Book.java

public void book(TripReq input) {
    Flow f = Flows.currentFlow();

    FlowFuture<BookingRes> flightFuture =
        f.invokeFunction("./flight/book", input.flight, BookingRes.class);

    FlowFuture<BookingRes> hotelFuture =
        f.invokeFunction("./hotel/book", input.hotel, BookingRes.class);

    FlowFuture<BookingRes> carFuture =
        f.invokeFunction("./car/book", input.carRental, BookingRes.class);

    flightFuture.thenCompose(
        (flightRes) -> hotelFuture.thenCompose(
            (hotelRes) -> carFuture.whenComplete(
                (carRes, e) -> EmailReq.sendSuccessMail(flightRes, hotelRes, carRes)
            )
            .exceptionallyCompose( (e) -> cancel("./car/cancel", input.carRental, e) )
        )
        .exceptionallyCompose( (e) -> cancel("./hotel/cancel", input.hotel, e) )
    )
    .exceptionallyCompose( (e) -> cancel("./flight/cancel", input.flight, e) )
    .exceptionally( (err) -> {EmailReq.sendFailEmail(); return null;} );
}