OPC UA supports two communication patterns: Client/Server (the classic Request/Response model) and Pub/Sub (publish/subscribe over MQTT or UDP multicast/unicast), the latter introduced in version 1.04 to address Industrie 4.0 use cases.

OPC UA is dual-mode by design. Client/Server has been in the standard since version 1.0 and exposes bidirectional services such as Read, Write and Subscribe. Pub/Sub was added in 1.04 (2018) with transport bindings over MQTT, AMQP or UDP, which scales much better for IIoT and cloud telemetry. Both modes can coexist inside the same device, and the typical split is Client/Server for configuration and engineering access, Pub/Sub for high-volume telemetry.

When sizing a new architecture, do not pick one mode dogmatically: keep Client/Server for engineering/SCADA reads and writes, and add Pub/Sub only when you need cloud-bound telemetry or many-to-many distribution.