MQTT vs OPC-UA vs Sparkplug B for IIoT comparison

MQTT is a lightweight publish-subscribe protocol designed for constrained devices and unreliable networks. It's popular for IIoT because it is extremely lightweight (2-byte fixed header), works over TCP/IP with TLS, supports millions of simultaneous device connections on cloud brokers, handles intermittent connectivity well via QoS levels and Last Will and Testament messages, and has a large ecosystem of brokers (Mosquitto, EMQX, HiveMQ) and cloud integrations (AWS IoT Core, Azure IoT Hub). Its main limitation is the lack of semantic structure — payloads are opaque, and topic naming is unstandard unless combined with Sparkplug B.

Sparkplug B (Eclipse Sparkplug specification) adds four critical capabilities to MQTT that raw MQTT lacks: a standardised topic namespace (GroupID/MessageType/EdgeNodeID/DeviceID), device state management via Birth and Death certificate messages (NBIRTH/NDEATH/DBIRTH/DDEATH), typed metric payloads using Protobuf encoding (each metric has a name, value, datatype, timestamp, and quality), and system-level state management via SCADA HOST STATE messages. Together, these additions make MQTT behave like a structured industrial protocol while retaining MQTT's broker infrastructure, scalability, and ecosystem.

Use OPC-UA Client-Server when you need bidirectional communication with industrial machines — reading current values, writing setpoints, calling control methods, and browsing the machine's data model for auto-discovery. OPC-UA is the standard for direct PLC/SCADA integration. Use MQTT (ideally Sparkplug B) when you need to aggregate data from many devices, publish to multiple consumers, scale to hundreds or thousands of devices, or integrate with cloud IoT platforms. The most effective architectures use both: OPC-UA at the field level (PLC ↔ edge gateway) and MQTT/Sparkplug B at the plant-to-enterprise level (edge gateway ↔ SCADA/cloud).

Sparkplug B defines a mandatory topic namespace: spBv1.0/[GroupID]/[MessageType]/[EdgeNodeID]/[DeviceID]. GroupID groups related edge nodes (e.g. by factory or production line). MessageType identifies the message purpose (NBIRTH, NDEATH, NDATA, NCMD for edge node messages; DBIRTH, DDEATH, DDATA, DCMD for device messages; STATE for SCADA host state). EdgeNodeID identifies the Sparkplug Edge Node (typically the edge gateway). DeviceID identifies a specific device connected to the edge node. This standardised structure allows any Sparkplug-compliant SCADA or analytics platform to correctly interpret messages from any Sparkplug-compliant device.

For plant-level (on-premises) MQTT brokers: EMQX Enterprise and HiveMQ offer the best Sparkplug B support, clustering for high availability, and enterprise security features including RBAC, audit logging, and TLS mutual authentication. Mosquitto is suitable for smaller deployments and edge broker roles. For cloud-based MQTT: AWS IoT Core, Azure IoT Hub, and Google Cloud IoT Core provide managed MQTT infrastructure with deep integration into their respective cloud analytics services. HiveMQ Cloud offers a managed MQTT broker with strong Sparkplug support for teams that want MQTT broker SLA without cloud lock-in.

OPC-UA Pub/Sub (Part 14 of the OPC-UA specification) extends OPC-UA's communication model with a publish-subscribe pattern that can use MQTT or AMQP as its transport layer. It allows OPC-UA's semantic information model — with typed nodes, engineering units, and relationships — to be published to an MQTT broker in either JSON or binary (UADP) format. This combination gives you MQTT's scalability and broker ecosystem with OPC-UA's semantic richness. Cloud platforms like AWS IoT SiteWise and Azure Digital Twins support OPC-UA Pub/Sub over MQTT as a preferred data ingestion path for digital twin scenarios.

Sparkplug B's Birth certificate mechanism ensures data consistency on reconnection. When an edge node connects or reconnects, it publishes an NBIRTH (Node Birth) message containing the complete current state of all metrics — not just changes. If a SCADA host was offline during the disconnection and misses NDATA messages, it receives the complete current state in the NBIRTH when connectivity resumes. The SCADA host's STATE message handles the reverse: when the SCADA host reconnects, it publishes its ONLINE state, which triggers all edge nodes to re-publish their NBIRTH messages, ensuring the SCADA system has a complete, consistent view of all connected devices.

For cloud IoT platform connectivity, MQTT is the dominant protocol — all major cloud IoT platforms (AWS IoT Core, Azure IoT Hub, Google Cloud IoT Core) support MQTT natively. For structured industrial data that needs semantic richness in the cloud, OPC-UA Pub/Sub over MQTT is increasingly supported, particularly by AWS IoT SiteWise (which has native OPC-UA ingestion) and Azure Digital Twins (which integrates with the Azure IoT Edge OPC Publisher module). Sparkplug B is well-supported by industrial IoT platforms like Ignition SCADA, Inductive Automation's cloud offerings, and HiveMQ Cloud, but less natively supported by general-purpose cloud IoT platforms compared to raw MQTT.