Triggering
Triggering links MonitoredItems so that when one triggering item reports, a set of linked items release whatever they have queued. Set with SetTriggering, the linked items sit in MonitoringMode Sampling, costing acquisition but no traffic until the trigger fires. It is the server-side way to capture context around an event.
How the link works
You nominate a triggering item, already in Reporting mode, and a list of linked item handles in Sampling mode. When the triggering item produces a notification, the linked items' queued notifications are included in the same NotificationMessage. The relationship is N to M - one trigger can release many items, and an item can be linked to several triggers - and links are managed incrementally by adding and removing handles.
The classic use case
A batch-complete or part-ejected boolean triggers thirty process values, so the collector receives a coherent record per part without polling and without streaming thirty tags continuously. The same pattern serves fault capture: a fault bit triggers the surrounding analogue values, giving you the context of the event at the moment it happened rather than at the next scheduled read.
Support is real but uneven
SetTriggering is implemented in the UA-.NETStandard V2 subscription engine and in open62541, but OPC Publisher does not use server-side triggering at all - a genuine interop gap if your edge layer is Publisher-based. Links also live with the subscription: after a reconnect that recreates subscriptions rather than transferring them, the links must be re-established, and clients frequently forget.
Example
Capturing process context on a part-complete pulse
triggering item ns=3;s="Line3"."PartComplete" MonitoringMode Reporting
linked items MeltTemperature, MeltPressure, CycleTime, ScrewSpeed, ...
MonitoringMode Sampling, SamplingInterval 100 ms, QueueSize 1
SetTriggering(subscriptionId, triggeringItemId, linksToAdd=[...], linksToRemove=[])
On each PartComplete notification, the linked items' queued values ride along
in the same NotificationMessage. Between parts they generate no traffic at all.Defined in: Part 4 - Services, SetTriggering and the triggering model for MonitoredItems.
Last verified 7 September 2026 against OPC UA 1.05.07. OPC UA Connect is an educational resource and is not affiliated with the OPC Foundation.