An event template is a construct that allows you to specify qualifying or matching criteria based on values of one or more of an event's fields. In event templates, you can qualify only on those event fields whose type is a primitive type. Event templates are used with on statements. See The on statement.

An event template begins with the name of an event type that is to be matched.

Event templates can be either positional (see By-position qualifiers) or named (see By-name qualifiers) or a combination of both. Further, the criteria can be omitted entirely, in which case any event of the same event type will match. When both positional and named qualifiers are present in an event template qualifier expression list, the positional matches must come first.

Optionally, a colon and an identifier can follow the event expressions. This is called an event coassignment and specifies a variable whose value will become (that is, will be assigned) a reference to the matched event structure when the correlator detects a matching event and listener, and invokes the actions defined in the listener.

See also Stream source templates.