private void checkScanNextCallSeq(ScanRequest request, RegionScannerHolder rsh)
    throws OutOfOrderScannerNextException {
    // if nextCallSeq does not match throw Exception straight away. This needs to be
    // performed even before checking of Lease.
    // See HBASE-5974
    if (request.hasNextCallSeq()) {
      long callSeq = request.getNextCallSeq();
      if (!rsh.incNextCallSeq(callSeq)) {
        throw new OutOfOrderScannerNextException(
          "Expected nextCallSeq: " + rsh.getNextCallSeq() + " But the nextCallSeq got from client: "
            + request.getNextCallSeq() + "; request=" + TextFormat.shortDebugString(request)
            + "; region=" + rsh.r.getRegionInfo().getRegionNameAsString());
      }
    }
  }
The returned String is used as key doing look up of outstanding Scanners in this Servers'
this.scanners, the Map of outstanding scanners and their current state.
@param scannerId A scanner long id.
@return The long id as a String.
hbase-server/src/main/java/org/apache/hadoop/hbase/regionserver/RSRpcServices.java