Random rand = new Random(random().nextInt());
@Override
public void run() {
try {
while (operations.decrementAndGet() >= 0) {
// bias toward a recently changed doc
int id = rand.nextInt(100) < 25 ? lastId : rand.nextInt(ndocs);
// when indexing, we update the index, then the model
// so when querying, we should first check the model, and then the index