What are schedulers in Reactor?

In Reactor (a reactive library for Java), schedulers are responsible for managing the threads on which reactive stream operations are executed.

Main types of schedulers:

• Schedulers.immediate() — executes tasks in the current thread.
• Schedulers.single() — uses a single thread for sequential task execution.
• Schedulers.elastic() — creates a new thread as needed, caches and reuses them, suitable for blocking operations.
• Schedulers.parallel() — a thread pool optimized for parallel computations.

Using schedulers allows controlling where exactly the code will run, for example, switching from an IO thread to a computation or UI thread.

Example:

Flux.range(1, 5)
    .subscribeOn(Schedulers.parallel()) // data generation in a parallel pool
    .map(i -> i * 2)
    .publishOn(Schedulers.single()) // switch to a single thread for further processing
    .subscribe(System.out::println);

Thus, schedulers help manage asynchrony and parallelism in reactive applications.