* @throws com.alibaba.wasp.client.RetriesExhaustedException
* wrapping a ConnectException if failed putting up proxy.
*/
private AdminProtocol getServerConnection(final ServerName sn)
throws IOException {
AdminProtocol admin = this.serverConnections.get(sn);
if (admin == null) {
LOG.debug("New connection to " + sn.toString());
admin = this.connection.getAdmin(sn.getHostname(), sn.getPort());
this.serverConnections.put(sn, admin);
}