10 MQTT Interview Questions and Answers

MQTT (Message Queuing Telemetry Transport) is a lightweight messaging protocol designed for constrained devices and low-bandwidth, high-latency networks. It is widely used in IoT (Internet of Things) applications due to its efficiency and reliability in ensuring message delivery. MQTT’s publish-subscribe model makes it ideal for scenarios where devices need to communicate asynchronously and in real-time.

This article offers a curated selection of MQTT interview questions and answers to help you prepare effectively. By familiarizing yourself with these questions, you will gain a deeper understanding of MQTT’s core concepts and practical applications, enhancing your readiness for technical discussions and assessments.

MQTT Interview Questions and Answers

1. Explain the publish/subscribe model used in MQTT and how it differs from traditional client-server models.

The publish/subscribe model in MQTT involves three main components: the publisher, the subscriber, and the broker. The publisher sends messages to a topic, the broker receives these messages, and then forwards them to all subscribers who have subscribed to that topic. This model allows for a many-to-many relationship between publishers and subscribers, enabling efficient and scalable communication.

In contrast, the traditional client-server model involves a direct connection between a client and a server. The client sends a request to the server, and the server processes this request and sends back a response. This model is typically synchronous and requires a direct connection between the client and server, which can be a limitation in scenarios requiring high scalability and flexibility.

Key differences between the publish/subscribe model and the traditional client-server model include:

• Decoupling: In the publish/subscribe model, publishers and subscribers are decoupled, meaning they do not need to know about each other’s existence. In the client-server model, the client and server are tightly coupled.
• Scalability: The publish/subscribe model is more scalable as it allows multiple publishers and subscribers to communicate through a single broker. The client-server model can become a bottleneck as the number of clients increases.
• Asynchronous Communication: The publish/subscribe model supports asynchronous communication, allowing messages to be sent and received independently. The client-server model is typically synchronous, requiring a direct and immediate response.
• Flexibility: The publish/subscribe model is more flexible, allowing for dynamic changes in the number of publishers and subscribers. The client-server model is less flexible and requires predefined connections.

2. What are the three Quality of Service (QoS) levels in MQTT, and when would you use each one?

MQTT supports three Quality of Service (QoS) levels to ensure message delivery reliability:

• QoS 0 (At most once): The message is delivered according to the best efforts of the underlying TCP/IP network. There is no acknowledgment, and the message is not stored or retransmitted if the delivery fails. This level is suitable for scenarios where occasional message loss is acceptable, such as environmental sensor data where updates are frequent and losing a single message has minimal impact.
• QoS 1 (At least once): This level guarantees that the message is delivered at least once to the receiver. The sender stores the message until it receives an acknowledgment from the receiver. However, this can result in duplicate messages if the acknowledgment is lost and the message is retransmitted. This level is appropriate for use cases where message delivery must be guaranteed, but duplicates can be handled by the application, such as in financial transactions or critical alerts.
• QoS 2 (Exactly once): This is the highest level of service. It ensures that the message is delivered exactly once by using a four-step handshake process between the sender and receiver. This level is suitable for scenarios where message duplication is unacceptable, such as in billing systems or order processing systems where each message must be processed only once.

3. How can you secure MQTT communication beyond using TLS?

Beyond using TLS, securing MQTT communication can be achieved through several additional measures:

• Authentication: Implementing robust authentication mechanisms ensures that only authorized clients and brokers can connect to the MQTT network. This can be done using username/password combinations, OAuth, or other authentication protocols.
• Authorization: Authorization controls what actions authenticated clients can perform. Using access control lists (ACLs) or role-based access control (RBAC) can help manage permissions effectively.
• Message Encryption: Encrypting the payload of MQTT messages adds an extra layer of security. This can be done using symmetric or asymmetric encryption algorithms.
• Client Certificate Authentication: Using client certificates for authentication provides a higher level of security compared to username/password combinations. This involves issuing and managing certificates for each client.
• Network Security: Implementing network security measures such as firewalls, VPNs, and intrusion detection/prevention systems can help protect the MQTT infrastructure from unauthorized access and attacks.
• Regular Security Audits: Conducting regular security audits and vulnerability assessments helps identify and mitigate potential security risks in the MQTT setup.

4. What is a retained message, and how is it used?

A retained message in MQTT is a special type of message that the broker stores and delivers to any future subscribers of the topic. When a client subscribes to a topic, it immediately receives the last retained message on that topic, if one exists. This is particularly useful for scenarios where you want new subscribers to receive the most recent status or configuration information without waiting for the next update.

Retained messages are used in various scenarios, such as:

• Status Updates: Devices can publish their current status as a retained message, ensuring that any new subscriber immediately knows the device’s state.
• Configuration Information: Configuration settings can be published as retained messages so that new devices or clients can quickly get the necessary configuration upon subscribing.
• Last Will and Testament (LWT): Retained messages can be used to notify other clients about the unexpected disconnection of a client.

To publish a retained message, the client sets the retained flag to true when sending the message. The broker then stores this message and delivers it to any future subscribers of the topic.

5. Explain the concept of a Last Will and Testament (LWT) message and provide a use case scenario.

A Last Will and Testament (LWT) message in MQTT is a feature that allows a client to notify other clients about its unexpected disconnection. When an MQTT client connects to a broker, it can specify an LWT message, which the broker will store. If the client disconnects unexpectedly (e.g., due to a network failure), the broker will publish the LWT message to a specified topic, informing other clients about the disconnection.

Use Case Scenario:
Consider a scenario where multiple IoT devices are monitoring environmental conditions in a smart home. Each device publishes its data to an MQTT broker. To ensure reliability, each device can set an LWT message that will be published if it goes offline unexpectedly. For example, a temperature sensor can set an LWT message to notify the broker that it is offline. Other devices or systems subscribed to the LWT topic can take appropriate actions, such as alerting the user or switching to a backup sensor.

6. How do topics work, and what are the differences between single-level and multi-level wildcards?

In MQTT, topics are used to route messages between clients. A topic is a hierarchical string that allows clients to subscribe to specific types of messages. When a client publishes a message to a topic, all clients subscribed to that topic receive the message. Topics are case-sensitive and use a forward slash (/) to separate levels in the hierarchy.

There are two types of wildcards in MQTT: single-level and multi-level.

• Single-level wildcard (+): This wildcard matches exactly one level in the topic hierarchy. For example, if a client subscribes to “home/+/temperature”, it will receive messages from “home/livingroom/temperature” and “home/kitchen/temperature”, but not from “home/livingroom/temperature/inside”.
• Multi-level wildcard (#): This wildcard matches any number of levels, including zero. It must be the last character in the topic filter. For example, if a client subscribes to “home/#”, it will receive messages from “home/livingroom/temperature”, “home/kitchen/humidity”, and “home/livingroom/temperature/inside”.

7. Explain the concept of shared subscriptions and provide an example of when they might be used.

Shared subscriptions in MQTT allow multiple clients to share the load of processing messages from a single topic. This is achieved by using a special subscription syntax that includes a shared subscription group. When a message is published to the topic, it is delivered to only one of the subscribers in the group, effectively balancing the load.

For example, consider a scenario where multiple instances of a service need to process incoming sensor data. By using shared subscriptions, the load can be distributed among these instances, ensuring that no single instance is overwhelmed.

Example:

import paho.mqtt.client as mqtt

def on_message(client, userdata, message):
    print(f"Received message: {message.payload.decode()}")

client = mqtt.Client()
client.on_message = on_message

# Subscribe to the shared subscription group 'group1' for the topic 'sensor/data'
client.connect("mqtt.example.com")
client.subscribe("$share/group1/sensor/data")
client.loop_forever()

In this example, multiple instances of the client can subscribe to the shared subscription group ‘group1’. When a message is published to the ‘sensor/data’ topic, it will be delivered to only one of the clients in the group, thereby distributing the processing load.

8. Describe a systematic approach to troubleshooting connectivity issues in an MQTT setup.

To systematically troubleshoot connectivity issues in an MQTT setup, follow these steps:

1. Verify Network Connectivity: Ensure that the network connection between the MQTT client and broker is stable. Check for any network outages, firewall rules, or routing issues that might be blocking the connection.
2. Check Broker Availability: Confirm that the MQTT broker is running and accessible. Verify the broker’s IP address and port number, and ensure that the broker is not overloaded or experiencing downtime.
3. Authentication and Authorization: Ensure that the MQTT client has the correct credentials to connect to the broker. Check for any authentication or authorization issues that might be preventing the client from establishing a connection.
4. Client Configuration: Verify the MQTT client’s configuration settings, including the client ID, keep-alive interval, and clean session flag. Ensure that these settings are correctly configured to match the broker’s requirements.
5. Protocol Version: Ensure that both the MQTT client and broker are using compatible protocol versions. Mismatched protocol versions can lead to connectivity issues.
6. TLS/SSL Configuration: If using TLS/SSL for secure communication, verify that the certificates are correctly configured and not expired. Ensure that the client and broker trust each other’s certificates.
7. Logs and Diagnostics: Check the logs on both the MQTT client and broker for any error messages or warnings. These logs can provide valuable insights into the root cause of the connectivity issues.
8. QoS Levels: Ensure that the Quality of Service (QoS) levels used by the client are supported by the broker. Mismatched QoS levels can lead to message delivery issues.
9. Network Latency and Bandwidth: Monitor network latency and bandwidth to ensure that they are within acceptable limits. High latency or low bandwidth can impact the performance of the MQTT connection.
10. Test with a Different Client/Broker: If possible, test the connection with a different MQTT client or broker to isolate the issue. This can help determine whether the problem lies with the client, broker, or network.

9. How does MQTT handle offline clients to ensure message delivery?

MQTT handles offline clients through several mechanisms to ensure message delivery:

1. Quality of Service (QoS) Levels: MQTT provides three levels of QoS to ensure message delivery:

• QoS 0: At most once delivery, where the message is delivered once or not at all.
• QoS 1: At least once delivery, where the message is delivered at least once but may be delivered multiple times.
• QoS 2: Exactly once delivery, where the message is guaranteed to be delivered exactly once.

2. Persistent Sessions: When a client connects to an MQTT broker, it can request a persistent session. This means that the broker will store all messages and QoS subscriptions for the client. If the client goes offline, the broker will keep these messages and deliver them when the client reconnects.

3. Last Will and Testament (LWT): This feature allows a client to specify a message that will be sent by the broker if it unexpectedly disconnects. This ensures that other clients are aware of the disconnection and can take appropriate action.

4. Retained Messages: When a client publishes a message with the retained flag set, the broker stores the last retained message and its QoS for that topic. When a new client subscribes to that topic, the broker sends the retained message to the client.

10. Provide a real-world use case where MQTT is effectively utilized and explain why it was chosen over other protocols.

A real-world use case where MQTT is effectively utilized is in smart home automation systems. In such systems, various devices like lights, thermostats, and security cameras need to communicate with each other and with a central hub to provide a seamless user experience.

MQTT was chosen over other protocols for several reasons:

• Lightweight and Efficient: MQTT is designed to be lightweight, making it ideal for devices with limited processing power and memory. This is beneficial in a smart home environment where many devices are resource-constrained.
• Low Bandwidth Usage: MQTT minimizes the amount of data transmitted over the network, which is advantageous for environments with limited bandwidth or where data transmission costs are a concern.
• Quality of Service (QoS) Levels: MQTT offers different QoS levels, allowing developers to choose the appropriate level of reliability for message delivery. This ensures that critical messages, such as security alerts, are delivered reliably.
• Publish/Subscribe Model: The publish/subscribe model of MQTT allows for efficient and scalable communication between devices. Devices can subscribe to specific topics and receive updates only when relevant data is published, reducing unnecessary data transmission.
• Persistent Sessions: MQTT supports persistent sessions, which means that even if a device goes offline, it can receive messages that were sent while it was disconnected once it reconnects. This is particularly useful for devices that may have intermittent connectivity.

In comparison to other protocols like HTTP or CoAP, MQTT’s lightweight nature and efficient use of bandwidth make it more suitable for the constrained environments typical of smart home devices. HTTP, while widely used, is more resource-intensive and not optimized for low-bandwidth scenarios. CoAP, another lightweight protocol, is designed for constrained environments but lacks some of the advanced features of MQTT, such as persistent sessions and multiple QoS levels.