MQTT

MQTT is a protocol that is based on the broker topology, and that implements a publish-subscribe message pattern. MQTT was invented in 1999 by Andy Stanford-Clark from IBM and Arlen Nipper from Arcom. Today, MQTT is an Oasis standard.

MQTT is often used in situations where events should be sent from many sensors and broadcast to several applications. IBM and others for instance use MQTT so that IoT devices can send updates into their cloud. When Facebook introduced their Messenger application as a standalone application, they also relied on MQTT to push messages to the clients. We consider MQTT in detail because this protocol is relatively easy to understand, introduces concepts that are used in various form also in other protocols, and because its patterns and topology is complementary to those of HTTP. When you have understood MQTT and HTTP, many other application-level protocols will be easy to understand, too. There are also some more practical reasons why we recommend you to use MQTT for the semester project:

• MQTT is simple to debug, since more than one entity can subscribe to the same topic. This means, you can work on the communication between two entities, but observe all communication by subscribing to these topics with a debugging tool.

• You only need to handle the IP address of the MQTT broker. All other addressing happens indirectly via topics.

• Application startup is simple. You can startup the MQTT broker first, but clients can then connect in any order after that. The MQTT broker can also be hosted on a server and be always-on.

• MQTT works also behind a NAT, in both directions. This means you can push a message from any location to a computer that is connected to your router at home.

Roles

There are three roles in MQTT. The Publisher sends messages to the Broker, which then forwards them to a set of interested Subscribers.

A typical interaction looks as follows:

There can be any number of subscribers and any number of publishers in a system. Because of the publish-subscribe pattern, the subscribers do not have to know about the publishers, and the publishers do not have to know of the subscribers. They only have to know the address of the MQTT broker and connect to it.

The brokers only implement generic functionality, this means, they do not depend on the specific application. Therefore, the broker is a component in a system that can be reused. There are several MQTT brokers available. Examples are Mosquitto, RSMB, HiveMQ.

Topics

Usually, subscribers are not interested in all messages that all publishers send. Subscribers therefore only subscribe to specific topics, which depend on the application. The topics are organized in a hierarchy, separated by a dash ("/"). The following is an example for topics that an application for home automation can use:

The light l1 for instance subscribes to the topic house/garage/lights/l1 so that it can receive messages that switch it on or off. The passive infrared sensor pi1 publishes messages to the topic house/garage/sensors/pi1 every time it detects a movement. An application to switch on the lights whenever a movement is detected can then work like this: (In pseudo code)

Topics can include wildcards, which make it possible for a subscriber to subscribe to several topics with a single pattern:

• The "+" is used as a wildcard for a single level

• The "#" is used as a wildcard for several levels. It must only be placed at the end of a topic pattern.

Examples:

• To receive all messages sent within the house, a subscriber can subscribe to house/#

• To receive all messages for lights in any zone (garage or kitchen), a subscriber can subscribe to house/+/lights/+

Exercise:

A publisher sends a message to the topic a/b/c/d. Which of the following 15 subscription topics will receive this message?

Retained Messages

Imagine a temperature sensor that sends the temperature every 10 minutes. This is a sensible intervall, since temperature does not change too quickly, and the sensor should preserve its battery. When we start an application that needs the temperature in a room, it would have to wait until the sensor sends its temperature, which can be as long as about 10 minutes in the worst case.

Instead, the sensor can mark its temperature message as a retained message. The broker will then store a copy of this message until it is replaced with a new message sent to the same topic. Whenever a new subscriber subscribes to the topic, it will immediately receive the retained message from the broker. In our example this means that the application does not have to wait until the sensor wakes up and sends again, but that it immediately receives the last temperature reading. (We show the reainted message with a trailing R.)

Last Wills

The MQTT broker monitors the connection to all clients, and detects once they do not respond anymore. Any subscriber, however, by default does not get notified once another client disappears. For instance, our temperature application would not be able to detect immediately once a temperature sensor disconnects. (It would only indirectly, because there would be no temperature readings anymore.)

Instead, a client can define a "last will" with the broker at the connection startup. This last will defines a message payload and a topic to which the message will be sent if the broker detects that the client suddenly disconnects. Any interested subscribers can then get notified about the sudden disconnection. If the client, however, disconnects properly from the broker, the last will message is not sent.

Quality of Service

Messages can be sent with three different quality-of-service (QoS) flags, which determine how much effort the broker and the clients spend on sending them:

• QoS=0 is also called At most once. The message can get lost, and there will be no attempts to resend it.

• QoS=1 is also called At least once. This means that the message will be eventually received, but that several copies of the message may appear due to duplication. The receiver has to detect any such duplicates.

• QoS=2 is also called Exactly once. This guarantees the delivery and avoids any duplication.

You may immediately ask: If QoS=2 is available, why would one ever use any of the lower QoS levels? The answer is that the highest quality of service is also more expensive with regards to transmission effort. To send a single QoS=2 message, several messages on the underlying channel are necessary. Therefore, an application should always choose the lowest QoS level it can work with. Below you see three diagrams that show how many control packages are involved to just send a single MQTT message with the different QoS levels. (The grey message arrows that start end end at the same lifeline illustrate only local operations, not communication.)

Sequence Diagrams on Many Levels

Maybe you wondered how some of the sequence diagrams we have seen on the last pages relate to those that you develop for a specific application. Assume the application uses MQTT. What should you show then in a sequence diagram? The answer: It depends on what you want to show.

• The diagrams Quality of Service X above show how MQTT is implemented internally and passes messages in order to publish one single message. These diagrams are only interesting when you need to understand if the MQTT protocol is suitable for your purposes, or you are concerned which QoS level to use. You will probably not use this level of detail for an application-level diagram.

• The diagram above that shows MQTT using a retained message above is already closer to the application level, since publish, subscribe and message are shown as single, compact messages.

• In some cases, it is probably interesting to show an MQTT broker as part of the sequence diagram. After all, it is a critical component in the system that requires consideration. But it may not be necessary to contain the broker in the diagrams of all use cases. Instead, the lifelines representing individual MQTT clients can directly send messages to each other, so that the application logic gets easier visible.

Solution of the Exercise