@Override
    public String toString() {
      return "clientIPAndPort=" + this.clientIPAndPort + ", userName=" + this.userName
        + ", regionInfo=" + this.r.getRegionInfo().getRegionNameAsString();
    }
Holder class which holds the RegionScanner, nextCallSeq and RpcCallbacks together.
hbase-server/src/main/java/org/apache/hadoop/hbase/regionserver/RSRpcServices.java