reactor-core/src/main/java/reactor/core/publisher/Flux.java
public static <T> Flux<T> mergeSequential(Publisher<? extends Publisher<? extends T>> sources) {
		return mergeSequential(sources, false, Queues.SMALL_BUFFER_SIZE,
				Queues.XS_BUFFER_SIZE);
	}
public static <T> Flux<T> mergeSequential(Publisher<? extends Publisher<? extends T>> sources) {
		return mergeSequential(sources, false, Queues.SMALL_BUFFER_SIZE,
				Queues.XS_BUFFER_SIZE);
	}
Merge data from {@link Publisher} sequences emitted by the passed {@link Publisher}
into an ordered merged sequence. Unlike concat, the inner publishers are subscribed to
eagerly. Unlike merge, their emitted values are merged into the final sequence in
subscription order.
<p>
<img class="marble" src="doc-files/marbles/mergeSequentialAsyncSources.svg" alt="">

@param sources a {@link Publisher} of {@link Publisher} sources to merge
@param <T> the merged type

@return a merged {@link Flux}, subscribing early but keeping the original ordering