I have published an InfoQ news post about the AWS team's solution for purely event-driven serverless workflows with manual steps and third-party integrations:
Amazon Web Services (AWS) recently announced that AWS Step Functions supports callback patterns to "automate workflows for applications with human activities and custom integrations with third-party services". Workflow executions can now be paused until applications return a token via the Step Functions API, which obsoletes the previously required polling or other workarounds.
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Continue reading the full post in AWS Step Functions Gains Callback Patterns to Resume Paused Workflows.