A Query Language for OPC UA Event Filters

The OPC UA standard combines industrial communication with information modeling. In order to remove the need for continuous polling, OPC UA clients can create subscriptions to be notified about data changes and events. OPC UA Event Filters provide powerful constructs for the server-side selection of relevant events. This feature is however underused in practice, also because of the complexity of Event Filters. Event Filters are expressed in a “byte-code” encoding. Assembling this encoding by hand is difficult and error-prone. This paper discusses the semantics of OPC UA Event Filters and proposes a query language to facilitate their definition. A formal grammar for the query language is provided in the BNF format. An implementation of a corresponding parser is available as part of the open62541 library.