"How do you make sure that a hotel room cannot be booked by more than one person at a time" Excellent question! You don't. Instead, assuming a globally consistent transaction ordering, eg Spanner's TrueTime, but any uuid scheme suffices, it becomes a tradeoff between reconciliation latency and perceived unreliability. A room may be booked by several persons at a time, but eventually only one of them will win the reconciliation process.

    A: T.uuid3712[X] = reserve X
    ...
    B: T.uuid6214[X] = reserve X  // eventually loses to A because of uuid ordering
    ...
    A<-T.uuid6214[X]: discard T.uuid6214[X]
    ...
    B<-T.uuid3712[X]: discard T.uuid6214[X], B.notify(cancel T.uuid6214[X])
    -----
    A wins, B discards