Nurkiewicz, Tomasz,Christensen, Ben. Reactive Programming with RxJava: Creating Asynchronous, Event-Based Applications (p. 131). O'Reilly Media. Kindle Edition.

// Scenario
// 1. The list can be potentially quite long
// 2. Sending an email might take several millionseconds or even seconds
// 3. The application must keep running gracefully in case of failures, but report in the end which tickets failed to delievered.

List<Ticket> failures = Observable.from(tickets)
				.flatMap(ticket ->
						rxSendEmail(ticket)
							.flatMap(response -> Observable.<Ticket>empty())
							.doOnError(e -> log.warn("Failed to send {}", ticket, e))
							.onErrorReturn(err -> ticket))
				.toList()
				.toBlocking()
				.single()

// above code collect tickets failed to be delievered

// cf. ignoreElements() : ignores all emitted values and forward onCompleted or onError.