MQTT, Sparkplug B and the Unified Namespace — Industry 4.0 & IIoT — Smart Manufacturing Software

4. MQTT, Sparkplug B and the Unified Namespace

Intermediate★ Must knowLesson 4 of 10

MQTT — lightweight publish/subscribe protocol over TCP (1883, TLS 8883). Clients publish to topics; the broker delivers to subscribers.

Feature	Meaning
Topics	Hierarchical strings: `acme/pune/assembly/line1/robot2/temperature`
Wildcards	`+` one level (`acme/+/assembly/#`), `#` all remaining levels
QoS 0 / 1 / 2	At most once / at least once / exactly once
Retained message	Broker keeps the last value for new subscribers
Last Will & Testament	Broker publishes it if a client disconnects unexpectedly (device offline)
Persistent session	Broker queues messages for a client while it is offline
MQTT 5	Reason codes, user properties, shared subscriptions, message expiry

Sparkplug B (Eclipse) adds an industrial standard on top of MQTT: fixed topic namespace (spBv1.0/group/NDATA/edge-node/device), Protobuf payloads with metrics, birth/death certificates (NBIRTH/NDEATH) so subscribers know state, and report-by-exception.

Unified Namespace (UNS) — a single, event-driven hub (usually an MQTT broker) where every system publishes its current state under an ISA-95-structured topic tree, and every consumer subscribes to what it needs — replacing point-to-point integrations.

csharp

// Publish a machine state with MQTTnet (v4/v5 style)
var client = new MqttClientFactory().CreateMqttClient();
await client.ConnectAsync(new MqttClientOptionsBuilder().WithTcpServer("broker.plant.local", 8883).WithTlsOptions(o => o.UseTls())
    .WithClientId("line1-edge").WithWillTopic("acme/pune/assembly/line1/edge/status").WithWillPayload("offline").WithWillRetain().Build());

var json = JsonSerializer.Serialize(new { state = "Running", speed = 42.5, ts = DateTime.UtcNow });
await client.PublishAsync(new MqttApplicationMessageBuilder()
    .WithTopic("acme/pune/assembly/line1/filler/state").WithPayload(json)
    .WithQualityOfServiceLevel(MqttQualityOfServiceLevel.AtLeastOnce).WithRetainFlag().Build());

MQTT vs OPC UA: OPC UA = rich information model and client/server access to machines (great at Level 1–2); MQTT = lightweight pub/sub for moving data to many consumers and the cloud. They are complementary (and OPC UA PubSub can run over MQTT).