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View topic: MQTT QoS Levels
Lakshmi Deepa S

QoS guarantees the delivery of the specific message sent between the client and the server. It communicates in unreliable networks and allows the client to choose a level of service. It also reduces packet loss and lowers latency on network infrastructure. Bevywise MQTT broker supports all levels of QoS such as QoS 0, QoS 1, and QoS 2.
QoS 0: Fire and Forget
QoS level 0 is the lowest and simplest overhead way where the message is delivered atmost once. It implies a fire-and-forget model. The messages are published and received faster but do not have a guarantee. It is used when there is a stable connection between the sender and the receiver. QoS 1 is used to send collected data of energy management in industrial automation.
QoS 1: Atleast Once
QoS 1 is the default mode of message transfer that is used when the client or broker needs to sure it receives all the messages. The messages are delivered at least once and guarantee a successful transmission of the message to the broker. The message might be delivered multiple times until acknowledgment is received from the receiver which leads to duplicate messages. To remove duplicate messages data pruning functionality is used on the server-side. QoS 1 is used in MQTT Brokers like AWS IoT, Azure, etc that greatly reduces transmission cost.
QoS 2: Exactly Once
QoS 2 is the highest and safest method that consumes more resources and guarantees each message delivery exactly once. Overhead occurs due to more acknowledgment sequence to guarantee no message duplication. By using Timestamps and cumulative data readings overhead in QoS 2 can be avoided. It is used for mission-critical applications like pharmacy manufacturing, medical monitoring, nuclear, and defense industry to do proper planning before implementation.