Bind each incoming PubSub DataSetMessage to the correct publisher/writer context and a validated DataSetMetaData snapshot before interpreting its fields. A plausible field count or a familiar datatype is not enough: reordered fields can decode successfully while acquiring the wrong engineering meaning. Configuration-version checks belong ahead of cache updates. This guide pins OPC UA Part 14,…
Validate the MQTT packet type and fixed-header flags before accumulating its advertised body. Decode Remaining Length with a four-byte limit and minimal-encoding check, then compare the complete packet size against the effective receive bound using checked arithmetic. An incomplete prefix should remain bounded; a fourth continuation byte is already invalid and does not justify waiting…
Keep an MQTT DISCONNECT reason attached to the connection that received it, classify the exact numeric code, and choose the next investigation from that evidence. Separate malformed encoding, unexpected protocol state, unsupported capabilities, administrative limits and transport closure without a reason. Reconnecting unchanged is not a repair for every category. This guide pins the MQTT…
Allocate MQTT Packet Identifiers from one outbound ownership pool shared by QoS-positive PUBLISH, SUBSCRIBE and UNSUBSCRIBE. Release an identifier only after the corresponding terminal acknowledgement has been validated and processed. A successful PUBREC advances QoS 2 state but does not release its identifier; a failed PUBREC ends that exchange. This guide pins the MQTT 5.0…
Separate an MQTT-over-WebSocket failure into four checkpoints: a valid HTTP upgrade, selection of the MQTT subprotocol, correct WebSocket transport of MQTT bytes, and MQTT session acceptance. After those succeed, investigate idle disconnects by the direction of missing traffic and the timer that observes it. A successful WebSocket connection alone does not establish an accepted MQTT…
Use MQTT 5 Correlation Data as an application-owned binary token, not as a reused Packet Identifier or an index into a mutable request array. Associate that token with one pending request, an expected Response Topic, a deadline and a requester generation. A late reply must fail that ownership check instead of completing a newer operation…
In Mosquitto 2.0.20, enabling persistence does not make every MQTT acknowledgement a synchronous database checkpoint. The broker can acknowledge a retained QoS 1 publication while its newest state is still only in memory. Define restart expectations from persistence configuration and observed checkpoint behavior, then test orderly exit and abrupt process termination separately. This guide pins…
Review a broker bridge as a directed graph of broker-and-topic states. For every forwarding rule, compute the destination topic after prefix removal and addition, then ask whether another rule can send it back into the original export space. Distinct ingress and egress prefixes make that analysis easier and reduce reliance on implementation-specific loop suppression. This…
MQTT wildcards match topic levels, including empty levels, and a leading wildcard does not cover Topic Names beginning with a dollar sign. Build a concrete filter-versus-topic truth table before declaring that a subscription covers every device or every broker diagnostic. The most revealing cases are often a parent topic, a trailing slash or a system…
Validate an MQTT topic in layers: encoded byte length, well-formed UTF-8, MQTT’s prohibited and discouraged code points, then Topic Name grammar. A generic UTF-8 decoder alone is insufficient because U+0000 is valid Unicode encoding but forbidden in MQTT UTF-8 strings. Conversely, some well-formed code points are discouraged rather than universally forbidden, so keep protocol requirements…