Client/Server vs PubSub over MQTT

OPC UA Client/Server and OPC UA PubSub over MQTT solve different problems. They share OPC UA data concepts, but they do not expose the same interaction model.

OPC UA Client/Server

In the standard Client/Server model, OPC UA is primarily an information modeling system. The server exposes an address space, and clients interact with that address space through services.

The address space can contain:

ObjectTypes and objects.
VariableTypes and variables.
Structures and enumerations.
Methods that clients can call.
References that describe relationships between nodes.

Clients use NodeIds and browse paths to discover and access data. This makes Client/Server a good fit when the client needs context, structure, metadata, type definitions, and remote method calls.

OPC UA PubSub over MQTT

PubSub over MQTT is message-oriented. A publisher creates dataset messages and sends them through an MQTT broker. A subscriber receives messages and decodes the payload.

At the application level, a PubSub message is closer to a Lua table, JSON object, or dictionary than to a browseable address space. Metadata can describe the fields, but subscribers do not browse a live server address space, create sessions, or call methods through MQTT PubSub.

This makes PubSub a good fit for distributing current values, telemetry, events, or sensor data to many consumers.

Comparison

Primary model: Client/Server exposes a browseable OPC UA address space. PubSub over MQTT publishes dataset messages.
Communication pattern: Client/Server uses request/response between a client and server. PubSub uses publish/subscribe through an MQTT broker.
Discovery: Client/Server clients browse nodes and references. PubSub subscribers know topics and decode messages.
Identity: Client/Server is NodeId-centric. PubSub is field-, topic-, and message-centric.
Methods: Client/Server supports remote method calls through the Call service. Methods are not part of the PubSub message flow.
Best for: Client/Server fits rich device models, semantic data, remote operations, and integration with OPC UA clients. PubSub fits telemetry, fan-out distribution, cloud ingestion, and simple sensor data streams.

Choosing the right model

Use Client/Server when:

Clients need to browse and understand the device model.
Data should carry strong OPC UA semantics.
The application needs Read, Write, Browse, or Call services.
You need sessions, user authentication, and endpoint security negotiation.

Use PubSub over MQTT when:

The goal is to publish values or events to many receivers.
MQTT infrastructure is already part of the system.
Subscribers do not need to browse a live OPC UA address space.
The data can be represented as fields in a message.

Many systems use both. A device or gateway can expose a full OPC UA server for engineering tools and local control, while also publishing selected values over MQTT for dashboards, cloud services, or data pipelines.