📚 Chapter 1: MQTT (Message Queuing Telemetry Transport) Protocol

1️⃣ What is MQTT

🌐 MQTT (Message Queuing Telemetry Transport) is a lightweight publish/subscribe messaging protocol designed for low‑bandwidth, high‑latency, or unreliable networks. It is widely used in IoT, embedded systems, and machine‑to‑machine communication.

MQTT is ideal when:

Devices have limited CPU, RAM, or battery
Network bandwidth is expensive or unstable
Many devices need to send small messages efficiently

MQTT was originally created in 1999 to monitor oil pipelines over satellite links — a perfect example of constrained communication.

2️⃣ MQTT Architecture

MQTT uses a publish/subscribe model where clients can publish messages to topics and subscribe to topics to receive messages. The architecture consists of:

Publishers: Devices that send messages
Subscribers: Devices that receive messages
Brokers: Servers that manage message routing

Clients

Any device or program that publishes or subscribes to messages:

Sensors
Mobile apps
Embedded systems
Command‑line tools (mosquitto_pub, mosquitto_sub)

Broker

A central server that:

Receives all messages
Filters them by topic
Delivers them to subscribers
Manages client sessions, last‑will messages, and retained messages

Examples: Mosquitto, HiveMQ, EMQX.

Publish/Subscribe Analogy

Think of MQTT like radio:

A station broadcasts on a frequency (topic)
Listeners tune in to that frequency
The broadcaster does not know who is listening

flowchart LR
    P1([Publisher]) -->|Publish on Topic| B((MQTT Broker))
    P2([Publisher]) -->|Publish on Topic| B
    B -->|Deliver to Subscribers| S1([Subscriber])
    B -->|Deliver to Subscribers| S2([Subscriber])

3️⃣ MQTT Over TCP/IP

MQTT always runs on top of TCP, which provides:

Reliable delivery
Ordered packets
Connection‑oriented communication

Once connected, clients stay connected until they disconnect or time out.

Keep‑Alive Clients send a keep‑alive ping (default ~60 seconds) so the broker knows they’re still alive.

4️⃣ Client ID and Sessions

Client ID

Must be unique
Used by the broker to track subscriptions
If two clients connect with the same ID, the older one is disconnected

Clean Session

Clean Session = True → broker forgets everything when the client disconnects
Clean Session = False → broker remembers subscriptions and may store messages

5️⃣ Last Will and Testament (LWT)

A client can register a “last will” message that the broker publishes if the client disconnects unexpectedly. Useful for:

Detecting offline sensors
Monitoring device health

6️⃣ MQTT Topics

Topics are UTF‑8 strings separated by /, similar to file paths.

Examples:

house/garage/door
nait/wc204/light1
sensors/temperature/livingroom

Rules

Case‑sensitive
No spaces
No wildcards when publishing
Topics are created dynamically when used

Topic Hierarchy Diagram

graph TD
    A[house] --> B[garage]
    A --> C[kitchen]
    B --> D[light]
    C --> E[temperature]
    C --> F[humidity]

7️⃣ Topic Wildcards (Subscription Only)

Single‑Level Wildcard: `+`

Matches exactly one level. Example:

house/+/temperature

Matches:

house/kitchen/temperature
house/garage/temperature

Does NOT match:

house/kitchen/main/temperature

Multi‑Level Wildcard: `#`

Matches everything from that point downward. Example: house/#

flowchart TD
    A[Subscribe: house/#] --> B(house)
    A --> C(house/garage)
    A --> D(house/garage/light)
    A --> E(house/kitchen/sensor/temp)

8️⃣ Quality of Service (QoS)

MQTT defines three delivery guarantees:

QoS	Meaning	Guarantee
0	At most once	No ACK, no guarantee
1	At least once	Guaranteed, duplicates possible
2	Exactly once	Guaranteed, no duplicates

sequenceDiagram
    participant C as Client
    participant B as Broker

    Note over C,B: QoS 0
    C->>B: PUBLISH (QoS 0)
    Note over B: No ACK

    Note over C,B: QoS 1
    C->>B: PUBLISH (QoS 1)
    B->>C: PUBACK

    Note over C,B: QoS 2
    C->>B: PUBLISH (QoS 2)
    B->>C: PUBREC
    C->>B: PUBREL
    B->>C: PUBCOMP

9️⃣ Retained Messages

A retained message is stored by the broker and delivered immediately to new subscribers.

Example:

mosquitto_pub -h 127.0.0.1 -t nait/wc204/light1 -m "status:on" -r

🔟 Moskitto Tools

Mosquitto provides command‑line tools for testing and debugging:

Start Broker

mosquitto -v

Publish

   
mosquitto_pub -h 127.0.0.1 -t nait/WC204/light1 -m "status:on" -d -r

Subscribe

mosquitto_sub -h 127.0.0.1 -t nait/# -d

Subscribe and Auto‑Disconnect After 3 Messages

mosquitto_sub -h 127.0.0.1 -t nait/# -d -C 3

1️⃣1️⃣ Connecting to Public Broker

You can connect to public brokers like test.mosquitto.org for testing.