anchor.diff:      {SrcInstanceStartTime, TgtInstanceStartTime} ->
           ?LOG_INFO("recording a checkpoint for ~s -> ~s at source update_seq ~p",
               [dbname(Source), dbname(Target), NewSeqNum]),
  -        SessionId = couch_uuids:random(),
           NewHistoryEntry = {[
               {<<"session_id">>, SessionId},
               {<<"start_time">>, list_to_binary(ReplicationStartTime)},
  
positive.code:         committed_seq = NewSeqNum,
          start_seq = StartSeqNum,
          history = OldHistory,
          source_log = SourceLog,
          target_log = TargetLog,
          rep_starttime = ReplicationStartTime,
  
negative.code: 
          start_seq = StartSeq,
          history = History,
          session_id = couch_uuids:random(),
          source_log = SourceLog,
          target_log = TargetLog,
          rep_starttime = httpd_util:rfc1123_date(),
  