Chapter 10. OPC UA ServerGFK-2224Q January 2017 197OPC UA Performance ConsiderationsAs mentioned in preceding notes, the OPC UA Server supports 5 concurrent clients and 12,500 addressableelements. Care is suggested both approaching and operating at the max-max condition: 5 clients accessingthe full complement of 12,500 elements simultaneously.The OPC UA Server has been designed and implemented to support the maximum number of clients readingthe maximum number of OPC UA elements. However, at maximum load OPC UA clients may experiencedegraded performance, evidenced by slower than usual response times when browsing, reading, and writingaddressable elements.If OPC UA Server response times are not adequate for your application, consider implementing one or more ofthe following mitigation strategies: Controller Communications Window – OPC UA traffic is processed in the controller communicationswindow. If the window’s duration is inadequate for the amount of processing required to servicemultiple client requests, increasing the comm window will improve response times by the PLC. OPC UA Subscriptions – OPC UA defines a subscription model for the server to transmit data only whenan element’s value updates, rather than having a client continuously poll the server to read an elementthat is otherwise unchanged in value. Determine whether your application can use a subscription tocollect data from the PLC. OPC UA Aggregating Server – An aggregating server can both allow multiple clients to connect as asingle instance, and combine read-requests for the same element(s) into one message.Sessions and Subscriptions for OPC UAThere may be up to five concurrent sessions for the OPC UA server and across these sessions there may be upto ten concurrent subscriptions of variables. The subscription limit is shared across all available sessions.
