Editing Architecture of Btrieve (section)

===Pre-image paging vs Shadow paging===
Until version 6.0, ''pre-image paging'' was used when performing updates to records. It involved the creation of a new "pre-image file" before the changes were made, and then the pages from the original data file were copied into this new pre-image file temporarily. Then the system would make the changes to the original file. Should the update be interrupted and only half the data written to the page, then the page would just be rolled back by the engine by copying the page from the pre-image file back into the corrupted page in the original database file, then the temporary pre-image file would be deleted. The preimage files were given the extension .PRE, so finding these files in the system usually indicated that a transaction had not happened correctly and recovery had not been successful.

Starting in version 6.0, ''shadow paging'' was used instead of pre-imaging, and it is still used to this day. Instead of copying the page into a temporary file, the next spare physical location in the database file was found and the page was written to this location. This page is called a ''shadow page'' because it still has not had its location written to the file's PAT. Once the update to the shadow page completed, the PAT was updated and the entry recorded in the PAT of the next available and current physical page in the file. However, if a system failure occurred while doing the update to the shadow page the PAT would not be updated and so the change would be dropped because the current and next entry was not updated in the PAT.

The change-over from pre-image paging to shadow-paging caused radical file format changes that broke compatibility between previous versions of Btrieve and version 6.x of the product.