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ExternalEventDef and ExternalEvent

In LittleHorse, an ExternalEventDef is a Metadata Object that defines some event or activity occuring outside of the LittleHorse WfRun. An ExternalEvent is an Execution Object that represents the occurrence of such an event. Common use-cases for an External Event would be encapsulating a webhook from github when a new branch is pushed, or representing an event from DocuSign that is fired when a document is completed.

An External Event can be recorded through the PutExternalEvent gRPC call. This can be accomplished using clients in any of our SDK's or through the use of lhctl. Future versions of LittleHorse will allow you to directly hook up a webhook or event streaming system (eg Kafka) to LittleHorse and send events in a hands-off manner.


Workflow engines aim to help automate business processes. Oftentimes, such business processes involve interacting with the outside world and listening for things to happen outside of the workflow engine before making a decision about what to do next. That is precisely what an ExternalEvent is for: while a TaskRun allows a WfRun to change the outside world, an ExternalEvent allows a WfRun to react to the outside world. Some common use-cases are to:

  • Integrate with asynchronous third-party API's.
  • Trigger Interrupts.
  • Wait for a person to sign a document in DocuSign.
  • Wait for a customer to respond to a text message using a callback from the Twilio API.

ExternalEvent Structure

An ExternalEvent has a composite ID consisting of:

  • The wfRunId of the associated WfRun
  • The name of the ExternalEventDef
  • A guid which is unique to the

Why the wfRunId? An ExternalEven is intended to affect the behavior of a WfRun; therefore, an ExternalEvent must be correlated to a specific WfRun.

ExternalEvents have a payload which is simply a VariableValue.

ExternalEventDef Structure

There is an ExternalEventDef API Resource. The relationship between an ExternalEventDef and an ExternalEvent is the same as the relationship between a WfSpec and a WfRun.

Currently, the only field in an ExternalEventDef is the name of the event type. The event name is used to isolate events of different types, for example "document-signed" and "document-rejected" events.

In future versions of LittleHorse, the ExternalEventDef will have more information:

  • An optional schema for the data type to enable type checking in the WfSpec.
  • Information on how to correlate an ExternalEvent to a WfRun without explicitly setting the wfRunId.

ExternalEvent Nodes

One use for an ExternalEvent is the EXTERNAL_EVENT Node Type. When a ThreadRun reaches an EXTERNAL_EVENT node, it will halt until an ExternalEvent of the specified type (and with the correct wfRunId) arrives. The output of the NodeRun is simply the payload of the ExternalEvent.

An EXTERNAL_EVENT node can have a timeout configured; this means that if the ExternalEvent does arrive within X seconds after the ThreadRun arrives at the node, then the NodeRun will fail with a TIMEOUT exception.

If the ExternalEvent arrives before the ThreadRun reaches the EXTERNAL_EVENT Node, that's ok! The ThreadRun will immediately pick up the ExternalEvent and move on to the next Node.

An ExternalEvent gets correlated to one and only one NodeRun. When posting an event, you can optionally specify a threadRunNumber to ensure that the ExternalEvent can only be assigned to a NodeRun on that specific ThreadRun.


An ExternalEvent can also be used to trigger an Interrupt in LittleHorse. For more information, see the Interrupt documentation.


If you configure an EXTERNAL_EVENT node with the ExternalEventDef foo, you cannot use the foo external event def as an Interrupt Trigger elsewhere in the same WfSpec.