OPC UA OPC Unified Architecture
Industrial standard for data exchange in factories — abstracts all underlying protocols (Modbus, Profinet, etc.) behind a unified object model. IEC 62541.
Replaces the old "OPC Classic" (Windows-DCOM-based, COM/DCOM dying). OPC UA is platform-independent, TLS-secured, has an address-space model with types + methods. Powers "Industry 4.0" initiatives + every modern SCADA system. The PubSub extension (2018) makes it possible over MQTT/AMQP brokers, not just client-server. Implementations: open62541 (C, open source), Eclipse Milo (Java), node-opcua. Heavy and complex — typically enterprise OT engineers, not web devs.