MQTT Common Questions

Common Questions

If you are familiar with the web and email then you will probably find, as I did, that MQTT is very different. These are some of the questions I had, and saw on other sites and forums that may clear things up a little.

Q, Can multiple client publish to the same topic?

A- Yes

Q- Is is possible to know the identity of the client that published a message?

A- No not unless the client includes that information in the topic or payload.

Q- What happens to messages that get published to topics that no one subscribes to?

A- They are discarded by the broker.

Q-How can I find out what topics have been published?

A- You can’t do this easily as the broker doesn’t seem to keep a list of published topics as they aren’t permanent.

Q- Can I subscribe to a topic that no one is publishing to?

A- Yes

Q- Are messages stored on the broker?

A- Yes but only temporarily. Once they have been sent to all subscribers they are then discarded. But see next question.

Q- What are retained messages?

A- When you publish a message you can have the broker store the last published message. This message will be the first message that new subscribers see when they subscribe to that topic. MQTT only retains 1 message.

MQTT (Message Queuing Telemetry Transport) is an extremely lightweight connectivity protocol designed to cater to the Internet of Things. It is a machine-to-machine (M2M) protocol which works on publish/subscribe principle of transport of message. The small code footprint for its implementation and open-endedness makes it an ideal candidate for its application in IoT.

There are two types of devices participating in the protocol – brokers and clients. Brokers are the devices which host and relays data. Clients are the devices which access and modify data by publishing topics and receive other devices’ topics by subscribing. Let us take an example of the bulletin board. On the notice board, you might go through the topics which you have interest in. If you have something interesting to put up on the notice board, you are free to do that. To get the attention of the people who are interested in the topic, it is usually done in big bold letters. This is exactly how MQTT works.
Quality of Service (QoS)

The MQTT also gives a provision for three Quality of Service (QoS) – 0, 1 and 2.

MQTT QoS

    For QoS = 0, it ensures at most one delivery of the topic. It is equivalent to a ‘fire and forget’ concept.
    For QoS = 1, it ensures at least one delivery of the topic. It receives an acknowledgment. It makes sure that the broker has received the message.
    For QoS = 2, it ensures exactly one delivery of the topic. It gives us an assurance to the delivery of the message.

Load Testing HAProxy (Part 1)

This is the first post in a 3 part series on load testing HAProxy, which is a reliable, high performant TCP/HTTP load balancer.

Load Testing? HAProxy? If all this seems greek to you, don’t worry. I will provide inline links to read up on everything I’m talking about in this blog post.