What is the difference between Mono and Flux in reactive programming?

Mono emits 0 or 1 element. Flux emits 0 or more elements (including infinitely many).

Usage examples:

• Mono: Result of an operation on a single object (e.g., saving a record in a database, retrieving a single item by ID).
• Flux: Stream of events (e.g., user clicks, data from a file, collection elements).

// Mono example: getting a single user
Mono<User> findUserById(Long userId) {
    return userRepository.findById(userId);
}

// Flux example: getting all users
Flux<User> findAllUsers() {
    return userRepository.findAll();
}

Main differences:

Characteristic	Mono	Flux
Number of elements	0 or 1	0 or more
Representation	Represents a single event	Represents a stream of events
Operations	Applied to a single element	Applied to a stream of elements