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Chapter 10. OPC UA Server
GFK-2224Q January 2017 197
OPC UA Performance Considerations
As mentioned in preceding notes, the OPC UA Server supports 5 concurrent clients and 12,500 addressable
elements. Care is suggested both approaching and operating at the max-max condition: 5 clients accessing
the full complement of 12,500 elements simultaneously.
The OPC UA Server has been designed and implemented to support the maximum number of clients reading
the maximum number of OPC UA elements. However, at maximum load OPC UA clients may experience
degraded performance, evidenced by slower than usual response times when browsing, reading, and writing
addressable elements.
If OPC UA Server response times are not adequate for your application, consider implementing one or more of
the following mitigation strategies:

Controller Communications Window – OPC UA traffic is processed in the controller communications
window. If the window’s duration is inadequate for the amount of processing required to service
multiple 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 when
an element’s value updates, rather than having a client continuously poll the server to read an element
that is otherwise unchanged in value. Determine whether your application can use a subscription to
collect data from the PLC.

OPC UA Aggregating Server – An aggregating server can both allow multiple clients to connect as a
single instance, and combine read-requests for the same element(s) into one message.
Sessions and Subscriptions for OPC UA
There may be up to five concurrent sessions for the OPC UA server and across these sessions there may be up
to ten concurrent subscriptions of variables. The subscription limit is shared across all available sessions.