How Does MQTT Route Messages Through a Broker?

• Publisher
• Broker
• Subscribers

What you will learn

• Describe the roles of publisher, broker and subscriber.
• Understand what topics and wildcards are for.
• Tell the three Quality of Service levels apart.

Before you start

It helps to know how IoT devices connect to a network.

How does a broker pass messages on?

MQTT is a Client Server publish/subscribe messaging transport protocol. The Server is a program or device that acts as an intermediary between Clients which publish messages and Clients which have made Subscriptions.

The broker routes messages to the appropriate subscribers based on the topic hierarchy. MQTT is light weight, open, simple and designed to be easy to implement.

• Publisher
• Message with a topic
• Broker
• Subscriber to the topic
• Party not subscribed

How do topics and wildcards work?

A topic name is the label attached to a message, matched against the subscriptions the server knows. A topic filter is an expression in a subscription that shows interest in one or more topics.

The forward slash separates topic levels. The + wildcard matches exactly one level and must occupy a whole level in a filter. The # wildcard matches the parent level and any number of child levels; it must appear alone or after a /, and be the final character in a filter.

• Root of the topic tree
• One topic level
• The + wildcard gathers topics at one level

How do the three Quality of Service levels differ?

MQTT has three Quality of Service levels: QoS 0 (at most once) uses best effort; QoS 1 (at least once) assures delivery but may produce duplicates; QoS 2 (exactly once) assures one delivery. These levels describe each delivery hop from one sender to one receiver; the broker treats each recipient independently.

• At most once: send only
• At least once: with confirmation
• Exactly once: multi-step confirmation

MQTT also decouples by time, allowing the broker to store messages for clients that are not online, while the session and subscription remain valid. Storage also depends on message QoS, message expiry and broker policy. If a client configured a Will Message, the server can publish it after an abnormal connection closure; timing also depends on Will Delay and the session.

Real-world example

With an adult, think of a temperature sensor at home. The sensor publishes a reading to a topic. A phone and a screen in the house both subscribe to that topic, so both receive the reading.

Common misunderstandings

Devices must connect straight to each other. With publish/subscribe, applications are decoupled and go through the broker.

Every message arrives exactly once. That is only one of three levels; at least once can produce duplicates.

Frequently asked questions

What is MQTT?

MQTT is a light, simple publish/subscribe messaging transport protocol.

What does a broker do?

The broker routes messages to the appropriate subscribers based on the topic hierarchy.

What are wildcards for?

The + wildcard matches exactly one topic level and must occupy that whole level in a filter; # matches the parent level and any number of child levels, must appear alone or after a /, and must be the final character in a filter.

What is a will message?

A message configured by a client for the server to potentially publish after an abnormal closure, depending on the delay and session state.

Summary

• MQTT uses publish and subscribe, with a broker routing messages by topic.
• Topics are layered with forward slashes, and wildcards match several topics.
• QoS 0 is at most once, QoS 1 is at least once, and QoS 2 is exactly once for each delivery hop between one sender and one receiver.