MQTT reference connector

The first concrete connector instantiating the framework’s contracts. This cluster :satisfies: Connector framework (FEAT_0030).

Feature: MQTT reference connector FEAT_0036
status: open
satisfies: FEAT_0030
is satisfied by: REQ_0250, REQ_0251, REQ_0252, REQ_0253, REQ_0254, REQ_0255, REQ_0256, REQ_0257, REQ_0258, REQ_0259, REQ_0260, REQ_0261
is refined by: ADR_0010

The first concrete connector instantiating the framework’s contracts: rumqttc-backed MQTT 3.1.1 plugin and gateway with bidirectional pub/sub, QoS 0+1, retained messages, wildcard subscriptions, and optional TLS.
Requirement: MqttConnector implements Connector REQ_0250
status: open
satisfies: FEAT_0036
is implemented by: BB_0004, BB_0020

The connector crate shall expose MqttConnector<C: PayloadCodec> that implements the Connector trait with type Routing = MqttRouting.
Requirement: MqttRouting carries topic, qos, retained REQ_0251
status: open
satisfies: FEAT_0036
is implemented by: BB_0004, BB_0020

The MqttRouting struct shall carry the MQTT topic name, the QoS level, and a retained-message flag. It shall implement the Routing marker trait.
Requirement: QoS 0 and 1 supported REQ_0252
status: open
satisfies: FEAT_0036
is verified by: TEST_0130, TEST_0131

The connector shall support MQTT QoS levels AtMostOnce (0) and AtLeastOnce (1). QoS 2 is deferred to a follow-on spec.
Requirement: Retained-message publish supported REQ_0253
status: open
satisfies: FEAT_0036
is verified by: TEST_0132

When MqttRouting::retained is true, the connector shall publish the envelope payload as a retained MQTT message.
Requirement: Wildcard subscriptions supported REQ_0254
status: open
satisfies: FEAT_0036
is refined by: ARCH_0011
is verified by: TEST_0102, TEST_0133, TEST_0134
links incoming: RISK_0005

The connector shall accept inbound subscriptions whose topic includes the MQTT wildcards + (single-level) and # (multi-level), and shall demultiplex received messages to the matching ChannelReader instance(s).
Requirement: Username/password authentication REQ_0255
status: open
satisfies: FEAT_0036
is verified by: TEST_0135

The connector shall accept username and password credentials in MqttConnectorOptions and present them on the MQTT CONNECT packet.
Requirement: TLS is optional via cargo feature REQ_0256
status: open
satisfies: FEAT_0036
is verified by: TEST_0136

The connector shall provide TLS support via rustls behind a default-off tls cargo feature. Client-certificate authentication is deferred to a follow-on spec.
Requirement: MQTT 3.1.1 baseline REQ_0257
status: open
satisfies: FEAT_0036

The connector shall target MQTT protocol version 3.1.1. MQTT 5.0 features (user properties, shared subscriptions, response topic) are deferred to a follow-on spec.
Requirement: Tokio sidecar inside the gateway crate REQ_0258
status: open
satisfies: FEAT_0036
is implemented by: BB_0004, BB_0021

The MQTT gateway shall host rumqttc::EventLoop on a tokio runtime contained inside taktora-connector-mqtt. Tokio shall not leak into taktora-executor’s WaitSet thread.
Requirement: Bridge channels are bounded REQ_0259
status: open
satisfies: FEAT_0036
is implemented by: BB_0021, BB_0022
is verified by: TEST_0160

The outbound (taktora-executor → tokio) and inbound (tokio → taktora-executor) bridges shall be bounded channels with configurable capacity in MqttConnectorOptions.
Requirement: Outbound bridge saturation surfaces as BackPressure REQ_0260
status: open
satisfies: FEAT_0036
is refined by: ARCH_0033
is implemented by: BB_0021, BB_0022
is verified by: TEST_0139

When the outbound bridge channel is full, ChannelWriter::send shall return ConnectorError::BackPressure and the connector shall report ConnectorHealth::Degraded.
Requirement: Inbound bridge saturation drops frames and signals Degraded REQ_0261
status: open
satisfies: FEAT_0036
is refined by: ARCH_0033
is implemented by: BB_0021, BB_0022
is verified by: TEST_0140

When the inbound bridge channel is full, the gateway shall (1) increment the per-channel inbound-drop counter exposed via InboundOutcome::Dropped { count } on the bridge’s try_send return, (2) drop the offending message for that delivery, and (3) emit a ConnectorHealth::Degraded { reason: "dropped N inbound frames" } health transition when the cumulative inbound-drop count crosses the connector’s configured inbound_drop_threshold (default 1). The Degraded transition is emitted at most once until the connector recovers to Up via the underlying stack’s recovery path; the cumulative drop count itself is observable through every subsequent InboundOutcome::Dropped return.