protected int getReplicaThreadIndexToUse(String datacenter) {
    return nextReplicaThreadIndexByDc.get(datacenter).getAndIncrement() % replicaThreadPoolByDc.get(datacenter).size();
  }
Select next available {@link ReplicaThread} in given datacenter.
@param datacenter the datacenter String.
ambry-replication/src/main/java/com/github/ambry/replication/ReplicationEngine.java