The LogEvent class has four methods named read_log_event. This is the base class for binlog events. I might prefer different names. I was extremely confused by servers on which the value of Seconds_Behind_Master rapidly fluctuated between 0 and a large value. This behavior is documented, I just had not read that page recently. Fortunately, the db operations experts explained it to me. So I am adding a few more variables to make it easier to monitor replication including:

• Counters for the number of binlog events written by the IO thread and run by the SQL thread.
• Counters for the number of bytes written by the IO thread and run by the SQL thread.
• A timer for the number of seconds that the SQL thread waits for the IO thread to provide more events.

One:
  static Log_event* read_log_event(IO_CACHE* file,
                                   pthread_mutex_t* log_lock,
                                   const Format_description_log_event
                                   *description_event);