Basic FAQs on MQTT

Question 1

What is MQTT in IoT?

Accepted Answer

MQTT is the standard protocol for messaging and data exchange for the Internet of Things. The protocol uses a publish/subscribe architecture. The technology provides a scalable and cost-effective way to connect devices over the Internet. It is able to deliver data over the Internet in near real-time and with guarantees of delivery. MQTT is designed for IoT devices - lightweight, which enables low-cost device communication.

Question 2

Who uses MQTT?

Accepted Answer

MQTT is used by many major companies, especially in the automotive, industry 4.0, transport, and entertainment sectors. MQTT is used for data exchange between constrained devices and server applications. It keeps bandwidth requirements to an absolute minimum, handles unreliable networks, requires little implementation effort for developers, and is, therefore, ideal for machine-to-machine (M2M) communication.

Question 3

How does MQTT work?

Accepted Answer

MQTT follows the Publish/Subscribe paradigm. The sender (Publisher) and receiver (Subscribers) of messages communicate via so-called topics and are decoupled from each other. The connection between them is handled by the broker. The task of the broker is to filter all incoming messages and distribute them correctly to the subscribers. A client doesn’t have to pull the information it needs, the broker pushes the information to the client whenever something new is available.

Question 4

What is an MQTT client?

Accepted Answer

An MQTT client is any device (from a microcontroller to a full-fledged server) that operates an MQTT library and connects to an MQTT broker over a network. Each MQTT client can be both publisher and/or subscriber.

Question 5

What does an MQTT broker do?

Accepted Answer

An MQTT broker is at the heart of any MQTT deployment. Depending on the implementation, a broker can handle up to millions of concurrently connected MQTT clients. The broker is responsible for receiving all messages, filtering the messages, determining who is interested in each message, and sending the message to these subscribed clients.

Question 6

What is an MQTT topic?

Accepted Answer

Communication in MQTT is based on the topic principle. An MQTTtopic is a UTF-8 string that the broker uses to filter messages for each connected client. To receive messages, the client must subscribe to the topic. A topic can have one or more topic levels. Each topic level is separated by a slash (Topic Level Separator).

Question 7

Is MQTT secure?

Accepted Answer

MQTT is designed to allow very secure communication. As an application layer protocol, it introduces extensive device authentication and authorization possibilities. The underlying TCP/IP transport protocol can add additional security via TLS encryption.

Question 8

Is MQTT open source?

Accepted Answer

MQTT is an open protocol that is standardized by OASIS and ISO (ISO/IEC 20922:2016).

Question 9

Does MQTT use WebSockets?

Accepted Answer

With the right broker implementation that supports native WebSockets, MQTT with 100% of its features set can be used via WebSockets. As they provide a bi-directional, ordered, and lossless communication channel via TCP.

Question 10

Does MQTT use TCP or UDP?

Accepted Answer

MQTT uses TCP. Due to ordering requirements, MQTT over UDP is not possible. For a detailed answer, watch the MQTT Q&A Video: https://youtu.be/zvthFJ-sigA?si=5jlaJcVsKZSelRsx

Question 11

Does MQTT require internet?

Accepted Answer

Yes, to send or receive messages, the MQTT client must establish a TCP connection to the broker. However, MQTT comes with features specifically designed to cope with unstable network connections, like the broker buffering incoming messages for disconnected clients.

Question 12

What is the difference between HTTP and MQTT?

Accepted Answer

MQTT is a data-centric, extremely lightweight, binary protocol. Due to its minimal packet overhead, MQTT excels when transferring data over the wire in comparison to document-centric protocols such as HTTP. Unlike HTTP, which is based on a request/response pattern, MQTT provides push-based communication. This push is made possible via standing TCP connections.

Question 13

What is the difference between AMQP and MQTT?

Accepted Answer

AMQP is bi-directional, synchronous peer-to-peer messaging protocol that has high demands for implementation complexity and a larger network overhead. It is a binary wire protocol, build as replacement for message middleware with main focus on interoperability between different vendors via rich featuring and several exchange patterns. MQTT is a binary protocol with strength in simplicity to be ideal for mobile IoT app and M2M. It provides the pub/sub messaging pattern and is designed for resource-constrained devices, low bandwidth and high latency networks. MQTT is specified by the official OASIS Standard.

Question 14

Does MQTT use WebSockets?

Accepted Answer

With the right broker implementation that supports native WebSockets, MQTT with 100% of its features set can be used via WebSockets. As they provide a bi-directional, ordered, and lossless communication channel via TCP.

Question 15

Does MQTT work with Kafka?

Accepted Answer

Yes, MQTT and Kafka can be integrated with one another. The most efficient way to do so is to use the HiveMQ Enterprise Kafka Extension.

Question 16

Is MQTT restful?

Accepted Answer

No. MQTT is not stateless and does not provide a request/response pattern. It is a publish/subscribe application layer messaging protocol that requires a standing TCP connection and transmit messages in an instantaneous, push way.

Question 17

Can MQTT session be resumed on any node?

Accepted Answer

Yes, an MQTT client can connect to any cluster node and resume its session.

Question 18

Does HiveMQ Support MQTT Load Balancers?

Accepted Answer

Yes, all prevalent load balancing solutions, both hardware and software, are fully supported by HiveMQ.

Question 19

What is Clean Start in MQTT?

Accepted Answer

Clean Start is an MQTT 5 feature that allows the MQTT client to define whether the MQTT broker continues a previous session for the client or starts a fresh session. The clean start flag is set in the CONNECT package of the client.

Question 20

Why do I need Client ID in MQTT?

Accepted Answer

The client identifier (ClientId) identifies each MQTT client that connects to an MQTT broker. The broker uses the client ID to identify the client and the current state of the client. This ID should be unique per client and broker.

Question 21

What is Keep Alive in MQTT?

Accepted Answer

Keep-Alive is the number of seconds that the broker permits between when a client finishes sending one MQTT packet and starts to send the next.

Question 22

What is Last Will Message (LWT) in MQTT?

Accepted Answer

The last will message is part of the Last Will and Testament (LWT) feature of MQTT. The will message notifies other clients when a client disconnects ungracefully.

Question 23

What is Message Expiry in MQTT?

Accepted Answer

Message Expiry is an MQTT 5 feature that allows the MQTT client to set the period of time that the MQTT broker stores a specific PUBLISH message for any matching subscribers that are not currently connected. For more details, read our article on Session and Message Expiry Intervals .

Question 24

What are the different MQTT Packet Types?

Accepted Answer

The MQTT protocol defines a series of predefined MQTT control packets (messages) to manage the communication between devices and the broker. MQTT Packet Types Each MQTT packet contains a mandatory 2-byte header with an optional variable header and/or payload. The minimum MQTT packet size is 2 bytes. The maximum packet size is 256MB. AUTH: A packet that MQTT brokers and clients can send after connection establishment to realize advanced authentication methods such as challenge/response or OAuth. (MQTT 5 only) CONNECT: The packet the MQTT client sends to the MQTT broker to request a connection. CONNACK: The packet the MQTT broker sends to the MQTT client to acknowledge receipt of the connection request. DISCONNECT: The packet the MQTT broker or client sends upon disconnection. PINGREQ: The packet the MQTT client sends to the MQTT broker to establish the status of the server connection. PINGRESP: The packet the MQTT broker sends in response to a PINGREQ message from an MQTT client to indicate that the server is alive. PUBLISH: The packet the MQTT broker or client sends to publish a message to a topic. PUBACK: The packet the MQTT broker or client sends in response to a PUBLISH packet with QoS 1. PUBREC: The packet the MQTT broker or client sends in response to a PUBLISH packet with QoS 2. PUBREC is the second packet of the QoS 2 protocol exchange. PUBREL: The packet the MQTT broker or client sends in response to a PUBREC message. PUBREL is the third packet of the QoS 2 protocol exchange. PUBCOMP: The packet the MQTT broker or client sends in response to a PUBREL message. PUBCOMP is the fourth and final message of the QoS 2 protocol exchange. SUBSCRIBE: The packet that the MQTT client sends to the MQTT broker to subscribe to a topic. A SUBSCRIBE message can contain multiple subscriptions for a client. Each subscription is made up of a topic and a QoS level. SUBACK: The packet the MQTT broker sends to the MQTT client in response to a SUBSCRIBE packet. The message contains the packet identifier of the original subscribe message and a list of return codes. UNSUBSCRIBE: The packet the MQTT client sends to the MQTT broker to delete a subscription to a specific topic on the broker. UNSUBACK: The packet that the MQTT broker sends to the MQTT client to acknowledge the deletion of a subscription. The UNSUBACK message contains the packet identifier of the original UNSUBSCRIBE message.

Question 25

What is a Packet Identifier in MQTT?

Accepted Answer

The packet identifier uniquely identifies a message as it flows between the MQTT client and broker. The client library and/or the broker are responsible for setting this internal MQTT identifier.

Question 26

What is a Payload in MQTT?

Accepted Answer

The application-specific content of an MQTT packet. The payload contains the actual content of the message. MQTT is data-agnostic. It is possible to send images, text in any encoding, encrypted data, and virtually every data in binary.

Question 27

What is Payload Encryption in MQTT?

Accepted Answer

The encryption of application-specific data on the application level (typically, the MQTT PUBLISH packet payload or the CONNECT LWT payload). Payload encryption allows end-to-end encryption of application data and adds another layer of security for topics that deliver confidential information.

Question 28

What are Persistent Sessions in MQTT?

Accepted Answer

A client session for which the MQTT broker saves all relevant client information. When the client reconnects, information such as subscriptions is immediately available. The session is identified by the client ID that the client provides during connection establishment. To know more, read our article on Persistent Session and Queuing Messages .

Question 29

What is Publish/subscribe (Pub/Sub)?

Accepted Answer

An architectural pattern that decouples the clients that send messages (the publishers) from the clients that receive messages (the subscribers). In the pub/sub model , publishers and subscribers never contact each other directly. The connection is handled by a broker that filters all incoming messages and distributes them correctly to subscribers

Question 30

What are the different Quality of Service (QoS) in MQTT?

Accepted Answer

The Quality of Service (QoS) level is an agreement between the sender of a message and the receiver of a message that defines the guarantee of delivery for a specific message. There are 3 QoS levels in MQTT. Quality of Service levels QoS 0: This service level guarantees a best-effort delivery. There is no guarantee of delivery. The recipient does not acknowledge receipt of the message and the message is not stored and re-transmitted by the sender. QoS 1: This service level guarantees that a message is delivered at least one time to the receiver. The sender stores the message until it gets a PUBACK packet from the receiver that acknowledges receipt of the message. The message can be sent or delivered multiple times. QoS 2: This service level guarantees that each message is received only once by the intended recipients. QoS 2 is the safest and slowest service level. The guarantee requires at least two request/response flows (a four-part handshake) between the sender and the receiver.

Question 31

What are Reason Code in MQTT?

Accepted Answer

Reason Codes are the information the MQTT protocol supplies to indicate the result of the numerous operations. The SUBACK and UNSUBACK packets provide a list of reason codes per topic in the payload. Since MQTT v5, reason codes for CONNACK, PUBACK, PUBREC, PUBREL, PUBCOMP, DISCONNECT, and AUTH packets appear in the variable header.

Question 32

What is a Reason String in MQTT?

Accepted Answer

Optional human-readable content that can be provided with a specific MQTT reason code to further explain the outcome.

Question 33

What are Retained Messages in MQTT?

Accepted Answer

A normal MQTT message with the retained flag set to true. The MQTT broker stores the last retained message and the corresponding QoS for the topic. Each client that subscribes to a topic pattern that matches the topic of the retained message receives the retained message immediately after they subscribe. The broker stores only one retained message per topic.

Question 34

What is Session Expiry in MQTT?

Accepted Answer

Session Expiry is an MQTT 5 feature that allows the MQTT client to define how long after the client disconnects until the session of the client expires. When a session expires, the broker removes all subscriptions, queued messages, and incomplete message transmissions that are associated with the session.

Question 35

What are Shared Subscriptions in MQTT?

Accepted Answer

A special type of subscription that allows multiple MQTT clients to share the same subscription . The clients in the shared subscription group receive messages in an alternating fashion.

Question 36

What is a Topic in MQTT?

Accepted Answer

Topic is an MQTT-specific form of addressing that allows MQTT clients to share information.

Question 37

What is Topic Name in MQTT?

Accepted Answer

An MQTT 5 feature that allows MQTT clients to define an integer value that the client can use as a substitute for the name of a topic. Use of a topic alias in place of the UTF-8 string topic name can reduce the size of the PUBLISH package.

Question 38

What is Topic Alias in MQTT?

Accepted Answer

A label with UTF-8 encoding that the publisher of the MQTT message assigns to the MQTT message. The MQTT broker matches this label against the subscription requests that MQTT clients submit.

Question 39

What is Topic Filter in MQTT?

Accepted Answer

A text string with UTF-8 encoding that MQTT clients use to subscribe to topics and receive publications. The MQTT broker uses to filter to distribute matching messages to interested MQTT clients. An MQTT topic filter consists of one or more topic levels. Each topic level is separated by a forward slash (topic level separator). MQTT topic filters support the use of wildcards. Topic Filters MQTT Wildcards: Predefined symbols that can be used in an MQTT topic filter to subscribe to multiple topics simultaneously. Plus sign (+): A single-level wildcard that matches any name for a specific topic level. Hash (#): A multi-level wildcard that matches all topics that match the topic levels that are specified in front of the # symbol in the topic filter.

Question 40

What are $Topics in MQTT?

Accepted Answer

MQTT topics that start with a $ symbol are reserved for internal statistics of the MQTT broker. Clients cannot publish messages to $ topics and $ topics are excluded from wildcard subscriptions.

Question 41

What are User Properties in MQTT?

Accepted Answer

An MQTT 5 feature that appends UTF-8 string key-value pairs to MQTT packets (excluding PINGREQ and PINGRESP). Within the limits of message size, an unlimited number of user properties can be added as metadata to MQTT messages to pass information between the MQTT publisher, broker, and subscriber.

Question 42

What is the difference between MQTT 5 and HiveMQ 4?

Accepted Answer

MQTT 5 and HiveMQ 4 were released at about the same time. MQTT 5 - released March 2019 - is a binary protocol for transferring data. The centerpiece of every MQTT communication is the MQTT broker. HiveMQ itself is a first-class MQTT broker. Since release version 4 all MQTT 5 features are fully integrated in HiveMQ. HiveMQ 4 provides full MQTT 5 support.

Question 43

Does HiveMQ Support MQTT Load Balancers?

Accepted Answer

Yes, all prevalent load balancing solutions, both hardware and software, are fully supported by HiveMQ.

Question 44

What happens if one node goes down?

Accepted Answer

The rest of the cluster will overtake that node's duties. By default at least one other node replicates all the relevant information.

Question 45

How to implement HiveMQ?

Accepted Answer

HibeMQ can be implemented on-premise or on the cloud. Check out HiveMQ documentation on getting started with HiveMQ.

Question 46

Is it possible to upgrade all cluster nodes without downtime?

Accepted Answer

Yes, HiveMQ supports Rolling Upgrades, which means that zero-downtime upgrades are possible when using a HiveMQ cluster. All nodes can be upgraded one-by-one.

Question 47

How is the load distributed between the cluster nodes?

Accepted Answer

HiveMQ is designed with a true distributed and masterless cluster architecture. All persistent data and session information is distributed between the cluster nodes. Messages are being handled by the node a client is connected to and distributed to the other nodes when needed.

Question 48

Does HiveMQ lose messages when one or more nodes go down?

Accepted Answer

HiveMQ implements a sophisticated replication system. By default each node has 1 replicate within the cluster but that value can be configured. As long as a node's replicate(s) doesn't go down at the same time, the node goes down, no messages will be lost.

Question 49

How does HiveMQ distribute messages and queued messages in the cluster?

Accepted Answer

Sessions, including queued messages, are stored by multiple nodes. The number of replicates is configurable. Every node in the cluster can access that information.

Question 50

How often are new versions of HiveMQ released and how long is each version supported?

Accepted Answer

Major HiveMQ versions do not have a predefined release cadence. Check out our Release Notes section.

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