External Events

info

Before you can use an ExternalEventDef in a WfSpec, you need to create the ExternalEventDef metadata object in LittleHorse. You can do that following our metadata management docs.

You can use WorkflowThread#WaitForEvent() to add an EXTERNAL_EVENT Node that causes the WfRun to block until an ExternalEvent arrives. It's simple:

Java

public void threadFunction(WorkflowThread thread) {
    NodeOutput eventOutput = thread.waitForEvent("my-event-name");
}

Go

func someThreadFunction(thread *littlehorse.WorkflowThread) {
    eventOutput := thread.WaitForEvent("my-event-name")
}

Python

def thread_function(thread: WorkflowThread) -> None:
    ext_event_output = thread.wait_for_event("my-event-name")

Accessing Event Content

Lastly, just as with TASK nodes you can use the NodeOutput from WorkflowThread::waitForEvent() to mutate a WfRunVariable:

Java

public void threadFunction(WorkflowThread thread) {
    WfRunVariable myVar = thread.addVariable("my-var", VariableType.JSON_OBJ);
    NodeOutput eventOutput = thread.waitForEvent("my-event-name");
    thread.mutate(myVar, VariableMutationType.ASSIGN, eventOutput);
}

Go

func someThreadFunction(thread *littlehorse.WorkflowThread) {
    myVar := thread.AddVariable("my-var", lhproto.VariableType_JSON_OBJ)
    eventOutput := thread.WaitForEvent("my-event-name")
    thread.Mutate(myVar, lhproto.VariableMutationType_ASSIGN, eventOutput);
}

Python

def thread_function(thread: WorkflowThread) -> None:
    myVar = thread.add_variable("my-var", VariableType.JSON_OBJ)
    event_output = thread.wait_for_event("my-event-name")
    thread.mutate(myVar, VariableMutationType.ASSIGN, event_output)

note

If you use an ExternalEventDef for any EXTERNAL_EVENT node as shown in this tutorial, you cannot re-use that ExternalEventDef as a trigger for Interrupts.