lmxopcua/docs/drivers/Mqtt.md

MQTT Driver

In-process MQTT broker-subscribe driver. It holds one MQTTnet 5 client against one broker, subscribes to the topics its authored tags name, and forwards each PUBLISH straight to ISubscribable.OnDataChange. Ingest is push, not poll, and one-way — there is no IWritable.

Sparkplug B is P2 and not implemented: mode: SparkplugB is a placeholder everywhere it appears. See Mqtt-Test-Fixture.md for the harness and ../plans/2026-07-15-mqtt-sparkplug-driver-design.md for the design.

Project Layout

Project	Holds
Driver.Mqtt.Contracts	Config records, enums, MqttTagDefinition + the pure MqttTagDefinitionFactory, and the one shared MqttJson.Options. Deliberately transport-free — no MQTTnet reference
Driver.Mqtt	Runtime: MqttDriver, MqttConnection (connect + reconnect supervisor), MqttSubscriptionManager, LastValueCache, factory registration, MqttDriverProbe
Driver.Mqtt.Browser	MqttDriverBrowser + MqttBrowseSession — the AdminUI address picker's #-observation session

.Browser references .Driver (not just .Contracts) — a documented, deliberate exception so browse can reuse MqttConnection.BuildClientOptions (TLS / CA-pin / credentials / protocol mapping). Its cost is pulling Core into the AdminUI graph. The csproj carries a boxed warning; do not copy the pattern into a new driver's browser. The clean fix (deferred) is a leaf Driver.Mqtt.Transport project.

Capability Surface

public sealed class MqttDriver
    : IDriver, ITagDiscovery, ISubscribable, IReadable, IHostConnectivityProbe, IRediscoverable, IAsyncDisposable, IDisposable

Capability	Entry point	Notes
IDriver	InitializeAsync / ReinitializeAsync / ShutdownAsync / GetHealth	Order register → attach → connect is contractual. ReinitializeAsync applies a tag-only delta in place, and rebuilds the session when the broker identity changed
ITagDiscovery	DiscoverAsync	Replays only the authored rawTags into an Mqtt folder. SupportsOnlineDiscovery => false, RediscoverPolicy => Once — a broker's live topic set is not a tag catalogue
ISubscribable	SubscribeAsync / OnDataChange	One SUBSCRIBE per distinct authored topic. publishingInterval is ignored — MQTT is push-based; this driver neither polls nor throttles
IReadable	ReadAsync	Serves the last received/retained value from LastValueCache; never throws. A never-seen tag reads BadWaitingForInitialData
IHostConnectivityProbe	GetHostStatuses	1-second poll of MqttConnection.State; HostName is "{host}:{port}"
IRediscoverable	OnRediscoveryNeeded	Seam only — nothing raises it in Plain mode. Sparkplug DBIRTH will

Not implemented: IWritable, IPerCallHostResolver, IAlarmSource, IHistoryProvider. Materialized nodes are SecurityClassification.ViewOnly, IsAlarm: false, IsHistorized: false — "MQTT ingest is one-way in P1 … ViewOnly rather than an Operate node whose writes would silently vanish."

Driver-type string: Mqtt (DriverTypeNames.Mqtt).

Configuration

Bound through MqttJson.Options — case-insensitive, enums by name, unknown members skipped. Author the connection on the device (DeviceConfig) and the mode on the driver (DriverConfig); DriverDeviceConfigMerger merges them, and injects rawTags.

// DriverConfig
{ "Mode": "Plain", "Plain": { "TopicPrefix": "", "DefaultQos": 1 } }

// DeviceConfig
{
  "Host": "10.100.0.35",
  "Port": 8883,
  "UseTls": true,
  "Username": "otopcua",
  "Password": "",                 // supply via ${secret:} / env — never commit
  "CaCertificatePath": null       // pins the chain; null = OS trust store
}

Every key has a default, so nothing is strictly required by the binder. Notable ones: port 8883, useTls true, protocolVersion V500, cleanSession true, keepAliveSeconds 30, connectTimeoutSeconds 15, reconnectMinBackoffSeconds 1 / reconnectMaxBackoffSeconds 30, maxPayloadBytes 1 MiB, allowUntrustedServerCertificate false.

⚠️ Leave clientId unset on a redundant pair. Both nodes of a pair run the driver. MQTT requires client ids to be unique per broker, so a fixed clientId makes the two nodes evict each other forever — the broker logs Client <id> already connected, closing old connection and both nodes reconnect every few seconds while still reporting Healthy. Unset (the default) lets MQTTnet generate a unique id per connection, which is correct. This was observed live during the P1 gate. (The probe and browser already self-uniquify with -probe- / -browse- suffixes.)

Tag Configuration

Key	Type	Default	Notes
topic	string	—	Required. Absent/blank rejects the tag ⇒ BadNodeIdUnknown
payloadFormat	Json | Raw | Scalar	Json	Strict — a typo'd value rejects the tag
jsonPath	string	"$"	Absent or empty ⇒ document root. Json only
dataType	DriverDataType	String	Strict. Float64, not Double — see below
qos	int 0–2	driver's defaultQos	Strict — a malformed value rejects rather than silently weakening delivery
retainSeed	bool	true	Seed this tag from the broker's retained message on subscribe

There is no mode key — the AdminUI's "Tag shape" selector is UI-only, inferred from the blob and never serialized.

dataType uses DriverDataType, which has Float64 and has no Double. Authoring "Double" is a strict-enum rejection, not a fallback.

Payload handling: Json selects via JSONPath then coerces (miss ⇒ BadDecodingError, coerce fail ⇒ BadTypeMismatch); Raw returns the payload's UTF-8 text verbatim and ignores dataType; Scalar decodes then coerces. The JSONPath subset is $, $.a.b, $['a'], $.a[0] — no filters, slices, wildcards or recursive descent.

Wildcards

The runtime accepts a +/# topic (it is ambiguous, not unparseable) and the deploy-time Inspect pass warns. The AdminUI editor rejects it outright — the one place the editor is deliberately stricter than the driver, because one Tag holds one value and a wildcard would feed it from many source topics. Wildcard tags are indexed by filter, not by concrete topic, so they survive a reconnect; a message fans out to every matching tag, and unauthored topics are dropped.

Connection + Failure Semantics

MqttConnectionState: Disconnected → Connected → Reconnecting → Faulted → Disposed. A broker that is merely down stays Reconnecting indefinitely — unreachable is never a fault. Backoff is exponential between the two reconnectBackoff bounds.

MQTTnet 5 does not throw on a rejected CONNACK — it returns the code and leaves the client disconnected. Left unhandled, a wrong password produced a Healthy driver that never received a value. MqttConnection therefore raises MqttConnectRejectedException:

• Unrecoverable ⇒ Faulted, supervisor stops (retrying cannot help): MalformedPacket, ProtocolError, UnsupportedProtocolVersion, ClientIdentifierNotValid, BadUserNameOrPassword, NotAuthorized, Banned, BadAuthenticationMethod, TopicNameInvalid, PacketTooLarge, PayloadFormatInvalid, RetainNotSupported, QoSNotSupported, ServerMoved.
• Everything else is transient and keeps retrying — including UnspecifiedError and any future code. (ServerMoved is permanent, UseAnotherServer is temporary — per the MQTT 5 spec.)

A reconnect that completes without re-subscribing would be a healthy-looking connection that receives nothing forever, so OnReconnectedAsync throws when zero filters are granted, tearing the session down rather than swallowing it. A SUBACK failure degrades just the affected RawPaths to BadCommunicationError.

Browse

MqttDriverBrowser opens a read-only observation session: it subscribes at QoS 0 to # (or {topicPrefix}/#) under a -browse--suffixed client id and builds a /-segment tree from whatever publishes. Nothing is ever published. It is inherently incomplete — a topic that stays silent during the window is invisible — so manual entry remains the escape hatch.

Bounds: 50 000 nodes (then ⚠ Observation truncated), 1024-char topics / 256-char segments (then ⚠ Some topics were too long), 512-byte payload inspection. Sessions are reaped after 2 minutes idle. SparkplugB browse throws NotSupportedException.

The bespoke browser wins over the universal DiscoveryDriverBrowser, which would decline MQTT anyway (SupportsOnlineDiscovery is false).

Test Connect

MqttDriverProbe performs one CONNECT (no SUBSCRIBE, no publish) under a -probe- client id and disconnects. Success message: "MQTT CONNECT OK". A rejected CONNACK is described by the same DescribeConnackRejection the running driver uses, so the button and the driver report a rejection in identical words. See TestConnectProbes.md.

Testing

• Unit — tests/Drivers/…Driver.Mqtt.Tests (266 tests): tag-config mapping, JSONPath subset, strict-enum rejection, subscription indexing (incl. the wildcard-by-filter case), reconnect/rebuild branches, CONNACK classification.
• Live (env-gated) — tests/Drivers/…Driver.Mqtt.IntegrationTests (7 tests) against the Mosquitto fixture; skips cleanly when MQTT_FIXTURE_ENDPOINT is unset. See Mqtt-Test-Fixture.md and infra/README.md §3.

Operational Notes

• Read-only. No writes reach the broker; every node is ViewOnly.
• No alarms, no driver-side history. Historization is a server-side concern (isHistorized).
• Both pair nodes subscribe independently — MQTT fan-out is per-connection, and the Primary gate applies downstream, not to ingest.
• Unauthored traffic is ignored. A busy broker is normal; the driver never auto-provisions a tag.