Mono<T> vs Flux<T>.

Mono<T> and Flux<T> are Reactor's two Publisher implementations, and the only difference is cardinality: Mono emits 0 or 1 element then completes; Flux emits 0 to N (possibly infinite) elements then completes. Choosing the right one is a typing decision that documents intent — a method returning Mono<User> promises at most one user.

When to use which

Use Mono<T> for a single asynchronous result: fetch one entity by id, an HTTP response, a save() that returns the saved row, or a side-effecting call with no payload (Mono<Void>). Use Flux<T> for a stream: query results, server-sent events, a Kafka topic, a paginated feed.

Mono<User>  user   = userRepo.findById(id);      // 0..1
Flux<Order> orders = orderRepo.findByUser(id);   // 0..N
Mono<Void>  done   = mailer.send(message);        // completion signal only

Converting between them

This is the part interviewers probe, because it's where people get stuck.

// Flux -> Mono
Mono<Long>      count = orders.count();          // a single aggregate
Mono<Order>     first = orders.next();           // first element (or empty)
Mono<List<Order>> all = orders.collectList();    // BEWARE: buffers everything

// Mono -> Flux
Flux<Order> many = user.flatMapMany(u -> orderRepo.findByUser(u.id()));

flatMapMany is the canonical "one input, many outputs" bridge. Note collectList() defeats streaming — it materialises the whole stream into memory, so avoid it on large or unbounded sources.

Both are lazy and reusable

Neither does anything until subscribed. The same Mono/Flux can be subscribed multiple times; whether each subscription re-runs the work depends on whether the source is cold or hot (covered separately). A Mono<T> is not a CompletableFuture<T> — the future has already started running, while the Mono is a deferred recipe.

A subtle empty-vs-error point

Mono.empty() completes with no value (onComplete with no onNext) — distinct from an error. Downstream you handle "nothing found" with switchIfEmpty(...) or defaultIfEmpty(...), not with error handling. Conflating empty and error is a common bug.