Message Queuing Telemetry Transport (MQTT)

Topic parameters

Khamdamboy Urunov, a Ph.D. student.

Special Communication Research Center.,

Graduate School of Financial Information Security., Kookmin

University Seoul, South Korea

2

Contents and comments

• Topic instructions• Message type and examples

3

MQTT TopicPublish / SubscribeThe publish / subscribe model lets you build a network of nodes that don’t need to know each other to function.Instead, nodes only know of topics to publish or subscribe to.For example, you might have a topic structure like this:

oinside/bedroom/temperatureoinside/kitchen/temperatureoinside/bathroom/temperature

The various temperature sensors in your house would publish to their own topic, while a display showing temperatures in the house might subscribe to something like:inside/+/temperatureThe “+” acts as a wildcard, allowing a system to subscribe to a group of similar topics. publishe

rsubscribe

rtopic

sensor

sensorTemperature

Message B = light (On/Off)

Temperature Message A = 270

4

MQTT Topic and WildcardsTopics and wildcardsMessages are addressed through the use of topics. Topics are separated by a “/“ allowing topics to be grouped in a tree

structure.

Messages Address

topics

https://hackaday.io/project/1183/logs

How you design your topics is important to allow efficient use of wildcard matching.There are two wildcard characters “#” and “+” , level spared “/”

“#” is the multi-level wildcard and matches zero or more levels.

“+ ” is single – level wildcard “/” spared level

How you design your topics?

Name: identifying the topic within the domain

5

MQTT Topic Name

http://www.slideshare.net/InduSoft/indusoft-web-studio-and-mqtt-for-internet-of-things-applications

The MQTT Topic Name is included in every PUBLISH message In general Topic name have the format:

•site/line/machine/data Notice that the device or appliance ID is useful to include to be

able to subscribe to the flow coming from a specific device, the refrigerator, as opposed to all instance of a given device

As such the length of the topic name, in real application is on the order of tens of bytes.

site/line/machine/dataHow to make Topic Name:

inside/bedroom/temperature/value ?value

How size of Topic Name?

Topic Name = 10 byte

appliance ID is usefully to include to be able to subscribe to the flow •Refrigerator•Smart door

6

MQTT Topic Name Topic is a UTF-8 string, which is used by the broker to filter messages for each connected client. A topic consists of one or more topic levels. Each topic level is separated by a forward slash (topic level

separator).

For example a subscription to myhome/groundfloor/+/temperature would match or not match the following topics:

Multi Level: #While the single level wildcard only covers one topic level, the multi level wildcard covers an arbitrary number of topic levels. In order to determine the matching topics it is required that the multi level wildcard is always the last character in the topic and it is preceded by a forward slash.

http://www.hivemq.com/blog/mqtt-essentials-part-5-mqtt-topics-best-practices

7

MQTT Topic Name (cont….)

Multi Level: #

Client subscribing to a topic with a multi level wildcard is receiving all messages, which start with the pattern before the wildcard character, no matter how long or deep the topics will get.

If you only specify the multilevel wildcard as a topic (#), it means that you will get every message sent over the MQTT broker.

If you expect high throughput this is an anti pattern.

8

Topics beginning with $

oin general you are totally free in naming your topics, but there is one exception. oeach topic, which starts with a $-symbol will be treated speciallyofor example not part of the subscription when subscribing to #othese topics are reserved for internal statistics of the MQTT broker. oTherefore it is not possible for clients to publish messages to these topics. oAt the moment there is no clear official standardization of topics that must be

published by the broker. oIt is common practice to use $SYS/ for all these information and a lot of brokers

implement these, but in different formats. oOne suggestion on $SYS-topics is in the MQTT GitHub wiki and here are some

examples:$SYS/broker/clients/connected$SYS/broker/clients/disconnected$SYS/broker/clients/total$SYS/broker/messages/sent$SYS/broker/uptime

MQTT Topic Name (cont….)

9

MQTT Topic UTF-8The third column of the following table shows the appearance of the space character, in the sense that the cell contains the words “foo” and “bar” in bordered boxes separated by that character. It is possible that your browser does not present all the space characters properly. This depends on the font used, on the browser, and on the fonts available in the system.

http://www.cs.tut.fi/~jkorpela/chars/spaces.html

10

MQTT Topic and Wildcards

http://www.slideshare.net/InfoQ/embedded-java-and-mqtt

11

1. One topic for each deviceFor each device a topic is defined. Its state can be controlled by publishing a

message with payload “ON” or “OFF”.Pro:• the user must not know about the address code of the Intertechno device • changes of the address must not be published• the message is simply “ON” or “OFF to control the device

Contra:• the user must know the topic for each device • the user can only control configured devices

2. One topic for a JSON messagePro:• very flexible to control the devicesContra:• the user must know about the syntax of the JSON and the coding of devices

Solution:Provide both options 

http://www.jensd.de/wordpress/?p=1833

Home Control with Raspberry Pi and MQTT

This MQTT client basically follows two design patterns:

12

My configuration is very simpleOn start-up the Client is searching for sweethomehub-config.xml in the users home directory which is then unmarshalled from JAXB.This configuration contains the codes and the topic for each device and the MQTT settings for the broker connection:

Home Control with Raspberry Pi and MQTT (cont…)

And there is one additional topic awaiting the JSON commands:sweethome/devices/jsoncommand

http://www.hivemq.com/blog/mqtt-essentials-part-5-mqtt-topics-best-practices

13

MQTT Topic Architecture A Topic is a hierarchical structured string, which is used for message filtering and routing and determines which message gets to which client.

http://www.slideshare.net/InduSoft/indusoft-web-studio-and-mqtt-for-internet-of-things-applications

14

MQTT QoS value

15

MQTT QoS value (cont…)

16

MQTT TOPIC Summary (1/2)

Don’t use spaces in a topicA space is the natural enemy of each programmer, they often make it much harder to read and debug topics, when things are not going the way, they should be. So similar to the first one, only because something is allowed doesn’t mean it should be used. UTF-8 knows many different white space types

Each topic will be included in every message it is used in, so you should think about making them short and concise. When it comes to small devices, each byte counts and makes really a difference.

Keep the topic short and concise

Using non-ASCII UTF-8 character makes it really hard to find typos or issues related to the character set, because often they can not be displayed correctly. Unless it is really necessary we recommend avoid using non ASCII character in a topic.

Use only ASCII characters, avoid non printable characters

http://www.slideshare.net/alexmorenocano/mqtt-slides

17

In some cases it is very helpful, when the topic contains a unique identifier of the client the publish is coming from. This helps identifying, who send the message. Another advantage is the enforcement of authorization, so that only a client with the same Client Id as contained in the topic is allowed to publish to that topic. So a client with the idclient1 is allowed to publish to client1/status, but not permitted to publish to client2/status.Topics are a flexible concept and there is no need to preallocate them in any kind of way, regardless both the publisher and subscriber need to be aware of the topic. So it is important to think about how they can be extended in case you are adding new features to your product. For example when your smart home solution is extended by some new sensors, it should be possible to add these to your topic tree without changing the whole topic hierarchy.

Don’t forget extensibility

Embed a unique identifier or the Client Id into the topic

MQTT TOPIC Summary (2/2)

http://www.slideshare.net/alexmorenocano/mqtt-slides

18

Reference

1. https://github.com/aoabook/WroxAccessories/blob/master/src/com/wiley/wroxaccessories/MQTT.java

2. https://www.crossdart.info/p/mqtt/1.1.0/mqtt_message_publish.dart.html3. http://iicb.org/viewpaper/14. http://www.javased.com/?source_dir=Racenet-for-Android/src/com/albin/mqtt/messa

ge/RetryableMessage.java5. http://www.programcreek.com/java-api-examples/index.php?api=org.dna.mqtt.moq

uette.proto.messages.AbstractMessage.QOSType6. http://indigoo.com/

Link of the source code and useful data:

19

Thank you!hamdamboy.urunov@gmail.com