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Subscription limits for an OPC UA client

Yuri Bairros

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11/03/2025

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11/03/2025 - 06:12

Hi!

I’m currently facing an issue while using the OPC UA Client node in Node-RED to connect to a machine via the OPC UA protocol.

I’ve noticed that when creating multiple subscriptions, the client seems to hit a limit — after around 10 subscriptions, new ones either fail to establish or stop receiving data.

It’s important to mention that the issue is not related to the number of variables within a single subscription, since I can successfully add many variables to one subscription without any problems. The problem occurs specifically when trying to create additional subscriptions on the same client.

So, I’d like to ask:

Is there any limit on the number of subscriptions per client defined by the OPC UA specification or by server/client configuration?

If such a limit exists, is there a way to adjust or increase it (via session parameters, server settings, or client configuration)?

I want to ensure that in a production environment, the system can properly scale with the required number of subscriptions or sessions.

Thank you very much.

Best regards,
Yuri

Randy Armstrong

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11/03/2025 - 18:10

Every server has limits it publishes in its ServerCapabilities

https://reference.opcfoundatio.....docs/6.3.2

MaxSubscriptions is an unsigned integer specifying the maximum number of Subscriptions the Server can support. The value specifies the maximum the Server can support under normal circumstances, therefore there is no guarantee the Server can always support the maximum.

MaxMonitoredItems is an unsigned integer specifying the maximum number of MonitoredItems the Server can support. The value specifies the maximum the Server can support under normal circumstances, therefore there is no guarantee the Server can always support the maximum.

MaxSubscriptionsPerSession is an unsigned integer specifying the maximum number of Subscriptions per Session the Server can support. The value specifies the maximum the Server can support under normal circumstances, therefore there is no guarantee the Server can always support the maximum.

MaxMonitoredItemsPerSubscription is an unsigned integer specifying the maximum number of MonitoredItems per Subscription the Server can support. The value specifies the maximum the Server can support under normal circumstances, therefore there is no guarantee the Server can always support the maximum.