6.2. Callbacks

The Callback state performs an action and waits for an event that is produced as a result of the action before resuming the workflow. The action performed by a Callback state is an asynchronous external service invocation. Therefore, the Callback state is suitable to perform fire&wait-for-result operations.

From a workflow perspective, asynchronous service indicates that the control is returned to the caller immediately without waiting for the action to be completed. After the action is completed, a CloudEvent is published to resume the workflow.

{
        "name": "CheckCredit",
        "type": "callback",
        "action": {
            "functionRef": {
                "refName": "callCreditCheckMicroservice",
                "arguments": {
                    "customer": "${ .customer }"
                }
            }
        },
        "eventRef": "CreditCheckCompletedEvent",
        "timeouts": {
          "stateExecTimeout": "PT15M"
        },
        "transition": "EvaluateDecision"
}

name: CheckCredit
type: callback
action:
  functionRef:
    refName: callCreditCheckMicroservice
    arguments:
      customer: "${ .customer }"
eventRef: CreditCheckCompletedEvent
timeouts:
  stateExecTimeout: PT15M
transition: EvaluateDecision

The action property defines a function call that triggers an external activity or service. After the action executes, the Callback state waits for a CloudEvent, which indicates the completion of the manual decision by the called service.

After the completion callback event is received, the Callback state completes its execution and transitions to the next defined workflow state or completes workflow execution if it is an end state.