Understanding Archive Logs & Redo Logs

Archive logs are vitally important for Retail Pro database recovery.  But what exactly are archive logs?  To understand archive logs, you need to first understand redo logs.  Redo logs are key database files that store changes made to the database.  Every new sales transaction.  Every new or modified customer record.  Every time an item's description changes.  All such database changes are stored in one of three redo logs, each with a 100MB file size.  So what do redo logs have to do with archive logs?  Well, archive logs are basically copies of redo logs. 

Let's take a closer look:

• As changes are made to the database, those changes are first stored in Redo Log 1. 
• When Redo Log 1 is full, Redo Log 2 is used to store database changes.
• When Redo Log 2 is full, Redo Log 3 is used to store database changes.
• When Redo Log 3 is full, the system does not create a Redo Log 4. 
• Rather, it reuses Redo Log 1.  But remember, Redo Log 1 is currently full of data.  So before reusing Redo Log 1, the system copies that data to an archive file, which resides outside the database.  Let's call this Archive File 1.  Now Redo Log 1 is free to store database changes again.
• When Redo Log 1 is full, Redo Log 2 has to be reused.  As before, the system copies any data currently in Redo Log 2 to another archive file - Archive File 2 - before reuse. 
• This process of using and reusing the three redo logs will ultimately result in the creation of numerous archive logs. 

Given their role in database recovery, here are a few best practices relating to archive logs:

• Do Not Disable Archiving - You should never disable the archiving function in Retail Pro, even when applying a software update (maintenance pack).  The only acceptable time to disable archiving is during the initial data migration to Retail Pro 9.  Just make sure to enable archiving after data migration is complete.
• Store Archive Logs on External Media - Like the backup, archive logs should be stored on an external storage device.  This will prevent the loss of archive logs that have yet to be backed up.  Using the Technician's Toolkit, you can easily modify the location (log path) where archive logs should be stored.

Note: To maximize Retail Pro performance, initially save archive logs to the same computer as the primary data source and then copy or move (manually or via batch job) to removable media.

Delete Archive Logs as Needed - Archive logs can easily get out of hand and fill up your hard drive.  For this reason, they should be periodically purged.  Ideally, archive logs should be deleted as part of the Technician's Toolkit-created backup process.  Never delete archive logs that have not been backed up.