code: @Nonnull
  	public String getNodeId(final InetSocketAddress socketAddress) throws IllegalArgumentException {
  		if ( socketAddress == null ) {
  			throw new IllegalArgumentException("SocketAddress must not be null.");
  		}
  		final String result = _address2Ids.get( socketAddress );
  		if ( result == null ) {
  			throw new IllegalArgumentException("SocketAddress " + socketAddress + " not known (registered addresses: " + _address2Ids.keySet() + ").");
  		}
  		return result;
  	}
docs: Return the nodeId for the given socket address. Returns <code>null</code>
  if the socket address is not known.
  @throws IllegalArgumentException thrown when the socketAddress is <code>null</code> or not registered with this {@link MemcachedNodesManager}.