Celix PushSteams are based on the OSGI Promises (OSGi Compendium Release 7 Specification, Chapter 706). It follows the specification as close as possible, but some adjustments are mode for C++17.
NOTE: this implementation is still experiment and the api and behaviour will probably still change.
OSGi Compendium Release 7 Promises Specification (HTML)
OSGi Compendium Release 7 Specification (PDF)