.2.0/2,M11.1.0", 1);
tzset();

In case of a failure, the IoT device will keep retrying until it can successfully sync its time with the NTP server. Also, multiple NTP servers can be added to ensure redundancy.