boolean update(SAMPLE x, Updater<AGGREGATOR, SAMPLE> updater) {
        synchronized (lock) {
            current = updater.update(current, x);
            return isRegistered;
        }
    }
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