Reactive Programming vs Message Broker
Reactive:
Asynchronous programming approach have their nativity of the program architecture skeletoned via reactor pattern that is adherent to the OS level even driven architecture.
Message Broker:
The messaging channels are more a short term message storage systems that delays the processing and catalyst the Data level Even Driven Architecture.
Conclusion:
Hence, reactive programming language is a process execution deferer (with OS) while mesaage brokers are processing data deferers.
Usage:
In API development async programming cam be used to address the user interaction request bottleneck while message brokers aid in internal data communication and external server integrations.