In this episode of MQTT Essentials series, we explain MQTT QoS - the different Quality of Service Levels in MQTT.

👉 What is MQTT QoS?
The Quality of Service (QoS) level is an agreement between the sender of a message and the receiver of a message that defines the guarantee of delivery for a specific message. There are 3 QoS levels in MQTT:

QoS 0: At most once delivery
QoS 1: At least once delivery
QoS 2: Exactly once delivery

👉 Why is Quality of Service important?
QoS is a key feature of the MQTT protocol. QoS gives the client the power to choose a level of service that matches its network reliability and application logic. Because MQTT manages the re-transmission of messages and guarantees delivery (even when the underlying transport is not reliable), QoS makes communication in unreliable networks a lot easier.

👉 QoS 0 - at most once delivery
The minimal QoS level is zero. This service level guarantees a best-effort delivery. There is no guarantee of delivery. The recipient does not acknowledge receipt of the message and the message is not stored and re-transmitted by the sender. QoS level 0 is often called “fire and forget” and provides the same guarantee as the underlying TCP protocol.

👉 QoS 1 - at least once delivery
QoS level 1 guarantees that a message is delivered at least one time to the receiver. The sender stores the message until it gets a PUBACK packet from the receiver that acknowledges receipt of the message. It is possible for a message to be sent or delivered multiple times.

👉 QoS 2 - exactly once delivery
QoS 2 is the highest level of service in MQTT. This level guarantees that each message is received only once by the intended recipients. QoS 2 is the safest and slowest quality of service level. The guarantee is provided by at least two request/response flows (a four-part handshake) between the sender and the receiver. The sender and receiver use the packet identifier of the original PUBLISH message to coordinate delivery of the message.

👉 Best practices:
QoS 0
- is recommended when message queueing is not required
- when message loss is acceptable
- when bandwidth is at a premium

QoS 1
- is the default level
- offers the best tradeoff between bandwidth / delivery guarantee

QoS 2
- when lower performance is acceptable and bandwidth is sufficient
- when delivery exactly once is required

** Chapters / Table of Contents **

00:00 Introduction
00:18 What are Quality of Service (QoS) Levels in MQTT?
01:51 QoS 0: delivery at most once in MQTT
02:21 QoS 1: delivery at least once in MQTT
03:05 QoS 2: delivery exactly once in MQTT
03:58 Best practices with QoS level in MQTT
05:21 Announcing part 8 - Persistent Sessions & Message Queueing in MQTT

👉 If you missed Part 1 - What is MQTT, watch it now:    • What is MQTT | MQTT Essentials Part 1  

👉 Continue watching the MQTT Essential Series now: MQTT Essentials part 8 - Persistent Sessions & Message Queueing    • Persistent Session and Message Queue ...  

👉 We also explain the core of MQTT concepts, its features, and other essential information on the HiveMQ blog: https://bit.ly/3F7l2pm

👉 Get your own MQTT Essentials in a comprehensive, free E-Book: https://bit.ly/3D17jhJ

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