code: public Mutation hintFor(Mutation mutation, long now, int ttl, UUID targetId)
      {
          assert ttl > 0;
  
          InetAddress endpoint = StorageService.instance.getTokenMetadata().getEndpointForHostId(targetId);
          // during tests we may not have a matching endpoint, but this would be unexpected in real clusters
          if (endpoint != null)
              metrics.incrCreatedHints(endpoint);
          else
              logger.warn("Unable to find matching endpoint for target {} when storing a hint", targetId);
  
          UUID hintId = UUIDGen.getTimeUUID();
          // serialize the hint with id and version as a composite column name
          CellName name = CFMetaData.HintsCf.comparator.makeCellName(hintId, MessagingService.current_version);
          ByteBuffer value = ByteBuffer.wrap(FBUtilities.serialize(mutation, Mutation.serializer, MessagingService.current_version));
          ColumnFamily cf = ArrayBackedSortedColumns.factory.create(Schema.instance.getCFMetaData(Keyspace.SYSTEM_KS, SystemKeyspace.HINTS_CF));
          cf.addColumn(name, value, now, ttl);
          return new Mutation(Keyspace.SYSTEM_KS, UUIDType.instance.decompose(targetId), cf);
      }
docs: Returns a mutation representing a Hint to be sent to <code>targetId</code>
  as soon as it becomes available again.