What are the differences between the three QoS levels in MQTT protocol?

The MQTT protocol defines three Quality of Service (QoS) levels to ensure reliable message delivery under different network conditions.

QoS 0 - At most once

• Features: Message delivered at most once, no delivery guarantee
• Acknowledgment: No acknowledgment required
• Use Cases: Stable network, scenarios where message loss is acceptable
• Pros: Minimal overhead, fastest transmission
• Cons: Possible message loss, no reliability guarantee

QoS 1 - At least once

• Features: Message delivered at least once, but may be duplicated
• Acknowledgment: PUBACK acknowledgment
• Flow:
  1. Publisher sends message (PUBLISH)
  2. Receiver confirms receipt (PUBACK)
  3. If publisher doesn't receive acknowledgment, message is resent
• Use Cases: Scenarios requiring guaranteed delivery but accepting duplicates
• Pros: Guarantees message delivery
• Cons: May produce duplicate messages

QoS 2 - Exactly once

• Features: Message delivered exactly once, no loss or duplication
• Acknowledgment: Two-phase acknowledgment (PUBREC + PUBREL + PUBCOMP)
• Flow:
  1. Publisher sends message (PUBLISH)
  2. Receiver confirms receipt (PUBREC)
  3. Publisher releases message (PUBREL)
  4. Receiver completes confirmation (PUBCOMP)
• Use Cases: Scenarios with extremely high reliability requirements, such as financial transactions
• Pros: Most reliable, guarantees no loss or duplication
• Cons: Highest overhead, slowest transmission

QoS Level Selection Guidelines

• QoS 0: Real-time sensor data, status updates where loss is acceptable
• QoS 1: General message notifications, logging where delivery is required
• QoS 2: Critical business data, payment commands, control commands where loss and duplication cannot be tolerated

Performance Comparison

QoS Level	Message Overhead	Network Round Trips	Reliability	Use Cases
QoS 0	Minimal	1	Low	Loss acceptable
QoS 1	Medium	2	Medium	Delivery required
QoS 2	Maximum	4	High	No loss or duplication

In practical applications, the appropriate QoS level should be selected based on business requirements, network environment, and performance requirements.