MonitoredItem behaviour on OPC UA Server restart

[chrismarsh]

Hello Support,
We have a case where we use UAExpert to subscribe to (live updating) monitored items, on our OPC UA Server (this is all good).
When the server restarts, with UAExpert remaining connected, the monitored items no longer update.
Is this a case that the OPC UA Server should handle (i.e. persist and restore monitored items list over a server restart?)

Thank you.

[Support Team]

Hi,

the functionality depends on the server's implementation. A server that is just a protocol wrapper and lost connection to the data source would probably not be able to persist data, and even when owning the source, an embedded server would probably not have enough space to persist. The Client instead has a defines procedure to reconnect, and reactivate, and if not possible to reactivate (because server was down) just recreate the subscription and all the monitored items.

There are two ways in OPC UA technology to persist data (both optional and depend on server's implementation):
1) historizing: the server controls which and how many tags are historized (persisted), including duration and frequency, independently of the client's desire.
2) durable subscription: the client(s), each on its own, ask the server to persist the last value of the subscribed items with .

Whereas 1) is good to control and to handle by the server (by configuration, predictable size), each item has history independently of the number of clients asking for this particular hiszorized tag,
but 2) is more "flexible" but controled by the client(s) and may require huge (unpredictible) memory/storage (each client has private subscriptions).

[chrismarsh]

Thank you Support