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Retire a device’s retained state through an approved list of exact topic names, after stopping every writer that can recreate those values. For each owned topic, MQTT removes the retained value when it accepts a publication with RETAIN set and a zero-byte payload. Unsubscribing, ending the device session or publishing an empty payload without RETAIN…

Choose Retain Handling for the subscription’s initial stored values, Retain As Published for the RETAIN flag on later deliveries, and No Local for suppressing publications from the same ClientID. These are separate controls. Retain Handling two does not turn a subscription into a filter that rejects every future publication sent with RETAIN set. This guide…

When one publication matches several subscriptions in the same client session, multiple application deliveries can be legitimate even without a retransmission. Record the matching filters and MQTT 5 Subscription Identifiers before blaming QoS retries. Preserve identifier multiplicity: repeated identifiers can describe multiple matching subscriptions rather than a malformed packet. This guide pins the OASIS MQTT…

A shared subscription distributes matching publications among subscribing sessions; it does not promise that every message from one device stays with one worker or finishes in device order. If order matters, introduce an explicit per-device sequencing boundary before stateful effects occur. Increasing the worker count without that boundary can turn a valid input sequence into…

A delayed Will is eligible when its delay expires or its MQTT session ends, whichever comes first. A long Will Delay cannot provide a reconnect grace period if Session Expiry ends the session immediately. To suppress a Will during a brief interruption, the client must reconnect to the same surviving session before the relevant boundary….

Compare the peer’s limit with the complete encoded MQTT control packet, not just the application payload. Topic bytes, packet identifiers, properties and variable-length length fields all consume the budget. Validate incoming declared packet size before allocating its body, while retaining a separate local memory limit. This guide uses the OASIS MQTT 5.0 Standard, 7 March…

Receive Maximum limits outstanding MQTT QoS 1 and QoS 2 protocol work on one connection. It does not directly cap an application’s pending records, and it does not throttle QoS 0. To use it as part of a memory bound, identify exactly when the client library sends acknowledgements and which buffers remain owned after that…

Reset topic-alias state whenever a new MQTT network connection begins, even if the previous MQTT session resumes. Send a full topic name when establishing a mapping on that connection, and omit the topic only after the mapping has been established in the ordered send stream. An alias is connection-local compression state, not a durable topic…

If two MQTT clients repeatedly disconnect just after the other connects, compare their actual Client Identifiers before changing Wi-Fi or keepalive settings. Two accepted connections using the same identifier can replace one another at the server. The repair is an intentional identity and connection-ownership contract, supported by a provisioning uniqueness check. This guide uses the…