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In C and C++, you can register a handler function to handle an OS signal: when the signal is caught, the program gets interrupted and the interrupt handler is executed.

In LittleHorse, you can handle an ExternalEvent and trigger an Interrupt ThreadRun.


When an ExternalEvent is posted to a WfRun, and a ThreadRun has registered an Interrupt for that specific ExternalEventDef, then the affected ThreadRun is halted. Once the ThreadRun is halted, then a new Child ThreadRun (specifically, an Interrupt Handler) is created.

Once the Interrupt Handler (Child) completes, the Interrupted Thread (Parent) is resumed. If the Interrupt Handler fails, then the Parent also fails with a CHILD_FAILED exception. This error is unrecoverable.

ExternalEvent Payload

Recall that an ExternalEvent has a payload, which is a VariableValue. The Interrupt Handler thread can access that value through the "INPUT" Variable. Recall from the Exception Handler Docs that "INPUT" is a reserved Variable name used for the same purpose.


Interrupts are registerd at the ThreadSpec level. Only one ThreadSpec may register an Interrupt for a specific ExternalEventDef.

When a ThreadRun is Interrupted, it must first halt. As per the WfRun Documentation, a ThreadRun is not considered HALTED until all of its Children are HALTED as well. Therefore, interrupting a ThreadRun causes all of the Children of the Interrupted ThreadRun to halt as well.

The Interrupt Handler is a Child of the Interrupted Thread. Therefore, it has read/write access to all of the Interrupted Thread's Variables.

Use Cases

Interrupts may be used for various reasons, such as to:

  • Kill a running WfRun and perform some cleanup action (such as notifying a customer) before making the Interrupted Thread fail.
  • Update the value of some Variable in a running WfRun, such as to change contact info or add items to a shopping cart.
  • Send heartbeats from an external system and allow the WfRun to keep track of the last seen activity (such as when determining when to invalidate an access token due to inactivity).