Quality of Service (QoS) (EN)

Quality of Service (QoS)

Quality of Service (QoS) defines three levels (0, 1, 2) that control the reliability of message transmission in MQTT. QoS 0 provides 'at most once' delivery where messages are sent without acknowledgment and may be lost. QoS 1 guarantees 'at least once' through acknowledgment with possible duplicate delivery. QoS 2 ensures 'exactly once' with increased overhead using a four-stage handshake that prevents duplicates. The choice of QoS level depends on requirements for reliability, latency, and network load.

QoS Message Flow

sequenceDiagram     participant P as Publisher     participant B as Broker     participant S as Subscriber      Note over P,B,S: QoS 0 - At most once     P->>B: Nachricht senden (ohne Bestätigung)     B->>S: Nachricht weiterleiten      Note over P,B,S: QoS 1 - At least once     P->>B: Nachricht senden     B->>P: PUBACK     B->>S: Nachricht weiterleiten     Note over B,S: Mögliche Duplikatlieferung      Note over P,B,S: QoS 2 - Exactly once     P->>B: PUBLISH (QoS 2)     B->>P: PUBREC     P->>B: PUBREL     B->>P: PUBCOMP     B->>S: PUBLISH (QoS 2)     S->>B: PUBREC     B->>S: PUBREL     S->>B: PUBCOMP 

Im Kontext

• Wird typischerweise zusammen mit MQTT, TCP/IP und Netzwerkprotokollen verwendet
• Verwandt zu