corMqttBridge

The MQTT bridge for coraine: an mqtt.so that carries NGSI-LD over MQTT, through libmosquitto. Until this repo, libmosquitto was linked into every broker; now a broker started without --bridges mqtt never maps it.

Notifications

A Subscription whose notification.endpoint.uri is mqtt:// or mqtts:// (TS 104 243, NGSI-LD MQTT Notification Binding) is delivered by this plugin. Everything about the delivery is in the Subscription, as NGSI-LD defines it:

Where	What
endpoint.uri	mqtt[s]://[<user>[:<pass>]@]<host>[:<port>]/<topic>[/<subtopic>]* - default ports 1883 / 8883
endpoint.notifierInfo	MQTT-QoS (0, 1, 2 - default 0) and MQTT-Version (mqtt3.1.1, mqtt5.0 - default mqtt5.0)
endpoint.receiverInfo	copied into the message's metadata by the broker

The message - the { "metadata": ..., "body": ... } envelope - is built by the broker; this plugin sends it as it is given. With --insecureNotif, an mqtts:// endpoint's self-signed certificate is accepted.

Without --bridges mqtt, the broker refuses such a Subscription when it is created or updated, rather than accepting it and failing every notification.

Channels - a topic <-> an entity attribute

As for every bridge: a message on a Channel's topic becomes the attribute's value, and a write to the attribute is published on the topic. The Bridge is one connection, to mqtt.server in the --bridgeConfig file - a Subscription's MQTT URI without the topic, plus the connection's MQTT-Version:

{ "mqtt": { "server": { "uri": "mqtt://mosquitto:1883", "MQTT-Version": "mqtt5.0", "clientId": "coraine-plant" },
            "ngsild": { "topics": {
              "plant/tank1/level": { "entityId": "urn:ngsi-ld:Tank:T1", "entityType": "Tank", "attribute": "level",
                                     "channelInfo": [ { "key": "MQTT-QoS", "value": "1" } ] } } } } }

• A Channel's topic is exact - no + / #: a message finds its Channel by it.
• channelInfo uses notifierInfo's vocabulary: MQTT-QoS.
• A JSON payload is the value as it is; anything else becomes a JSON string.
• The bridge's own publications do not come back as samples (MQTT 5 no local; on 3.1.1 the last payload published is dropped once when it echoes).
• libmosquitto's threaded loop owns the connection: started in the background, reconnecting by itself, every inbound topic subscribed again on reconnect.

The user documentation is coraine's doc/mqtt-bridge.md.

The contract

corBridge holds it: BridgeDriver.h is what this fills in - for notifications notifySchemes and notify() (ABI 10), and for Channels channelAddInfo, channelDel and publish(), with sampleIn() on the broker's side.

Building

Needs libmosquitto (Debian/Ubuntu: libmosquitto-dev).

make                  # mqtt.so, debug (traces compiled in)
make BUILD=release
make install          # to /opt/seamware/plugins/bridge (PLUGIN_DIR)
make contract         # does it still satisfy BridgeDriver.h?

The sibling cor repos are expected beside this one (COR_LIBS ?= ..); the corLibs umbrella clones and builds it with the rest. Its tests live in coraine: a bridge is tested through the broker that loads it.