4C Logging - Intro

• Data Replication
• Crash Recovery
• Auditing
• Debugging an application
• Undo/Redo of logged changes

4C Logging - Files

• log_hdr - One rcd exists in this file for each defined log.
• log_det - One rcd exists in this file for each instance a data file is being logged.
• log_ctl - This file is used internally by 4C to serialize access to the log files when changes are part of a transaction.
• log_fdhdr - maintains information about the file definition of logged files
• log_fddet - maintains field information about logged files.
• log_err - This records errors in logging.

• log_idx - This is a normal 4C file and exists in each individual log directory.
• log_data - This is a sequential file and there is one of them for each day of the month. They are named log_data.01 through log_data.31. The log_data sequential files must exist in each individual log directory.

4C Logging - Programs

• LogName - unique log name
• Description
• Directory - You must specify the full path of a valid log directory. Use the newlog.sh script to create one if necessary.
• KeepFor - Enter the number of days you want to keep the log data for. This should be less than 31 but for most uses will be much smaller, somewhere between 3 and 7. The log.purge.1 program uses this number to determine which log_idx records to purge and which log_data files to truncate.
• NUpdates - This gets updated everytime a rcd gets written to this log. Set it to 0 anytime you want.
• SaveBeforeImage - Check this if you want the log to maintain a copy of the rcd before it was modified or deleted. Records being added never have a BeforeImage.
• SaveAfterImage - Check this if you want the log to maintain a copy of the changed rcd. Deleted rcds never have an AfterImage.
• LogAdd - Check this if you want the log to log rcd Adds.
• LogMod - Check this if you want the log to log rcd modifications.
• LogDel - Check this if you want the log to log deletes.
• LogFWrite - Check this you want the log to log forced writes. A forced write is an update where 4C cannot tell if the rcd is being added or modified since no read was done of the rcd.
• LogUnhanged - Check this if you want to log updates that result in no change to the rcd.
• Disabled - Check this to disable the log.

• Sequence - sequence number
• Filename - The 4c filename
• ExtName - The external file name
• Dis? - 'y' if this file is disabled from logging.
• DBType - Valid only for external files. This currently can be one of:

   Oracle
   Postgres
   Sybase
   For an ODBC database, it must be the same as that specified
   in the sys.ext_type file for that external type.
   

• DBName - Valid only for External files.
• DBServer - Valid only for External files. Currently this should only be set for Sysbase Database external files. It should be the same as the sys.et_server field in the sys.ext_type file. If the external database does not use this field, then it should be left blank.

4C Logging - System PCLs

• sys.log_open()
• sys.log_close()
• sys.log_seek()
• sys.log_read()
• sys.log_getattr()
• sys.log_getname()
• sys.log_getval()
• sys.log_copyflds()
• sys.log_error()

• 1 - LOG_ERRCONN - Connection errro
• 2 - LOG_ERROPEN - Open Failed
• 3 - LOG_ERRNF - The specified <lclname> was not found as a currently open log.
• 4 - LOG_ERREOF - End of file while reading log
• 5 - LOG_ERRNORCD - There is no current log rcd.
• 6 - LOG_ERRNOFILEDEF - No file definition. This is an internal error that you should never see.
• 7 - LOG_ERRARG - Invalid or missing argument.
• 8 - LOG_ERRNOIMAGE - No rcd image exists of the type needed.
• 999 - Unknown Error

4C Logging - Misc

• Because the log_data and log_idx files can get large, you should make sure you keep logs only as long as necessary and run the log.purge.1 program regularly.
• If you are using the log.purge.run program to purge log data, you must verify the doit() PCL. It used to not loop, then at one time it looped continuously without sleeping. The PCL should look like this:

       while(1) {
         if (curdate != sys.cur_date) {
           curdate = sys.cur_date;
           sys.push_prog("log.purge.1");
         }
         sleep(3600);
       }
       

  This has been fixed in 4C Server Version 4.4.6-3 and higher
• Multiple log definitions are allowed
• One file may be logged by more than one log.
• Even though it is possible to log the same 4cfile without different external names in the same log, it is not a good idea. When reading the log it will be impossible to tell which of the external files was being updated.
  
  Use different logs for this purpose.
• 4C will notice when file definitions change and the data written to the log will always reflect the correct file definition.
• Don't try to log any of the following files:

  log_idx
  log_fdhdr
  log_fddet
  

• It is possible to log database files as well as 4C files, but it is not possible for 4C to know when non 4c programs make changes to database files. Neither is it possible for 4C to know what changes were made by an SQLStatement in a 4C program. If you need to log database files, you really should be using the DBMS tools for this.
• If more than one instance of 4c shared mem is running on a system and the different instances log the same data files, then the log_hdr, log_det, log_err log_fdhdr, and log_fddet files must be the same. The best way to accomplish this is to make symbolic links for these files between the different system directories.
• Do not define more than one log using the same directory Currently there is no check for this but it is guaranteed not to work.
• After defining logs, you should monitor them for size and errors.
• The system log info is initialized only when the 4c system memory is first allocated, so changes made to a log definition have no affect until all 4c process have been stopped. It is not necessary to stop the 4csrvrd process.
• It is very important to make sure your filesystem has enough space for your log files.
• Even when BeforeImage is selected, it is not always possible to know the BeforeImage rcd. The most common reasons for this are:
  • The application uses F_NODEBLOCK when reading a rcd
  • The Application calls sys.upd_file() without previously calling sys.read_file().
  • An external program, like the 4c utilities, use FCupdfile() without calling FCreadfile()
• Currently there is no program that can be used for crash recovery.