Callbacks in OpenShift Serverless Logic

This document describes the Callback state and related examples. The Callback state performs an action and waits for an event, to be produced as a result of the action, to resume the workflow. The action peformed 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. Once the action is completed, a CloudEvent is published to resume the workflow.

For the workflow to identify the published CloudEvent it is waiting for, the external service developer includes the workflow instance ID in the CloudEvent header or uses the Event correlation. The following figure displays the process:

The workflow correlation described in this document focuses on the former mechanism that is based on the fact that each workflow instance contains a unique identifier, which is generated automatically.