Embodiments are directed to providing schedule-based processing using web service on-demand message handling threads and to managing processing threads based on estimated future workload. In an embodiment, a web service platform receives a message from a client that is specified for schedule-based, background handling. The web service platform includes an on-demand message handling service with processing threads that are configured to perform on-demand message processing. The web service platform loads the on-demand message handling service including the on-demand message handling threads. The web service platform implements the on-demand message handling service's threads to perform background processing on the received client message. The client messages specified for background handling are thus handled as service-initiated on-demand tasks.