20 MQTT Interview Questions and Answers

MQTT is a popular communication protocol used in many industries. It is a lightweight, publish-subscribe messaging protocol that is designed for constrained devices and low-bandwidth, high-latency networks. When applying for a position that uses MQTT, it is important to be prepared to answer questions about the protocol. In this article, we will review some of the most common MQTT interview questions and how you should respond.

MQTT Interview Questions and Answers

Here are 20 commonly asked MQTT interview questions and answers to prepare you for your interview:

1. What is MQTT?

MQTT is a lightweight messaging protocol that is often used in IoT applications. It is designed to be simple and efficient, and it uses a publish/subscribe model so that messages can be easily distributed to multiple clients.

2. Can you explain some of the applications of MQTT in IoT?

MQTT is a popular protocol for connecting IoT devices because it is lightweight and efficient. It is often used in applications where data needs to be transmitted quickly and reliably, such as in industrial monitoring and control systems.

3. Do you think MQTT is better than other protocols like HTTP, HTTPS, XMPP, and WebSockets? Why or why not?

There is no one-size-fits-all answer to this question, as the best protocol to use depends on the specific needs of the application. However, MQTT does have some advantages over other protocols. For example, MQTT is designed to be lightweight and efficient, making it well-suited for applications that need to send a lot of data. Additionally, MQTT offers built-in support for Quality of Service (QoS), which allows for guaranteed delivery of messages.

4. Are there any limitations to using MQTT? If yes, then what are they?

There are a few potential limitations to using MQTT. One is that it is a publish/subscribe protocol, so it may not be well suited for applications that require bi-directional communication. Additionally, MQTT is a relatively simple protocol, so it may not be able to support all the features that some applications may need.

5. How does one use a topic filter with MQTT?

The topic filter is used to specify which topics to subscribe to. This is done by subscribing to a specific topic, and then the messages for that topic will be delivered to the client.

6. Is it possible to use wildcards in a topic name in MQTT? Which ones work?

Yes, it is possible to use wildcards in a topic name in MQTT. The two wildcards that are supported are the plus sign (+) and the hash sign (#). The plus sign can be used to match a single level of a topic, while the hash sign can be used to match multiple levels of a topic.

7. Can you give me an example of how to secure MQTT communication?

There are a few ways to go about securing MQTT communication, but the most common is to use SSL/TLS. This involves setting up a secure connection between the client and the server, and then encrypting all communication that passes between them. This ensures that no one can eavesdrop on the communication and that all data is kept safe.

8. What happens if no client ID is provided when connecting to a broker over MQTT?

If no client ID is provided when connecting to a broker over MQTT, then the broker will automatically generate a client ID for the client.

9. What do you understand about Quality of Service (QoS) levels in MQTT?

The Quality of Service (QoS) levels are used by MQTT to control the delivery of messages between clients and servers. There are three QoS levels:

– QoS 0: The message is delivered at most once. This is the fastest and most efficient delivery mode, but it does not guarantee that the message will be delivered.
– QoS 1: The message is delivered at least once. This ensures that the message will be delivered, but it may be delivered more than once.
– QoS 2: The message is delivered exactly once. This is the slowest and most reliable delivery mode, but it guarantees that the message will only be delivered once.

10. What happens if a message is published without specifying QoS level?

The message will be delivered at the default QoS level, which is QoS 0.

11. Does MQTT allow for two-way communication?

Yes, MQTT allows for two-way communication between a client and a server. This means that the client can send messages to the server, and the server can send messages back to the client. This makes MQTT a very useful protocol for applications that need to exchange data in real-time.

12. Are publishers allowed to subscribe to topics as well?

Yes, publishers are allowed to subscribe to topics. This can be useful in a number of situations, such as if a publisher wants to receive updates on a topic that they are also publishing to.

13. What is the significance of a connection timeout in MQTT?

A connection timeout in MQTT is used to control how long the client will wait for a response from the server after sending a connect request. If the server does not respond within the specified time, the client will assume that the connection has failed and will close the connection.

14. What is the maximum size of a payload that can be sent over MQTT?

The maximum size of a payload that can be sent over MQTT is 256 MB.

15. What’s the difference between MQTT and CoAP?

MQTT is a publish/subscribe messaging protocol designed for lightweight M2M communications. CoAP is a request/response messaging protocol designed for use with constrained nodes and networks.

16. What are the different types of brokers available for MQTT?

The two most common types of brokers available for MQTT are the centralized broker and the federated broker. The centralized broker is typically used in smaller deployments where all clients connect to a single broker. The federated broker is used in larger deployments where there are multiple brokers that are interconnected.

17. What happens if my client is connected to multiple devices at once?

If your client is connected to multiple devices at once, then it will receive messages from all of those devices. It is up to the client to then decide what to do with those messages.

18. How do I handle errors while publishing messages over MQTT?

There are a few different ways that you can handle errors while publishing messages over MQTT. One way is to simply check the return code of the publish function. If the return code is non-zero, then there was an error and you can take appropriate action. Another way is to set up a callback function that will be called when the message is published. This callback function can then check for errors and take appropriate action.

19. How do I ensure that my publisher/subscriber is working properly?

The best way to test your MQTT publisher/subscriber is to use a tool like MQTT.fx. This tool will allow you to connect to your broker, subscribe to topics, and publish messages. This will give you a good idea of whether or not your publisher/subscriber is working properly.

20. How would you compare MQTT performance with other messaging protocols like STOMP?

MQTT is a lightweight messaging protocol that is designed for constrained devices and low-bandwidth, high-latency networks. STOMP is a simple text-oriented messaging protocol that is designed for use with web applications. MQTT is typically faster and more efficient than STOMP, but STOMP may be a better choice for applications that need to support a large number of concurrent connections or that need to be able to work with a variety of different message formats.