What Is the OPC UA Client-Server Model?

The Founder's Explanation (Teach Exactly This Way)

"A library and its readers. The library holds all the books — that is the server. The reader comes, gets a membership, browses, and can even say, whenever a new book on this subject arrives, inform me — that is subscribe. The library does not run to every house; readers come and subscribe. OPC UA works exactly this way."

Teaching the Concept

The Library and Its Readers A library holds the books; readers take membership, browse, and can ask to be told whenever a new book on their subject arrives. In OPC UA the server is the library and the client is the reader — it connects, browses, and subscribes to what changes. Get the roles right and the connection simply works. The Model ``` SERVER exposes data as an address space of nodes CLIENT connects, browses, reads/writes SESSION secured connection (certs, sign/encrypt) SUBSCRIPTION server pushes changes via monitored items ``` A Working Connection ``` 1. Client opens a SECURE session to the server 2. Client BROWSES the address space 3. Client SUBSCRIBES to points it cares about 4. Server PUSHES updates only when values change ``` The client-server model is a lending library for data — servers hold it, clients subscribe, updates arrive on change.

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Related Topics

• What is OPC UA in BMS and when is it needed? — when to reach for OPC UA
• What is a protocol in BMS? — the shared language controllers use to talk
• Browse all Protocols topics — more from this section of the EnSmart BMS Library