MQTT Questions and Answers – Message Queuing Telemetry Transport
Learn the MQTT questions and answers (Message Queuing Telemetry Transport) to get insights and solutions to enhance your understanding of MQTT.
Explore topics such as MQTT protocol, MQTT brokers, QoS levels, message structures, security considerations, and MQTT implementation in IoT applications.
MQTT Questions
Stay informed about the latest advancements in IoT communication and optimize your MQTT-based systems with our reliable Q&A resource. Expand your knowledge and confidently navigate the realm of MQTT with our comprehensive guide.
What does MQTT stand for?
MQTT stands for Message Queuing Telemetry Transport.
What is MQTT?
MQTT is a lightweight messaging protocol for small sensors and mobile devices, optimized for high-latency or unreliable networks.
Who developed MQTT?
MQTT was developed by IBM and Eurotech.
What is the main purpose of MQTT?
The main purpose of MQTT is to collect data from many devices and then transport that data to the IT infrastructure.
What type of protocol is MQTT?
MQTT is a publish/subscribe protocol.
How does MQTT work?
MQTT works by allowing devices to publish information on a topic on an MQTT broker. Other devices can then subscribe to these topics and receive the data when new information is published.
What is an MQTT broker?
An MQTT broker is a server that receives all messages from the clients and then routes the messages to the appropriate destination clients.
How does MQTT handle device connectivity issues?
MQTT uses a method known as “Last Will and Testament” (LWT) to notify other clients about an ungracefully disconnected client.
What is MQTT used for?
MQTT is used for machine-to-machine (M2M) communication and is particularly useful in IoT applications due to its lightweight nature and low power usage.
Why is MQTT considered lightweight?
MQTT is considered lightweight because it uses a small code footprint and minimal network bandwidth.
How is data transported in MQTT?
In MQTT, data is transported over the network in the form of messages.
What is the role of an MQTT client?
An MQTT client is any device (from a microcontroller up to a fully-fledged server) that runs an MQTT library and connects to an MQTT broker over a network.
What are the topics in MQTT?
Topics in MQTT are the routing information for the broker. Each client that wants to send messages publishes them to a certain topic, and each client that wants to receive messages subscribes to a certain topic.
What is the MQTT protocol used for in IoT?
In IoT, MQTT is used to facilitate communication between devices and servers, especially in conditions where network bandwidth is at a premium or network connections are unreliable.
Does MQTT require a persistent connection?
Yes, MQTT requires a persistent TCP connection to ensure message delivery.
What is QoS in MQTT?
QoS stands for Quality of Service in MQTT. It defines how hard the broker/client will try to ensure that a message is received. MQTT has three levels of QoS: QoS 0 (At most once), QoS 1 (At least once), and QoS 2 (Exactly once).
What is the use of the ‘retain’ flag in MQTT?
The retain flag in MQTT is used to tell the broker to keep the last message on a topic and to deliver it to all future subscribers.
What is MQTT-SN?
MQTT-SN is a variant of MQTT designed for wireless sensor networks with scale and power constraints.
Can MQTT work over WebSockets?
Yes, MQTT can work over WebSockets, which allows it to be used in web browsers and other environments that support WebSockets.
How secure is MQTT?
MQTT itself does not provide any built-in security features. However, it can be made secure by using it over secure sockets layer/transport layer security (SSL/TLS), which provides communication security over a computer network.
What is the role of a payload in MQTT?
A payload in MQTT is the actual data or the message that is sent from the publisher to the broker and from the broker to the subscribers.
What is an MQTT topic hierarchy?
An MQTT topic hierarchy is a structured way to organize and access the various topics in an MQTT system. It is constructed in a hierarchical format with different levels separated by a forward slash (/).
How does MQTT handle high-latency networks?
MQTT is designed to handle high latency networks by providing features such as keep-alive pings and Quality of Service levels that ensure message delivery even in unstable network conditions.
How does MQTT compare to HTTP for IoT applications?
MQTT is often preferred over HTTP for IoT applications because of its lightweight nature and its ability to handle intermittent and low-bandwidth network connections.
Can MQTT work with JSON formatted data?
Yes, MQTT can work with JSON formatted data. The payload of an MQTT message can be in any format, including JSON.
What is the purpose of the ‘+’ wildcard in MQTT?
The ‘+’ symbol is a wildcard used in the MQTT topic subscription. It denotes one hierarchy level within the topic structure. For instance, ‘home/+/temperature’ would match ‘home/kitchen/temperature’ and ‘home/livingroom/temperature’.
What is the purpose of the ‘#’ wildcard in MQTT?
The ‘#’ symbol is a wildcard used in the MQTT topic subscription. It denotes all remaining hierarchy levels within the topic structure. For example, ‘home/#’ would match any topic starting with ‘home/’.
Can MQTT messages be encrypted?
Yes, MQTT messages can be encrypted using SSL/TLS, which provides a secure transport layer for the messages.
What is the purpose of a clean session in MQTT?
A clean session in MQTT is used to determine whether a client session is maintained across connections. If a clean session is set, the client and server will not store any information about the session after it is disconnected.
What is the maximum length of an MQTT message?
The maximum length of an MQTT message, including the fixed header and variable header, is 268,435,455 bytes.
Can MQTT be used without a broker?
No, MQTT operates based on a client-server model where the broker acts as the server. A broker is required for MQTT communication.
Is MQTT a RESTful protocol?
No, MQTT is not a RESTful protocol. It is a lightweight, publish-subscribe network protocol.
What are the benefits of using MQTT over REST for IoT devices?
MQTT has several advantages over REST for IoT devices, including lower power usage, lower bandwidth requirements, and better support for unstable network connections.
Can MQTT be used for real-time applications?
Yes, MQTT can be used for real-time applications. Its lightweight nature and Quality of Service levels make it suitable for applications requiring real-time updates.
Can MQTT be used with SQL databases?
Yes, MQTT can be used with SQL databases. Data received via MQTT can be stored in a SQL database for further processing or analysis.
Is there an official standard for MQTT?
Yes, MQTT is a standard defined by the Organization for the Advancement of Structured Information Standards (OASIS).
What does MQTT-SN stand for?
MQTT-SN stands for MQTT for Sensor Networks.
What are the differences between MQTT and MQTT-SN?
MQTT-SN is a variant of MQTT that was designed specifically for wireless sensor networks. While the basic principles remain the same, MQTT-SN includes additional features to handle the limitations of sensor networks, such as small packet size and limited device memory.
What is the role of a client identifier (client ID) in MQTT?
The client identifier (client ID) is a string that uniquely identifies each client connecting to an MQTT broker. Each client that connects to the broker must have a unique client ID.
What is a will message in MQTT?
A will message in MQTT is a message that is stored by the broker and sent out if the client gets disconnected ungracefully. This helps other clients know if a client has unexpectedly disconnected.
Can MQTT handle large volumes of data?
Yes, MQTT can handle large volumes of data efficiently due to its publish/subscribe model, but it’s more suitable for small and frequent messages common in IoT applications.
Can an MQTT client subscribe to multiple topics?
Yes, an MQTT client can subscribe to multiple topics from an MQTT broker.
Can an MQTT broker send messages to clients without a subscription?
No, an MQTT broker can only send messages to clients that have subscribed to specific topics.
Can MQTT be used for location tracking?
Yes, MQTT can be used for location tracking by transmitting location data from devices to a broker in real-time.
Can MQTT be used over UDP?
The standard MQTT protocol uses TCP, but its variant MQTT-SN can be used over UDP, which is suitable for sensor networks where device and bandwidth constraints exist.
Can MQTT be used in a peer-to-peer architecture?
MQTT is typically used in a client-server architecture with a broker. However, with specific arrangements and implementations, it can be adapted for peer-to-peer communication.
Can an MQTT client be both a publisher and a subscriber?
Yes, an MQTT client can be both a publisher of messages and a subscriber to topics.
What is meant by the term ‘mosquitto’ in the context of MQTT?
Mosquitto is an open-source message broker that implements the MQTT protocol versions 5.0, 3.1.1, and 3.1.
What kind of applications typically use MQTT?
MQTT is commonly used in IoT applications, real-time analytics, mobile applications, instant messaging, and other applications requiring live updates.
Can MQTT work without the internet?
Yes, MQTT can work without the internet in a local network setup. However, for clients to communicate across different networks or globally, internet access is required.
How is data integrity ensured in MQTT?
MQTT relies on TCP for data transmission, which inherently provides error checking and correction. Moreover, its QoS levels provide additional mechanisms to ensure message delivery and thus data integrity.
How is MQTT different from CoAP?
Both MQTT and CoAP are used in IoT applications, but they have different architectures. MQTT is a many-to-many communication protocol using a broker, while CoAP is a one-to-one protocol using a client-server architecture. MQTT is based on TCP/IP, and CoAP is based on UDP/IP.
What is the significance of a ‘keep alive’ interval in MQTT?
The ‘keep alive’ interval is a time period communicated to the MQTT broker by the client. If the broker doesn’t receive any message within this time frame, it considers the client as disconnected.
What happens when an MQTT broker goes down?
If an MQTT broker goes down, all active connections are lost. Clients will try to reconnect based on their settings. However, message queuing will be interrupted, leading to potential loss of messages unless they are stored elsewhere or QoS level 2 was used.
How can an MQTT client unsubscribe from a topic?
An MQTT client can unsubscribe from a topic by sending an UNSUBSCRIBE packet to the broker with the topic from which it wants to unsubscribe.
What does it mean when an MQTT message is published with QoS 0?
When an MQTT message is published with QoS 0, it means the message is delivered “at most once”. This is the lowest level of Quality of Service, where message delivery is not guaranteed.
Is it possible for an MQTT client to publish and subscribe to the same topic?
Yes, an MQTT client can publish messages to a topic to which it is also subscribed.
What is an MQTT bridge?
An MQTT bridge is used to connect two MQTT brokers. This allows MQTT clients connected to one broker to communicate with clients connected to the other broker.
What is MQTT over SSL/TLS?
MQTT over SSL/TLS is an MQTT protocol running over a secure socket layer or transport layer security. It provides secure and encrypted connections between MQTT clients and brokers.
Can MQTT use a database?
MQTT itself doesn’t use a database, but the data transported via MQTT can be stored in a database. Also, some MQTT brokers can use a database to store session information or message queues.
What happens if an MQTT client doesn’t respond to a PINGREQ packet?
If an MQTT client doesn’t respond to a PINGREQ packet sent by the broker within the keep-alive time, the broker will disconnect the client assuming it’s not available anymore.
Can an MQTT client choose its QoS level?
Yes, an MQTT client can specify the desired QoS level when it publishes a message. However, the subscriber can also specify its own maximum QoS level during the subscription. The broker will then deliver the message at the lowest QoS level out of these two.
Can MQTT run over other transport protocols besides TCP?
Standard MQTT runs over TCP. However, its variant MQTT-SN is designed for sensor networks and can run over other transport protocols, such as UDP.
What is meant by the term ‘topic namespace’ in MQTT?
The ‘topic namespace’ in MQTT is the structured set of topic names, arranged in a hierarchy. It’s used for routing messages from publishers to subscribers.
What does the term ‘wildcard subscription’ mean in MQTT?
‘Wildcard subscription’ in MQTT refers to subscribing to more than one topic at once using wildcard characters. The ‘+’ symbol represents a single level, and the ‘#’ symbol represents all remaining levels in the topic hierarchy.
What is a retained message in MQTT?
A retained message in MQTT is a message that is held by the broker and sent to any client that subsequently subscribes to the topic. The broker only retains the last retained message and discards any previous ones.
Can MQTT handle bidirectional communication?
Yes, MQTT can handle bidirectional communication between the client and the broker. The clients can be both publishers and subscribers.
What is the purpose of the DUP flag in MQTT?
The DUP flag in the MQTT PUBLISH packet is set when a message is being resent due to a failure in confirming its delivery, indicating it might be a duplicate.
Can an MQTT message be sent to a specific client?
In the MQTT model, messages are published to topics, not to specific clients. However, a specific client can be the only subscriber to a particular topic, in which case it would be the only receiver of the message.
Can an MQTT client connect to multiple brokers?
Yes, an MQTT client can connect to multiple brokers. This is often used to provide redundancy in case one broker fails.
Is MQTT a push or pull protocol?
MQTT is more of a push protocol, as the broker pushes messages to the clients that are subscribed to specific topics. However, clients can also pull messages from the broker by subscribing to a topic.
Can MQTT work on a Local Area Network (LAN)?
Yes, MQTT can work on a Local Area Network. It does not necessarily require an internet connection if all communicating devices are on the same network.
How is MQTT different from ZigBee and Z-Wave?
ZigBee and Z-Wave are wireless protocols mainly used for home automation, while MQTT is a messaging protocol typically used for data transmission between devices. MQTT operates over a network (like LAN or internet), while ZigBee and Z-Wave are used for direct device-to-device communication.
What is an MQTT broker cluster?
An MQTT broker cluster is a group of MQTT brokers working together to provide higher availability, reliability, and scalability. If one broker fails, another broker in the cluster can take over its tasks.
Can an MQTT client be implemented in JavaScript?
Yes, an MQTT client can be implemented in JavaScript, using libraries like MQTT.js which provide an API for MQTT messaging in a web browser context.
Can an MQTT broker distribute messages to clients in a particular order?
MQTT does not inherently guarantee the order in which messages are delivered to subscribers. However, if QoS 2 is used, it ensures that all messages are delivered to the client in the order they were published.
Can MQTT be used for video streaming?
While technically possible, MQTT is not typically used for video streaming as it’s designed for small and frequent messages. Other protocols like RTSP or WebRTC are better suited for this task.
What does QoS -1 signify in MQTT?
QoS -1 in MQTT is not an officially defined level. It’s sometimes used in some MQTT libraries to denote a special case, like automatic determination of QoS level, but it’s not part of the MQTT standard.
Can an MQTT message be broadcast to all clients?
In MQTT, messages are not typically broadcast to all clients. Instead, they are published to topics. However, if all clients are subscribed to the same topic, then in effect, a message can be broadcast to all clients.
Can an MQTT broker reject a client connection?
Yes, an MQTT broker can reject a client connection based on certain criteria, like if the client fails to provide valid authentication or if the broker is configured to limit the number of connections.
Can MQTT be used for data transfer between two servers?
Yes, MQTT can be used for data transfer between two servers. The servers would act as MQTT clients, with one publishing data to a topic and the other subscribing to the topic.
Is MQTT supported by AWS IoT?
Yes, MQTT is supported by AWS IoT. AWS IoT Core supports MQTT versions 3.1.1 and 5.0.
How can an MQTT client know that it has successfully subscribed to a topic?
When an MQTT client sends a SUBSCRIBE request to the broker, the broker responds with a SUBACK packet, acknowledging that the subscription has been granted.
Can an MQTT broker initiate a connection with a client?
No, in MQTT, it’s always the client that initiates a connection with the broker.
Is MQTT a good choice for high-speed data transfer?
MQTT can handle high-speed data transfer, but it’s designed primarily for delivering small packets of data at a time. For applications that require high-speed transfer of large amounts of data, other protocols might be more appropriate.
What does an MQTT PUBLISH packet contain?
An MQTT PUBLISH packet contains the following: a fixed header with MQTT control packet type and flags, a variable header with the topic name and packet identifier, and a payload with the application message.
Can the broker decide not to retain a message even if a client has set the retain flag?
No, if a client has set the retain flag, the MQTT broker must retain the message according to the MQTT protocol.
What happens when two MQTT clients with the same ClientID try to connect to the broker?
According to MQTT protocol, if a second client with the same ClientID as an already connected client tries to connect, the broker must disconnect the existing client.
Can an MQTT client subscribe to a topic with QoS 2 if it initially subscribed with QoS 0?
Yes, an MQTT client can change its subscription QoS level by sending a new SUBSCRIBE message to the broker with the new QoS level.
Can MQTT be used for broadcasting?
Yes, MQTT can be used for broadcasting messages to multiple subscribers. This is achieved by having multiple clients subscribe to the same topic to which messages are being published.
Is MQTT suitable for battery-powered devices?
Yes, MQTT is designed to be lightweight and suitable for battery-powered devices. Its low bandwidth consumption, efficient use of data, and keep-alive feature make it ideal for such devices.
Can an MQTT client reject a message from the broker?
An MQTT client cannot directly reject a message. However, if the client disconnects or does not acknowledge a message with QoS 1 or 2, the message is not considered delivered.
How can a publisher know that its message has been successfully delivered to the subscribers?
The publisher doesn’t directly know whether the message was successfully delivered to the subscribers. However, for QoS 1 and 2, the broker acknowledges receipt of the message from the publisher.
Is MQTT used in edge computing?
Yes, MQTT is commonly used in edge computing as a communication protocol for transmitting data from edge devices to servers.
How secure is MQTT?
The basic MQTT protocol does not include encryption, but it can be made secure by running it over SSL/TLS. MQTT also includes optional username/password authentication for connecting clients.
How can MQTT be integrated with Node.js?
MQTT can be integrated with Node.js using MQTT libraries, such as MQTT.js, which provide APIs for connecting to an MQTT broker and publishing/subscribing to topics.
What happens if a message is published to a topic to which no client is subscribed?
If a message is published to a topic to which no client is currently subscribed, the message will be discarded by the broker unless the message has the retain flag set.
Can MQTT handle real-time data?
Yes, MQTT can handle real-time data effectively due to its lightweight nature and low latency characteristics.
Can MQTT handle non-ASCII characters in the topic name?
Yes, MQTT allows UTF-8 encoded strings for topic names, which include non-ASCII characters.
Is there a limit on the length of an MQTT message?
The MQTT protocol allows a maximum payload size of 256 MB. The practical limit may be lower due to network or system limitations.
Why am I not receiving messages from the MQTT broker?
There could be several reasons for this. You might not be properly subscribed to the topic, the publisher may not be sending messages, the MQTT broker may be down, or there could be network connectivity issues.
How can I troubleshoot my MQTT connection?
First, check your MQTT broker and client logs. These can give you insights into any connection issues or errors. You can also use MQTT debugging tools or network tools like Wireshark to analyze MQTT packets.
What could cause a delay in message delivery in MQTT?
Network latency, broker load, message QoS level, and client processing speed can all affect message delivery time in MQTT.
Why is my MQTT client constantly disconnecting?
This could be due to unstable network connectivity, incorrect keepalive settings, the broker rejecting the client due to incorrect credentials, or the client’s inability to process incoming data quickly enough.
You are having trouble connecting my MQTT client to the broker. What could be the issue?
Check your MQTT broker URL, port, and client credentials. If you’re using SSL/TLS, ensure your certificates are valid. Network connectivity issues or broker downtime could also be the problem.
How can you test MQTT message throughput?
You can use load-testing tools or scripts to publish and subscribe to a high volume of messages and measure how many messages per second your MQTT setup can handle.
What could cause high latency in an MQTT connection?
Factors like network congestion, long distances between client and broker, high broker load, or inefficient message processing on the client side could increase MQTT latency.
You are experiencing packet loss in my MQTT communication. What might be the problem?
This could be due to network instability, an overloaded MQTT broker, or the use of MQTT QoS level 0 which does not guarantee message delivery.
Can you check the number of subscribers to a topic in MQTT?
Standard MQTT protocol does not provide a direct way to check the number of subscribers to a topic. However, some MQTT brokers may provide this feature through their management interface or APIs.
How can you monitor the status of my MQTT broker?
Some MQTT brokers provide built-in management interfaces or APIs that allow you to monitor their status. You can also use monitoring tools that support MQTT.
Why are my MQTT messages being delivered in the wrong order?
MQTT guarantees message order only for messages published at QoS level 2. If you’re using QoS 0 or 1, message order might not be maintained. Network issues could also cause out-of-order delivery.
How can you check if my MQTT client is connected to the broker?
You can use the MQTT PINGREQ and PINGRESP packets to check the connection status. Some MQTT client libraries may also provide a function to check the connection status.
What should you do if my MQTT client fails to connect to the broker?
Check your network connectivity, broker address, port, and credentials. If these are correct, check the broker’s status or logs to see if it’s rejecting your connection.
Why are some of my MQTT messages being lost?
If you’re using MQTT QoS level 0, message delivery is not guaranteed. Also, network issues or an overloaded broker could result in message loss.
Can you use MQTT over a slow or unreliable network?
Yes, MQTT can work over slow or unreliable networks. However, you may need to adjust the keepalive interval and use a higher QoS level to ensure reliable message delivery.
How can you test the reliability of my MQTT communication?
You can perform stress testing by sending a high volume of messages or simulate network issues to see how your MQTT setup handles these conditions.
Why do you get a “Connection Refused” error when connecting to an MQTT broker?
This could be due to incorrect client credentials, the broker being down, or network connectivity issues.
How can you ensure that my MQTT messages are delivered even if the network is unstable?
Use MQTT QoS level 2 for guaranteed message delivery, even in case of network issues. You can also enable message persistence in your MQTT broker.
How do you troubleshoot MQTT SSL/TLS connection issues?
Check your SSL/TLS certificates and keys, make sure the client supports the same SSL/TLS version as the broker, and check your network firewall settings. You can also check the broker and client logs for error messages.
Can you recover lost MQTT messages?
If the messages were published with the retain flag set, the broker will keep a copy of them. However, if the messages were not retained or if they were sent with QoS level 0, there’s no built-in mechanism in MQTT to recover lost messages.
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