Hello Felix,

Thank you for the question. As a start, please see this blog post about the most important parameters related to sessions and subscriptions: https://prosysopc.com/blog/opc-ua-sessions-subscriptions-and-timeouts/

Usually multiple monitored items are grouped inside a single subscription. This way you don’t need to create 3 subscriptions per client, but you could create one subscription with three monitored items. A subscription is always related to one session, so you will necessarily need 25 subscriptions if the different micro service clients are different applications in practice.

So, after this change you will be left with 25 subscriptions and 75 monitored items. I think this is a relatively normal amount of subscriptions and monitored items, but of course you could check the OPC UA server vendor documentation to find out if there’s some documented limits. Also, it’s important to configure suitable settings for data sampling, meaning parameters PublishingInterval and SamplingInterval. Typically suitable value for PublishingInterval is 1 second.