AGGREGATOR copyCurrent(ObservableUpdater<AGGREGATOR, SAMPLE> updater) {
        AGGREGATOR view;
        synchronized (lock) {
            view = updater.copy(current);
        }
        return view;
    }
The current generation of data produced from a single thread, where
generation is the period between two subsequent calls to
ThreadLocalDirectory.fetch().
vespajlib/src/main/java/com/yahoo/concurrent/LocalInstance.java