OPC UA server historical access support

Question

We are looking for a solution for collecting data from different SCADA systems. It seems that OPC UA is a good approach for that. Data collection will be done from a single system to multiple SCADA systems over the internet (https). So, we are planning to develop a OPC UA client that can connect to multiple OPC UA servers. Data will be collected with a given interval. The system should be able to handle if the connection between client and server is lost for a period of time. In that case, I assume we need to get the data by looking into historical data. Hence, we need a server that support HA (Historical Access). Are there any servers supporting this or do we need to develop our own server implementation? Or is there a better approach than the one described above? Any help or hints on this would be appreciated.

Answer 1

How long would you expect the connection to be down for?

While leaning on HA is certainly one way to handle this, I think you'll have a hard time finding any products on the market right now that are actually implementing HA.

Luckily, you can probably deal with this scenario without HA. If you create your subscriptions with a generous lifetime, and create your monitored items with a queue size that, based on the sampling interval, could hold enough data changes to span up to that lifetime, then you upon reconnecting you should receive all of the data changes that happened while the connection between client and server was lost.

If the connection is expected be down for days/weeks/months, then this won't work without support for durable subscriptions, introduced in UA 1.03, but then again you're limited by finding a server that supports durable subscriptions. (Durable subscriptions are basically just a way to make the lifetime of target subscriptions much longer than normally allowed and to instruct the server that they are expected to persist these subscriptions to disk and restore them in the case of e.g. server restart)